The crystalline quartz-rich raw material from Olduvai Gorge (Tanzania): why is it called quartzite when it should be called quartz?

The major raw material documented in the archeological sites of Olduvai Gorge (Tanzania) is a geological material with crystalline appearance, white or colorless, foliated or seemingly massive only at the outcrop scale, with a very high quartz-rich composition, and apparently of metamorphic origin, named by us in this paper: Crystalline Quartz-rich Raw Material (CQRM). Since the early days of research in Olduvai Gorge, a long-lasting terminological imprecision has allowed defining this material in a confused way as quartz or quartzite. Stubbornness in terminological imprecision reflects the complexity and specificity of CQRM related to a protracted and complex geological history composed by quartz-bearing metamorphic rocks of varied types and origins from recycling and/or tectonic reworking of much older Precambrian orogens and cratons. Currently the term quartzite is preferred by most researchers, despite being materials that have an appearance macro- and microscopic similar to quartz and show a response to fracture mechanics, and cutting-edge functional response is closer to quartz. In our view, it is crucial to undertake a comprehensive analysis of the CQRM from the structural, metamorphic, and petrological perspectives. Bearing this in mind, the main objective of the present study is to build a robust and conclusive petrological background that will enable an accurate identification and classification of this quartz-rich mineral resource. This geological material should be identified as “quartz.” The most diagnostic features supporting this interpretation can be summarized as some of the microstructural relics identified concur undoubtedly with a hydrothermal origin of the quartz and the recognition of special deformational structures at macro and micro scale point to tectono-metamorphic overprint of the hydrothermal quartz under granulite-facies conditions during the Panafrican orogenesis about 640 Ma ago.


Introduction
Raw material studies carried out in the paleoanthropological complex of Olduvai Gorge (Tanzania) have led to the identification of about ten different rock types that were used by early humans in the numerous lithic assemblages recovered there (Blumenschine and Peters 1998;Blumenschine et al. 2012a;Díez-Martín et al. 2009a;Egeland et al. 2019;Favreau et al. 2020;Hay 1976;Kyara 1999;Leakey 1967;McHenry and de la Torre 2018;Stiles 1991Stiles , 1998Tactikos 2005). Source areas for all these rock types have also been easily identified (Flébot-Augustins 1997;Hay 1971;Jones 1994;Kyara 1999).
Among these rocks, one in particular stands out as the most characteristic and conspicuous raw material (Fig. 1). It consists of a light-colored, homogeneous, and anisotropic siliceous crystalline material. It usually exhibits a penetrative foliation with a well-defined mineral/stretching lineation at macroscopic scale and a microscopic fabric revealing hightemperature deformation. These macro-and microstructural features support its identification as "metamorphic quartz." Given that this quartz-rich material has a relatively homogeneous cm-scale crystal grain size, with SiO 2 contents > 96%, this paper will label it "Crystalline Quartz-rich Raw Material," or CQRM, for intended interdisciplinary descriptive purposes. This raw material has been alternatively referred 1 3 78 Page 2 of 28 to, with no clear reasons, either as "quartz," "quartz/quartzite," or "quartzite." Currently, as will be seen later, the term quartzite has been winning the terminological game.
The microstructure (already photographed by some authors but not specifically investigated from geological, petrostructural, and mineralogical viewpoints; e.g., Favreau et al. 2020;Sánchez-Yustos et al. 2012;Soto et al. 2020a, b) is well known in geological literature on quartz-bearing metamorphosed and/or tectonized rocks (e.g., Passchier and Trouw 1996). This microstructure is identifiable in rock thin sections normal to the macroscopic foliation and parallel to the lineation and consists of recrystallized quartz grains showing rectangular subgrains (Law 2014).
In the Olduvai Region, the closest bedrock outcrops of CQRM lithologies occur in the Naibor Soit inselberg. These raw materials were increasingly selected for use by hominins between 1.85 and 1.3 Ma (de la Torre and Mora 2005a, b;Kimura 2002;Kyara 1999;Leakey 1971): (a) they represent ≥ 65% of the lithic assemblages identified in the Bed I "Zinj" floor (Díez-Martín et al. 2021; (b) they predominate (74.5%) in the early Acheulean at FLK West, in lower Bed II ; and (c) they reach their highest proportion (≥ 93%) in uppermost Bed II, in sites such as TK and BK (Díez-Martín et al. 2009a;Santonja et al. 2014).
The terminological vagueness related to this particular rock type is a long-running trait of raw material and lithic studies in Olduvai Gorge. After the discovery of the paleontological sites carried out in 1911 by the expedition of the entomologist W. Kattwinkel, the volcanologist and paleontologist H. Reck completed the comprehensive paleontological and geological study of the region discovering the first Olduvai hominin skeleton. This author already identified the CQRM lithologies and labeled them with the dual term "quartz and quartzite" (Quarz und Quarzitrücken, in the original publication) (Reck 1914, p. 84). Subsequently, Louis Leakey's publications on paleoanthropological remains in Olduvai did not specify the raw materials used for production of stone tools (Leakey 1932;Leakey et al. 1933). After the break imposed by World War II, researchers used the terms "quartz," "quartz/quartzite," or "quartzite" equally.
The terminological ambiguity related to the CQRM in Olduvai Gorge bears an inescapable archeological implication. Most of the researchers involved in the characterization of this raw material and determination of its sources are not specialists in the field of metamorphic tectonite rocks. In fact, they have based their studies heavily on nonspecific bibliographic references. In our view, it is crucial to undertake a comprehensive analysis of the CQRM from the structural, metamorphic, and petrological perspectives. Bearing this in mind, in order to better identify and classify these materials, the main objective of the present study is to build a robust and conclusive background on the basis of (1) identification of the field relationships of these quartz-rich rocks with the surrounding geological materials, (2) study of the relevant microstructural features depicted by these rocks under the petrographic microscope, (3) geochemical characterization of significant minerals forming part of the mineral assemblage in equilibrium, and (4) petrofabric analysis of

Historiographical contextualization
An extensive bibliographic analysis of about one hundred and twenty scientific studies has been carried out in order to trace and understand the terminological evolution of the CQRM within the paleoanthropological research undertaken in Olduvai Gorge. This review allowed us not only to describe and systematize the different phases of the research devoted to these materials, but also to understand the origin of the recurrent mistakes that persist in the current state of the art. The following periods can be identified (Table 1).
