The chemical composition of minerals is revealed by analyzing images with different magnifications. These features are observed in the different nature of the fracture surface of specific areas of a single mineral phase, with a significant change in the average atomic number, when focusing on submicron regions.
For example, BSE (COMPO) images of olivine crystals from the early melt crystallization appear to have a normal zoning pattern. The normal zoning is dark cores surrounded by bright rims, implying that Fo% (Mg/Fe) decreases towards the rim (Jankovics et al. 2018). Large crystals with reverse zoning were found in basaltic lava crater facies. The COMPO mode shows a brighter core surrounded by a dark rim, implying that Fo% increases towards the rim. This is the reverse zoning of olivine (Jankovics et al. 2018). It turns out that compositional variations in the crystal appear in the fracture surface topography as well. Important features of olivine genesis