1. Descriptive period . The first studies carried out barely discussed raw material identification. The first reference addressing this issue can be found in Louis Leakey's interpretation of the archeological sequence in Olduvai, where CQRM was generically described as "irregular lumps of quartz and quartzite" (Leakey 1951, p. 34). Subsequent works always referred to this rock type through the terminological dichotomy "quartz/ quartzite" (Cole 1954;Evernden et al. 1965;Kleindienst 1959;Leakey 1967). The same situation applied to Mary Leakey's benchmark monograph, volume 3, "Olduvai Gorge. Excavations in Beds I&II, 1960-1963" (Hay 1971Leakey 1971) and other relevant works published afterwards (Leakey 1975;Leakey et al. 1972;Stiles et al. 1974) where CQRM was indistinctly referred to as "quartz," "quartz/quartzite," "quartz and quartzite," and "quartz or quartzite." 2. Early identifying period . In 1976 Richard L. Hay, a specialist in sedimentary petrology, published his referential monograph on the geology of Olduvai Gorge. Although his work preferentially focused on establishing the normative stratigraphic sequence in Olduvai, he was also interested in the description and identification of lithic raw material rock sources. Hay (1976, p. 9) was the first author who described these CQRM specifically as: "The quartzite is extremely coarse-grained and commonly micaceous in the northern and eastern parts of the Olduvai region, including Naibor Soit and the hills and highlands to the north of the gorge. Individual crystals of quartz are generally 1 to 2 cm in diameter in these rocks. Most of the quartzite exposed to the south of the gorge is medium-grained and exhibits primary sandstone textures …". Hay's definition already shows a number of contradictions. Firstly, the grain size in the Olduvai CQRM is disproportionately larger (one order of magnitude) than the upper limit admitted for classification of a sedimentary siliciclastic rock as sandstone (< 2 mm) or as quartzite if the sandstone is very rich in quartz detrital grains (Folk 1974;Robertson 1999). In fact, siliciclastic rocks with grain sizes > 2 mm should be classified as conglomerates or rudites in a broader sense. Secondly, the key argument for the identification of these rocks as quartzites is based on the recognition in them of a fine lamination parallel to the compositional bed layering at a larger scale ("laminated quartzites" of some authors), interpreted by R. L. Hay as a primary sedimentary rock texture. This interpretation is contentious because it is based upon incomplete observations that failed to identify (1) the presence of a penetrative lineation contained in the planar structure and (2) their complete recrystallization that resulted in a crystalloblastic texture (as already noticed by Saggerson 1966). If these two features had been recognized, the above-mentioned lamination would have been interpreted correctly as a metamorphic foliation containing a mineral/stretching lineation, that is, a secondary metamorphic texture and not a primary sedimentary texture.

Geological framework
Olduvai Gorge (Tanzania) is located in the Great East African Rift Valley, more specifically adjacent to the Oldoinyo Ogol highlands (south of the Oliondo Mountains and Loita Hills). The mountainous area to the north of Olduvai Gorge is geomorphologically constrained by hard lithologies with a significant presence of quartz-rich rocks that were affected by high-grade regional metamorphism and concomitant deformation during the Neoproterozoic. These rocks and the deformational structures developed in these materials form part of the so-called East African Orogen (EAO; Stern 1994), formerly known as the "Mozambique Belt" (Holmes 1951). This orogen extends over 8000 km from the Sinai Peninsula to South Africa and beyond (current Antarctica) forming a 250-350 km wide belt, though locally it can approach 1000 km. This is one of the largest ancient orogenic belts on Earth, formed by the closure of the "Mozambique Ocean" between 650 and 500 Ma ago (Ediacaran to early Paleozoic times; Thomas et al. 2013) and the subsequent collision of the Eastern and Western Gondwana subcontinents and their magmatic arcs. Notably in Kenya and eastern Tanzania, two major crustal units can be distinguished in the EAO (Fig. 2): the "Eastern Granulites" and the "Western Granulites" (Hepworth 1972). The area of interest here has usually been ascribed to the "Western Granulite" unit, although so far it has not been studied in detail. As a result, studies of neighboring mountain domains several tens to a few hundreds of km apart (e.g., the Pare and Usambara Mountains or the Taita and Loita Hills) usually consider the Oliondo highlands a part of the Western Granulites (e.g., Cutten et al. 2006;Fritz et al. 2013), whereas others regard them as inliers of lowgrade metamorphic rocks (different from granulites) of the Western Granulites (e.g., Fritz et al. 2009, Fig. 2). The Western and Eastern Granulites have different lithological composition, age range of the protoliths, metamorphic grade, age of metamorphism, structural style, and igneous rock inclusions , and references therein).  Meert and Lieberman (2008), and Miller et al. (2011) The Eastern Granulites terrane is tectonically emplaced onto the Western Granulites, which themselves are tectonically emplaced over the Archean Tanzania Craton, made of much older though lower-grade metamorphic rocks (Holmes 1951). The age of tectonic stacking (diachronous along the orogenic belt) is ascribed to the Neoproterozoic Era, between 1000 and 538.8 Ma (Cohen et al. 2022). Deformation and metamorphism peaks (the so-called Kuunga Orogeny) occurred ca. 640 Ma ago in the Western Granulite Belt (composed of psammitic and pelitic metasediments and their migmatized equivalents) and the final crustal consolidation somewhat later at 580-500 Ma (Tenczer et al. 2013). Abundance of metamorphosed sandstones ("quartzites") among the psammitic and pelitic metasediments suggests derivation of detrital quartz grains from source areas dominated by granitic and gneissic rocks, such as those cropping out so far in the Tanzania Craton (Manya et al. 2006;Schlüter 1997;Thomas et al. 2016).

A geological guide to identify quartz-rich rocks in Pleistocene lithic industry as raw material sources
Quartz (and its polymorphic varieties, all compositionally being SiO2) is one of the most frequent minerals in the Earth's crust. Quartz and quartz-rich rocks are important raw materials in the current high-tech industry, in particular for the production of semiconductors and photovoltaics (e.g., Götze and Möckel 2012). The label "quartz-rich rock" is used here to encompass those rocks dominated by the quartz mineral (without reference to their origin). The suitability to conchoidal fracture, hardness, and resistance of flaked cutting-edges in quartz-rich rocks made them sought-after resources by the genus Homo since the early stages of technological behavior, from beginning of the Pleistocene. As explained in detail in a previous section, geological terms such as "quartz" and "quartzite" are used to label the raw materials of those lithic artifacts. Its correct detection serves as a guide to search for their source areas.
Although several specialists in non-geological disciplines have made considerable efforts to incorporate these geological constraints to their research, there exists considerable confusion regarding the terms (mis)used and their actual meaning. For example, "quartz" is a mineral in the geological sense that has formed as a result of geologic processes (cf. Deer et al. 1966;Neuendorf et al. 2005). However, the term quartz has sometimes been reported, implicitly or explicitly, as if it were a rock (e.g., Mourre 1996, p. 207;Sánchez Yustos et al. 2012, p. 7, etc.). In fact, a "rock" is in its correct geological sense an aggregate of one or more minerals (Neuendorf et al. 2005).
Petrographic observations (with the help of polarizing microscopes) have also been used in paleoanthropology and archaeology to describe and constrain quartz-rich rock characteristics. However, studies on the raw materials at the Olduvai sites usually failed to recognize and/or misinterpret diagnostic rock microstructures recorded in classical reference books (e.g., Passchier and Trouw 1996;Tucker 2013) and their mechanical significance (Spry 1969;Nicolas and Poirier 1976;Vernon 2004). The microstructural approach used in these works to discriminate the origin of quartzrich rocks has also been taken into account to explain/infer mechanical properties that might be of archeological interest, such as rock strength/fragility, isotropic/anisotropic character, and the predictability of fracture geometry during knapping. However, in several instances, they failed in the recognition of the mono-or polycrystalline character of the aggregates as well as in the identification of widespread solid-state metamorphic recrystallization fabrics imposed on pre-existing quartz rocks whatever their type. These types of quartz crystals are usually sourced (or derived) from pegmatite cavities where they can show a large grain size (up to several meters; e.g., Dias and Wilson 2000) and from hydrothermal veins that can attain large dimensions (several m-wide and km-long; e.g., Hippert and Massucatto 1998;Lemarchand et al. 2012). As a rule, they exhibit microstructures that isolate seemingly homogeneous and relatively intact lattice domains not to be confused with the detrital "grains" of rocks with a sedimentary primary origin (i.e., pure-quartz sandstones or quartzites). The dimensions admitted for some quartz-rich rock deposits/units may also be considered so large that they might no longer be interpreted as veins, but as "sedimentary quartzite" formations, the large dimensions of which are familiar to most researchers.
In spite of the above, also in the geological context, the term "quartzite" has been used loosely to name (1) metamorphic rocks formed by metamorphism of an almost pure quartz sandstone, (2) very hard sandstones, that is, sedimentary rocks composed almost exclusively of quartz grains cemented with additional quartz, (3) granular metamorphic differentiates formed by quartz dissolution in aqueous fluids and posterior re-precipitation coeval with metamorphism, and (4) hydrothermal/pegmatitic quartz mineral aggregates occurring in veins and genetically related to magmatic intrusions and ore deposits. Only the first case corresponds truly with a real quartzite in the geological sense, and, therefore, the usage of the term "quartzite" should be restricted to designate these rocks. This terminological confusion has brought to light the old "quartzite problem" (Skolnick 1965), requiring undoubtedly a microstructural analysis to identify the textural features registered in these rocks and discriminate clastic textures associated with siliciclastic rocks from metamorphic features (Howard 2005).
Further complicating matters, additionally, prominent tectonic fabrics can be superimposed later on any of the rock types during syn-metamorphic solid-state deformation. Such fabrics are defined by penetrative (dominant at a given micro and meso-scale) planar features termed "foliations" and/or linear features termed "lineations" (Spry 1969). In the case of rocks that have undergone significant deformation and metamorphism, however, the lack of a comprehensive microstructural study may lead to the wrong interpretation of these planar penetrative tectonic foliations as sedimentary laminations, as in Hay (1976). This is not the only case where non-specialists may confuse metamorphic features with sedimentary structures/ microstructures. Even the sigmoidal shear zone foliations well-known to "hardrock geologists" may be wrongly attributed to cross-bedding without truncation surfaces (climbing ripple cross stratification and aggrading beds) by "softrock geologists" (in the sense of Hall 1988) and others. Though geometrical similarities may exist between structures of radically different origins (sedimentary stratification/lamination versus tectonic/metamorphic pseudostratification or foliation; e.g., Turner 1941), careful microstructural observations are able to discriminate them.

Quartz-rich rocks in metamorphic environments
Metamorphic processes may affect rock precursors of any type (igneous, pegmatitic, hydrothermal, sedimentary, and even metamorphic) and usually redistribute in them large amounts of SiO 2 that are first mobilized (dissolved) and then precipitated (recrystallized) to form veins and lenses (Oliver 1996;Wagner et al. 2010). These are termed "metamorphic quartz mobilisates or differentiates" in the geological literature and occur along preexisting mechanical anisotropies. In regional metamorphic contexts, anisotropies are dominated by tectonic foliations formed in the host rocks during solid-state deformation accommodation and mineral growth process (Chapman 1950;Spry 1969;Yardley 1983).
Quartz metamorphic segregations (either quartz veins or more irregular masses) are derived from the wall rocks during metamorphism and, therefore, undergo minor transportation. In these cases, quartz vein abundance is closely related to the abundance of quartz in the country rocks (higher in quartzites, lower in pelitic schists). These quartz bodies usually appear as blankets parallel to foliations and shear zones or as saddles in minor fold hinges. Smaller quartz veins occurring parallel to foliations mainly form by diffusional transport (with a thickness of up to 10 cm, since diffusion is ruled out as a source of much larger veins).
Quartz-kyanite veins in high-grade quartzo-feldspathic schists and gneisses are classic examples of metamorphic differentiation in quartzose rocks containing aluminous material (e.g., Dorr 1969;Müller et al. 2007Müller et al. , 2012. Less dissolvable minerals are passively concentrated there by the solution and selective removal of the more soluble phases. The blankets can be continuous for meters or up to hundreds of meters (Guild 1957) and sometimes are of pegmatitic character, in coherence with the medium-to high-grade pressure and temperature conditions undergone by their metamorphic country rocks.
Even veins of hydrothermal origin may be filled with fine and/or coarse-grained quartz crystals that exhibit a banded or layered structure (e.g., Fig. 1 in Fonseca et al. 2015) parallel to the host rock walls. These well-known arrangements, usually observed in large veins that can be traced for some meters at most, are related to progressive vein infilling processes by repeated crack-seal mechanisms (Bons et al. 2012;Ramsay 1980) and to mineralizing fluid diversion into shorter and wider cavities upon hydrofracture arrest (Bons 2001). The layering described, which can even be strengthened in metamorphic scenarios where the veins become overprinted by solid-state plastic deformation, can also be misinterpreted as sedimentary lamination/bedding. A note of caution is thus needed when this type of interpretation is suggested in metamorphic environments.
The metamorphosed and strongly deformed equivalents of "hydrothermal vein quartz" and "pegmatitic quartz" may give rise to quartz-rich rock units and even mappable formations (made of poly-crystalline quartz aggregates) parallel to metamorphosed "sedimentary quartzites" and to "metamorphic quartz mobilisates." Yet, new "hydrothermal vein quartz" can also be generated during the overprinting process. Eventually, all these rocks occur interleaved in nature, may exhibit similar aspects to the naked eye, and might be wrongly termed "quartzite." Nevertheless, strictly speaking, they have radically different origins that might be revealed by careful study of their microstructure under the petrographic microscope (e.g., Lychagin et al. 2020).
In particular cases, hydrothermal quartz in orogenic metamorphic environments can occur in giant veins up to 15-20 km in outcrop length and tens of m in width (e.g., Jia and Kerrich 2000;Lemarchand et al. 2012) or vein complexes hundreds of m thick. They are known worldwide from the Archaean (Kerrich and Feng 1992) to the Cenozoic (Fonseca et al. 2015), including notable examples in linear Paleoproterozoic orogens (Pati et al. 2008;Rout et al. 2022) and in Gondwanan orogens (Carvalho 1983;Chaves 2007;Chaves et al. 2003;Esteves and Faleiros 2021;Hippert and Massucatto 1998) coeval with the Mozambique Belt in Tanzania during the Neoproterozoic.
In a nutshell, regional metamorphic terrains usually contain quartz-rich lenses and layers with thicknesses varying between cm and some hm thick and dm to km in map extent. Metamorphic quartzites after sedimentary precursors are common among them. These may conform large quartzite units with several km 2 outcrops (fine examples exist to the N of the Olduvai Gorge Region) that co-exist with (1) quartz veins of hydrothermal origin formed after dissolution/precipitation processes during diagenesis/burial and low-grade regional metamorphism, (2) syn-tectonic metamorphic quartz mobilisates forming veins usually parallel to the host rock principal foliation during low-to high-grade metamorphism, (3) pegmatitic, hydrothermal veins or stockworks and irregular masses associated with larger plutonic rock intrusions, and (4) igneous quartz-rich rocks.

Materials
Diverse quartz-rich lithologies, reminiscent of the raw materials used by Olduvai hominins to produce artifacts, have been collected in outcrops of Precambrian rocks (Fig. 3) located within a radius of about 40 km around Olduvai Gorge. The study of the variability of the samples collected, in principle, would permit us to (1) identify the basic types of quartz-rich rocks that crop out on the surface of this territory; (2) resolve their key mineralogical composition, microstructure, and fabric with a two-fold geological and material characterization application; and (3) tag the correct rock name to the particular crystalline quartz-rich raw materials (CQRM).
The best and most extensive Precambrian rock outcrops occur to the north of the Olduvai Gorge (Fig. 3). The only previous geological survey existing so far (Pickering 1958) categorized the outcropping rocks into two groups: the Oldoinyo Ogol and the Serengeti Groups. The lithostratigraphic and structural relationships within each group remain obscure and unknown. The layered nature of most units may suggest that those groups are dominated by thick (hm to km) successions of metamorphic rocks derived from terrigenous protoliths (sandstones, siltstones, and mudstones), currently with gentle to moderate dips and an apparent structural simplicity. However, as Shackleton's observations in equivalent neighboring areas demonstrate: (1) the outcrops exhibit a profusion of tight to isoclinal folds, implying conspicuous and intricate succession reversals; (2) the deformation is intense, as proven by the development of a well-defined foliation parallel to the compositional banding, a pervasive lineation on the foliation surfaces and the presence of elongated minerals and mineral aggregates, and (3) shear zones occur parallel or at low angle to the foliation, possibly implying tectonic succession repetitions and/or discontinuities (Shackleton 1993). North of Olduvai Gorge these structures can be remotely perceived in aerial views (Fig. 3), strongly suggesting that the outcrop observations For the purposes of the current study, the Oldoinyo Ogol Group contains two formations of interest: the Kissele and the Loipukoi Quartzites. The "Kissele Quartzite" was originally described by Pickering (1958) in his brief geological map explanation as: "very coarse-grained, crumbly, red quartzite which generally overlies white or colourless, coarse-grained quartzites" (Fig. 4a). Our field observations confirm that reddish quartzites are the dominant lithology ("host rocks") in the Kissele Quartzite outcrops (Fig. 4b).
They appear to be extremely brittle and, from our viewpoint, unsuitable for knapping. The "white or colorless quartzites" (sensu Pickering 1958), in turn, resemble the milky quartz veins (Fig. 4c) so common in low-to medium-grade metamorphic areas worldwide (e.g., Bons 2001). In the study area, the white/colorless quartz rocks would correspond to hydrothermal quartz veins and metamorphic differentiates later reworked and metamorphosed under high-grade conditions (Fig. 4d). They occur not only in primary outcrops but also as loose fragments forming talus deposits on the hillslopes. These cobbles and boulders (sensu Krumbein and Sloss 1951) display sharp edges and usually flat surfaces. Their size, tabular morphology, and availability might have been appropriate for knapping by hominins in the area (Fig. 4e). This type of quartz-rich rock is common in bedrock outcrops in the region such as the Naibor Soit inselberg, so often cited in archeological literature (e.g., Egeland et al. 2019;Santonja et al. 2014), and indeed is the most conspicuous artifact raw material at archeological sites in Olduvai Gorge. In this study, white/colorless quartz samples were collected from outcrops at Kissele, Naibor Soit, and Lekongi ( Fig. 3 and Table 2).
The second formation of interest in the Oldoinyo Ogol Group, the "Loipukoi Quartzite," was mapped by Pickering (1958) to the NW of the Kissele Quartzite. The closest outcrops to Olduvai Gorge are located ca. 35 km away and form various inselbergs with individual dimensions between 500 and 2000 m in length aligned for a distance of ca. 25 km. These outcrops are formed by a gray (smoky)-colored polycrystalline quartz, whose color is due to the presence of large amounts of minute ferromagnesian mineral grains and microinclusions in quartz (Fig. 4f). This rock exhibits a penetrative metamorphic foliation/lineation and is mechanically compact. Its microstructural homogeneity confers excellent quality for knapping. Additionally, the relative proximity of these outcrops to Olduvai Gorge makes this rock a likely candidate, although in a minority way, as a raw material in the lithic industry. In this study, gray quartz samples were collected from the Loipukoi outcrop ( Fig. 3 and Table 2).
The Serengeti Group is poorly represented in the area of study. Pickering (1958) mapped a 6 km long inselberg (and three additional ones with lengths below 500 m) made of rocks ascribed to this group. The "Lemuta Quartzite" formation of the Serengeti Group is located to the west of those of the Oldoinyo Ogol Group, the closest of them being 20 km away from Olduvai Gorge. The "Lemuta Quartzite" consists of white, fully recrystallized, and foliated/lineated quartz-rich rocks, intercalated with quartz-schists (Fig. 4g). Some quartz-rich rock beds contain stunning, mm-cm-sized kyanite crystals elongated parallel to the foliation/lineation and occasionally broken and stretched (Fig. 4h). Occurrence of kyanite in the rock mineral assemblage is petrologically outstanding and points to a medium-to high-grade metamorphic overprint that may help to constrain the formation/ reworking conditions of other rocks in the area (metamorphosed quartz-arenites or metamorphic differentiates). However, these rocks are not as mechanically strong as those previously described. In fact, their knapping qualities are very poor and their alteration produces weak granular products not suitable for use. This likely explains from the archeological perspective why those rocks are not found among the Olduvai Gorge hominin artifacts. Bearing in mind all the above, we sampled this formation in the Lemuta outcrop ( Fig. 3 and Table 2).

Petrographic and geochemical analysis
Standard 30 µm thick polished rock sections were used for conventional petrographic and microstructural studies. The sections were cut parallel to the XZ or XY structural planes (XY defined by the orientation of the foliation plane, X defined by the orientation of the mineral and stretching lineation).
Mineral analyses were performed in the Scientific-Technical Services microprobe unit at the University of Oviedo (Spain) with Cameca SX-50 and SX-100 automatic microprobes, the latter equipped with five wavelength dispersive spectrometers, a dispersive energy spectrometer, and with secondary electron, back-scattered electron and cathodoluminescence detectors. The operating parameters included a 10 s integration time, a 10 nA beam current, and a 15 kV accelerating voltage. Mineral structural formulae were calculated by charge balance criteria following various procedures suggested in the bibliography for different phases (see Droop 1987 andSpear 1993, for further details).

Electron back-scattered diffraction
The Electron Back-Scattered Diffraction (EBSD) study was performed on selected thin sections cut as indicated above. These were ultra-polished with a colloidal silica suspension to remove surface damage and then carbon coated to prevent charging. A copper tape was attached surrounding the measurement area to reduce charging effects. Crystallographic preferred orientation measurements were performed at the University of the Basque Country (Electron Microscopy Facility-SGIker) with an automated Electron Back-Scattered Diffraction system attached to a JEOL JSM-7000F Field Emission Scanning Electronic Microscope (FE-SEM). Samples were mounted in this device on a stage tilted 70°, with the rock lineation parallel to the SEM X-axis. The beam working distance was 20 mm (Prior et al. 1999) and the detector was placed at 188 mm. An acceleration voltage of 20 kV and a beam current of ca. 3.5 nA were applied. Crystallographic orientations were obtained using the Chan-nel5 software package after automated EBSD analysis on a predefined sampling grid with a step of 20-30 µm, covering up to 80% of the thin sections. These steps are significantly smaller than the average grain size of the minerals. The "raw" indexation percentage ranged between 89 and 97%. The obtained data were processed with MTex and Matlab (Bachmann et al. 2011(Bachmann et al. , 2010. Crystallographic orientation solutions with mean angular deviation (MAD) values between detected and simulated patterns over 1.2° were rejected to assure EBSD measurement reliability. The data were filtered so that the orientation diagrams contain one orientation per grain. The grain detection technique considered a critical misorientation threshold of 10°. To avoid errors in the grain detection related to the eventual presence of non-indexed pixels and the large grain size of the analyzed minerals only those grains covering more than 5 pixels were considered. Fabric orientation distributions are presented in lower hemisphere, equal area stereographic diagrams. The projection plane always corresponds to structural XZ sections and the macroscopic foliation is represented there as the equatorial diameter (E-W). The lineation is horizontal within the same plane.
The modal proportions were estimated on the basis of the fraction of grains indexed during the EBSD measurement. The strength of the fabric was expressed by the J texture index (Bunge 1982), representing the mean square value of the orientation distribution function (ODF). The calculations were performed with the MTex texture analysis software (Bachmann et al. 2010;Mainprice et al. 2011).

Petrography
Among the seventeen rock specimens sampled in the Olduvai Gorge region (Table 2), eighteen correspond to quartz-rich raw material from the Loipukoi, Lemuta, Kissele and Naibor Soit areas (Fig. 3). Their source rocks are much stronger and more resistant to weathering and erosion than the surrounding/interbedded rock formations and, thus, define positive reliefs above the average altitude of the area. Those rocks always exhibit an outstanding planolinear fabric with a distinct foliation containing a clear stretching/mineral lineation, though sometimes seemingly massive (only at the outcrop scale, not at the microscope scale) cm-to m-thick layers, and lenses parallel to the country rock foliation also occur (Fig. 5a). The latter usually correspond to white/colorless quartz. They are mechanically hard and their erosional dismantling produces variable amounts of loose rock fragments (as described in a preceding section; Fig. 5b).
The white/colorless quartz rocks (Fig. 5a, b) contain a mineral assemblage in equilibrium composed of quartz (up to 98%) and muscovite (1.5%), accompanied by accessory phases (< 1%) such as gypsum, ilmenite and other opaque minerals, tourmaline, rutile, kyanite and zircon. Quartz grains appear as elongate large crystals with the largest dimension up to 1.5 cm. Elongate quartz grains exhibit a well-defined shape-preferred orientation that defines the rock macroscopic foliation and lineation (Fig. 5c). Quartz grain boundaries are irregular and exhibit abundant microstructures (e.g., bulging, convex boundary segments among crystal defect-free and strained grains, complete inclusion of other minerals of moderate size) that denote active migration during recrystallization under temperature conditions high enough to permit activation of quartz deformation mechanisms dominated by grain boundary mobility (Law 2014). The concomitant development of quartz subgrains and new grains is remarkable, as their boundaries show diagnostic geometrical arrangements with two sets of grain-boundary segments at a high angle to each other, consistently oblique Fig. 4 a Outcrop detail of "Kissele quartzites" (sensu Pickering 1958) in Naibor Soit showing a contact between very coarse-grained, crumbly, reddish quartzite, and a compact, white/colorless polycrystalline quartz vein. b Naibor Soit outcrop of very coarse-grained, reddish quartzite strongly foliated, and crumbly, thus unsuitable for knapping. c Naibor Soit outcrop of foliated, white/colorless polycrystalline quartz (several m thick) reminiscent of milky quartz veins common in low-to medium-grade metamorphic siliciclastic successions. d Outcrop of white/colorless deformed quartz veins. These likely correspond to hydrothermal and/or metamorphic differentiates later reworked and metamorphosed under high-grade conditions. e Hillslope of the Naibor Soit inselberg with climbing people, showing a white quartz rock outcrop at the summit and scattered loose white/colorless quartz fallen fragments, mainly with tabular formats. The inset shows a closer view of the ground with irregular shaped cm-scale white quartz fragments. f Outcrop of the "Loipukoi quartzites" (sensu Pickering 1958) formed by gray (smoky)-colored polycrystalline quartz rocks with a penetrative metamorphic foliation/lineation. The rock color is due mainly to the presence of large amounts of minute ilmenite and other opaque grain minerals. g Outcrop of the "Lemuta quartzite" (sensu Pickering 1958) consisting of white, recrystallized, and foliated/lineated quartz-rich layers interbedded with quartz-schists. h Quartz-rich bed containing mm/cm-sized kyanite (Ky) crystals elongated parallel to the rock foliation/lineation. These are sometimes broken and stretched and their presence (petrologically outstanding) characterizes a medium-to high-grade metamorphism ◂ to the rock foliation. This diagnostic microstructure ("chessboard microstructure"; Gapais and Barbarin 1986; Fig. 5d) denotes quartz plastic deformation under high-temperature (> 600 ºC), hydrous conditions, and high strain rates (Bouchez et al. 1984(Bouchez et al. , 1985Kruhl 1996;Mainprice et al. 1986;Okudaira et al. 1995, Passchier andTrouw 1996) (Table 3).
Two types of white mica inclusions in quartz can be distinguished in the white/colorless quartz. The first (1) consists of small flakes with average 400 μm length and a few tens of µm width. These can be either completely included within quartz (Fig. 5c) or pinned at their grain boundaries (Fig. 5d), which acquire higher curvatures in their proximity. In all cases they share a unique parallel orientation identical to the elongation direction of quartz grains and the macroscopic foliation. This microstructure attests to pervasive secondary recrystallization under a strain field by means of active grain boundary migration mechanisms. The second type of white mica inclusions (2) are minute crystals (up to a few tens of µm long and some µm thick) that can be appreciated in polarized light microscopic observations under crossed nicols, only if the host quartz crystal is taken to a position of optical extinction. These flakes occur in quartz grains showing subgrains and undulose extinction, that is, in plastically deformed quartz grains not obliterated by complete recrystallization. There, the micro-inclusions appear as bright flakes (upper red circle in Fig. 5d) that usually exhibit identical orientations parallel to certain quartz host crystallographic directions (usually two or three; cf. Figure 5e) that may coincide with fine orientation bands along quartz rhombohedral planes {r} and {z} (Derez et al. 2015). These microstructures are diagnostic features that indicate a hydrothermal vein quartz origin (Götze 2012). During such quartz vein development, the mica flakes would have been attached to quartz crystal faces during free growth.
The study of fluid inclusions in quartz can cast light upon the origin of the rocks under study. Thus, although fluid inclusions can be found enclosed in quartz grains, they are not as ubiquitous as in the case of hydrothermal quartz (Götze 2012;Johnston and Butler 1946). In fact, they are rare in the interiors of recrystallized new grains. In turn, the subgrain and grain boundaries may contain some fluid inclusions, but they are also scarce and confer a very clean aspect. In our samples, grains with relic undulose extinction show a few intragranular healed microcracks, recognizable as fluid inclusion planes (Van den Kerkhof and Hein 2002; Fig. 5f-h). These can also be considered microstructural relics, with a twofold implication. On the one hand, their presence demonstrates a former brittle response of stressed quartz rocks under low to moderate temperatures followed by an incomplete recrystallization under higher temperatures. On the other hand, they exemplify recrystallization and grain boundary mobility/reorganization processes that result in a concomitant reorganization of the quartz solid and fluid inclusions, leaving clean grain interiors (Schmatz and Urai 2011;Vityk and Bodnar 1995). These microstructures also support that hydrothermal quartz protoliths, rich in aqueous fluids, facilitated the operation of ductile deformation mechanisms under moderate to high temperatures (Palazzin et al. 2018;Stünitz et al. 2017) that actually resulted in the outstanding planolinear macroscopic fabric and microstructure of this white/colorless quartz.

Quartz petrofabric
Lattice preferred orientation (LPO) patterns of rock-forming minerals are designed in Geology to illustrate the orientation of certain significant crystallographic elements. Usually, these patterns are represented in stereographic diagrams using the macroscopic foliation and lineation recognized in the rocks (resulting from strain-induced microstructural reorganization) as an external referential. The presence of a preferred orientation can be unraveled in appropriate fabric diagrams (e.g., Fig. 6) by clusters or concentrations of crystal axes and/ or planes around specific orientations. The "J texture index" quantifies the intensity and strength of such a preferred crystallographic orientation. In the case of quartz, LPO fabric diagrams are used to identify intracrystalline slip systems consisting of a slip plane and a slip direction. As a rule, mineral crystallographic planes with orientations approaching that of the macroscopic foliation are most likely to operate as slip planes. Moreover, mineral crystallographic directions close to Olduvai Gorge Chert the orientation of the macroscopic lineation are good candidates to depict the intracrystalline slip direction. The recognition of an LPO defined by the existence of a preferred orientation for certain crystallographic slip planes and axes can shed light on several deformation characteristics and thus provide valuable information on the active deformation mechanisms (brittle/plastic), deformation regime (coaxial/rotational), and thermobaric conditions (especially under lower or higher temperature (Law 2014;Passchier and Trouw 1996). Five fresh and well-preserved specimens from the Kissele, Loipukoi, Naibor Soit, and Lemuta areas have been selected for quartz petrofabric analysis. They are all characterized by high J texture index values (up to 4.6). This points to the existence of a fair quartz texture, that is, the presence of a remarkable quartz preferred crystallographic orientation that might support a sound petrofabric interpretation. Figure 6 portrays the LPO patterns determined for quartz c-axes and < a > -axes in sample KSL12.Qtz (Kissele area, see Fig. 3), a white/colorless quartz taken as a representative example. A distinct preferred orientation maximum of c-axes is present at a position forming an angle of 17-20º with the lineation orientation (X structural direction). Simultaneously, the < a > -axes scatter along a wide girdle perpendicular to the X direction. This LPO pattern denotes operation of the prism-[c] slip system in quartz by intracrystalline slip on prismatic planes (that contain the c-axis) along the c-axis direction (e.g., Blumenfeld et al. 1986;Kruhl 1996;Lister and Dornsiepen 1982;Schmid and Casey 1986). Here and in most previous studies worldwide, it has been identified in deformed quartz mineral aggregates that additionally exhibit chessboard microstructures. The association of these fabric and microstructural features supports the tectonic interpretation that the hosting quartz underwent high-temperature deformation under temperature conditions above 600-650 °C Mainprice et al. 1986;Passchier and Trouw 1996), likely in the range 700-800 °C (granulite facies of regional metamorphism) if the stable mineral assemblages present in related rocks is considered (e.g., Barth et al. 2010;Fernandez et al. 2003;Mainprice et al. 1986;Okudaira et al. 1995). The clear obliquity that exists between the LPO pattern and the external reference framework provided by the foliation and lineation implies that high-temperature deformation included non-coaxial (rotational) deformation components during strain accommodation, which further indicates that rock foliations acted as ductile flow planes, the flow direction being marked by the mineral-stretching lineation.

Discussion
Petrological observations made in this study of the Olduvai Gorge archetypal white/colorless quartz have revealed the presence of chessboard recrystallization microstructures fully overprinting rocks with a non-sedimentary primary microstructure. The scarce relics preserved concur with a hydrothermal origin of quartz layers. These include muscovite oriented microinclusions (µm-sized) inside large quartz crystals, the minor presence of euhedral/subhedral crystals of hydrothermal minerals such as tourmaline, gypsum and muscovite forming a paragenetically stable association, and the distribution of clouded domains rich in fluid inclusions in some quartz crystals. It is commonplace during metamorphism and recrystallization of vein quartz that a complete rearrangement (and/or leakage) of the fluid inclusions from grain interiors (clouded areas) and grain boundaries takes place, especially when recrystallized under high temperatures (Wheeler et al. 2004). Quartz veins likely formed a regional vein network developed during the prograde stages of regional metamorphism in the Mozambique orogenic belt. Some of the veins are large (several m-thick and hundreds of m long in current outcrops) and can be easily identified in the field and in panoramic views (Fig. 4) of the Precambrian inselbergs present to the north of Olduvai Gorge (including the proximate Naibor Soit and Kissele areas).
As commented above, Hay (1976) identified this rock type as quartzite and interpreted as primary sedimentary texture the macroscopic and visible mineral alignments produced by the preferred spatial arrangement of quartz mineral aggregates containing small muscovite crystals and other accessory minerals. From the petrological point of view, however, this premise is untenable because in terrains affected by high-grade metamorphism (like the area to the north of Olduvai), rock primary structures were erased due to the strong reworking associated with elevated pressures and temperatures prevalent during metamorphism.
R. L. Hay relied, notwithstanding, on previous geological surveys accomplished by experienced hard-rock geologists such as the 1/125,000 geological map, sheet 37 "Moru" completed by Pickering (1960). In this study he reported: "Unmetamorphosed granites of the shield are unconformably overlain by relatively unmetamorphosed quartzites, sandstones, and shales of the late Precambrian Bukoban System only about 25 km west of Lake Ndutu (Pickering 1960)" (Hay 1976, p. 11). However, in fact, the closest outcrops are really more than 50 km away from the Olduvai archeological sites. That is, they are located more than 25 km in a straight line towards the west of Lake Ndutu, in the headwaters of the gorge, which in turn is located another 25 km to the west of the archeological sites (Fig. 3). Those slightly metamorphic quartzites of the Bukoban System (actually the Bukoban Supergroup) that caught Hay's attention are currently known as the "Kinenge quartzites.". They are part of the Ikorongo group and lie unconformably on top of the Archaean craton rocks (Pickering 1960), in a tectonic domain different from the one in which Olduvai is located.
These materials were the subject of later studies (Kasanzu 2016;Kasanzu and Manya 2010;Kasanzu et al. 2008) where they were identified correctly as sandstones forming part of sedimentary rock successions containing also quartzites, shales, dolomitic limestones and basalts, and regarded as quartz-arenites by Kasanzu and Manya (2010, p. 365).
The white/colorless quartz hosted in the Kissele quartzite formation of the Oldoinyo Ogol Group (Pickering 1958) and similar rocks cropping out nearby in the Olduvai area are the best fitting candidates as mineral resources for the hominin artifacts. This CQRM was the result of the high-grade metamorphic overprint of protoliths that actually lack any primary sandstone texture vestiges but, in turn, exhibit fabric, microstructural and field characteristics (at outcrop and map scales) indicative of hydrothermal quartz veins. Thus, they should not be classified as "quartzites" in a normative petrological/geological sense. However, the reddish quartzrocks hosting the aforementioned CQRM can be classified as amphibolite-to granulite-facies metamorphic quartzites, identifiable attending to their color, mineral assemblage and microstructure. Both geological materials (white/colorless quartz representing hydrothermal veins and reddish quartzrock originally comprising part of a sedimentary succession where the former were emplaced), although different in their genesis, they have shared a common posterior metamorphic/ deformational overprint and, currently, depict comparable quartz petrofabric LPO patterns.
After Hay's work, reluctance to use the term quartzite by most authors led to prevalence of the tandem terminology for decades, using quartz and quartzite, as aforementioned, in varied forms that paid little or no attention to the geological origin of artifact raw materials. Kyara's (1999) attempt to overcome this imprecision remarked that: "…rock types are not still strictly categorized. At that time the names of the artifacts made on metamorphic siliceous rocks are interchangeably referred to as «quartz and quartzite» or «quartz/ quartzite»" (Kyara 1999, p. 115). This intended to find a consensual term binding together all the metamorphic raw materials identified at Olduvai: "For ease of data analysis, raw material types were grouped under three main working categories based on the three major rock types, namely; (i) Quartzites (inclusive of all metamorphic rocks: quartz, quartzite, purple quartzite, and gneiss); (ii) volcanics, in concurrence with Blumenschine and Masao (1991) instead of igneous rocks, which incorporate green phonolite, porphyritic phonolite, basalt, and trachyandesite, and, (iii) chert, which is the sole representative of sedimentary rocks in the region" (Kyara 1999, p. 116). However, this categorization was not grounded on any geological criterion and gave priority to terminological simplification without considering petrologic aspects that might have helped. The lack of robust geological identification criteria was probably the cause of this continuity in long term terminological imprecision.
The incorporation of new research teams working in Olduvai in the second decade of the twenty-first century has recently opened up a period in which the use of the term "quartz" lapsed into disuse without any petrological justification. In return, the term "quartzite" seems to acquire currently a widespread use (i.e., Díez-Martín et al. 2021;Fig. 5 a Microphotograph (crossed nicols) showing rutile (Rt) and muscovite with various habits (Mus) as accessory minerals included in a large quartz (Qtz) crystal that occupies the entire field of view. b Microphotograph (crossed nicols) showing gypsum (Gp) and muscovite (Mus) as accessory mineral inclusions within quartz. c Microphotograph (crossed nicols) showing the shape-preferred orientation of elongated quartz grains (Qtz) in a white quartz. Elongated quartz grains exhibit highly irregular boundaries in detail, denoting their high mobility during recrystallization, and contain nearly parallel scattered muscovite flakes (Mus1, e.g., those in the red circle) a few hundreds of µm long and 2-30 µm thick that also contribute to define the foliation. d Microphotograph (crossed nicols) showing a chessboard microstructure characterized by the formation of rectangular subgrains with nearly orthogonal boundaries and formed as a result of solid-state intracrystalline deformation. Two types of muscovite inclusions can be identified (red circles): larger muscovite flakes (Mus1) with a parallel orientation that defines the foliation, and much smaller muscovite flakes (Mus2) observed as bright spots and thin traits with diverse orientations oblique with respect to Mus1. e Close view of the interior of a quartz grain taken under crossed nicols in a position close to its optical extinction. The bright spots (e.g., those seen in the red circle) correspond to µm-sized muscovite grains (Mus2) and are aligned along two directions that coincide with fine extinction bands (FEB1 and FEB2; cf. Derez et al. 2015) parallel to quartz host rhomb crystallographic planes. f Close view of the interior of a quartz grain (crossed nicols) of the type imaged in (d). The image shows irregular bands rich in fluid inclusions inside a monocrystal that confer a clouded appearance to the domain enclosed by the dashed lines (by contrast with much cleaner quartz crystal domains outside). In the interior of the closed bands several solid and fluid inclusions are elongated and/or aligned parallel to the direction of the red segments, which is controlled by host quartz prismatic crystallographic planes. g High magnification image (parallel plane polarized light) of a quartz crystal interior showing a curved band (delimited by white dashes) containing µm sized (spherical to ellipsoidal) and elongated-parallel fluid and solid (opaque and irregular) inclusions. The band is a curved surface tilted ca. 45º with respect to the plane of the image. It can correspond to a relic primary hydrothermal quartz growth feature (not a healed microcrack). The quartz host contains a much smaller proportion of micro-and nanoinclusions and, thus, exhibits a clean aspect. h High magnification image (crossed nicols) of a quartz crystal interior showing a slightly curved band (delimited by white dashes) defined by µm sized (spherical to ellipsoidal) elongated fluid inclusions. The fluid inclusion band is nearly orthogonal to the plane of the image and the elongation direction of the inclusions (red segments) is parallel to the orientation of fine extinction bands (FEB) of the host quartz grain, that is, parallel to a specific crystallographic direction. This concurs with these being relic primary microstructures (of hydrothermal origin), since recrystallization would lead to a complete reorganization (and even decrepitation or removal) of the fluid inclusions, leaving clean quartz grain interiors and fluid accumulation/migration along grain boundary paths. Microphotographs (a), (g) and (h) were taken from Kissele (KSL16.Qtz in Table 2 and 3), whereas (b) to (f) correspond to similar samples from Naibor Soit (NBS13.Qtz in Table 2 and 3, see also   . At present, Egeland and colleagues seem to be the sole authors adhered to the term "quartz," based on the solid and precise petrologic argument that: "geological work on the Tanzanian Craton reveals that these inselbergs are too metamorphically evolved to be quartzite and, thus, are probably best characterized as resistant, quartz-rich remnants of heavily weathered granulites (Dawson 2008;Begg et al. 2009), which is the label we adopt here. Nevertheless, we think it is reasonable for Olduvai lithic artifacts presumably harvested from these outcrops and flaked largely or exclusively from their quartz constituents to be referred to as «quartz» artifacts." (Egeland et al. 2019, p. 101).
In the framework of this new consensual and imposition period Santonja et al. (2014), aware of the quandary around this CQRM, specifically defined these materials by the local name of "Naibor quartzite," stressing the exceptional particularism of this white/colorless quartz raw material. The "Naibor quartzite" is described by them as: "…a metamorphic rock whose almost exclusive constituent mineral is quartz with the shape of phenocrysts and a distinctive lamination that responds to knapping very differently from the other fine-grained quartzites present in Olduvai (Jones 1994), and more similarly to quartz (Mourre 1997)" (Santonja et al. 2014, p. 186). Although, the authors identify a metamorphosed rock SiO 2 -rich composition (97.97%) and differentiate a "distinctive lamination," this does not imply that the raw material should be classified as quartzite. Furthermore, this distinctive lamination the authors mention, as said before, is in fact a tectonic/metamorphic foliation rather than a sedimentary bedding/lamination (Boggs 1987). Additionally, the term phenocryst is not used properly. This term is used in igneous petrology to identify isolated larger crystals surrounded by a finer grained groundmass of either identical or different mineral composition, which does not concur with the homogeneous grainsize distribution of Olduvai quartz with a granoblastic texture (Licker 2003). Finally, although they indicate that the CQRM respond to knapping very differently from the other fine-grained quartzites present in Olduvai, these authors seem to feel comfortable with this contradiction. They do not question why such fracture dynamics are at odds with what is expected in quartzites. This contradiction could be properly defined as the "quartzite paradox," a characteristic trait of CQRM in Olduvai Gorge.
The extremely relevant contradiction in breakage response emerging from the Olduvai paradox is actually simple to answer: if this raw material is composed of large  Table 2) presented in lower hemisphere, equal area stereographic projections of XZ structural sections (foliation is the equatorial diameter E-W and the lineation is horizontal within that plane). In the stereograms the color patterns represent multiples of mean uniform distribution. b Idealized lower hemisphere stereographic projections showing the relationships between quartz [c] and < a > crystallographic axis lattice preferred orientations and intracrystalline slip systems operating under increasing temperatures (from ca. 300 °C at the left to > 600 ºC at the right) in a non-coaxial deformation regime (after Schmid and Casey 1986) quartz monocrystal grains, then it will exactly respond to knapping as quartz, which is an anisotropic material with substantial crystallographic symmetry. No classical quartzite has a grain size > 2 mm (if so, it would be quartz-rudite). On the contrary, quartzite is usually a microcrystalline mineral aggregate with conchoidal fracture that behaves in breakage in a different way from quartz, which exhibits in any CQRM homogeneous crystalline domains larger than 2 mm until 10-20 mm are reached. This oddity was already identified by Kimura (1997) when she pointed out that: "quartz fractures unevenly, while the quartzite of Olduvai is extremely coarsegrained and does not form conchoidal fracture" (Kimura 1997, p. 33). Maté-González et al. (2018) show that the cut-marks made by CQRM artifacts in Olduvai on fossil bones exhibit similar marks to those produced by quartz. However, their final terminological choice is driven by a rather deterministic rationale when they state that "… in a strict geological definition, these materials are clearly quartzites, and not quartz (Hay 1976;Santonja et al. 2014), regardless of their quartz-like behaviour" (Maté-González et al. 2018, p. 449). Thus, Maté-González and colleagues are arguing that the Olduvai CQRM is a raw material that looks, fractures and interacts with animal tissues like quartz but is "clearly quartzite" based on the authority judgment pronounced by Hay and more recently followed by Santonja and colleagues. This is circular reasoning leading to a logical fallacy that is not supported by scientific data.

Conclusions
The major hominin artifact lithic raw material documented in the Olduvai Gorge region is a geological material with a very high quartz-rich composition, and a metamorphic origin. In this work we propose the term CQRM (Crystalline Quartz-rich Raw Material) to designate these materials that with these petrological features support its identification as "Quartz." Stubbornness in terminological confusion reflects the complexity and specificity of CQRM at the Olduvai Gorge sites. Such uncertainties relied on the following issues, related to a protracted and complex geological history. Firstly, those materials crop out widely in nearby inselbergs (in some cases, several km 2 in area) composed by quartz-bearing metamorphic rocks of varied types and origins. Second, these metamorphic rocks proceed in part from sedimentary recycling and/or tectonic reworking of much older orogens and cratons (Mesoproterozoic, Paleoproterozoic and even Archean, > 2.5 Ga). Third, the metamorphic rocks were transformed in the solid state by intense ductile deformation under high-grade pressure and temperature (granulitic facies), reaching temperatures in excess of 750-800 °C during the Neoproterozoic Era, 1.0-0.5 Ga.
In this study we show that the terminological inaccuracy/ambiguity related to the CQRM conundrum and the related paradox originated in a complex geologic/petrologic context that had not been addressed so far from a multidisciplinary geological perspective. These white/ colorless quartz-rich material occur in areas dominated by outcrops of quartzites (sensu Pickering) with somewhat different mineral content, microstructure and macroscopic aspect than classic quartzites. However, they have been recently classified as quartzites by researchers who are not specialists in hard rock petrology, instead of having been classified as quartz. Therefore, the concurrence of structural/metamorphic petrologists is essential to describe these materials, identify their origin and eventually discuss the implications of their presence in the regional geological context.
This raw material was formed as hydrothermal veins and dykes, as well as in quartz metamorphic differentiates made up exclusively of quartz and they are hosted in quartzitic country rocks also made up exclusively of quartz. Logically, they share a similar chemical and mineralogical composition. After penetrative ductile deformation, both the host rocks and the hosted veins acquired a penetrative fabric (with convergent microstructure and petrofabric), but relics of their original texture can still be differentiated. The mineralogical similarity between these quartz-rich geological products (host rocks and hosted minerals) can largely explain the persistence and even the imposition of the name of quartzite for these CQRM by researchers inexperienced in the study of raw materials.
The most diagnostic features supporting this interpretation can be summarized as follows. First, some of the microstructural relics identified in the CQRM concur undoubtedly with a hydrothermal origin. Among them it is worth noting: (1) presence of oriented muscovite microinclusions (µm-sized) inside large quartz crystals, (2) occurrence of minor euhedral/subhedral crystals of hydrothermal minerals such as tourmaline, anhydrite (currently stabilized to gypsum) and muscovite forming a paragenetically stable association, and (3) existence of a distribution of clouded domains rich in fluid inclusions in some quartz crystals. Second, the recognition of special deformational structures/microstructures (elongate quartz grains defining the macroscopic foliation and lineation, irregular quartz grain boundaries with concave geometries, small oriented muscovite flakes pinned to quartz boundaries, chessboard recrystallization microstructures fully overprinting rocks with a non-sedimentary primary microstructure…) and intense quartz LPOs (characterized by concentrations of < c > -axes around the lineation orientation) point to operation of tectonometamorphic processes under granulite-facies conditions.
Undoubtedly, the precise characterization of the CQRM will allow us to speak with propriety about these raw materials used in the Olduvai Gorge sites and it can also help cast light on the supply and management strategies for these exceptional and unique mineral resources used by the hominin species who inhabited the Olduvai region during the Stone Age.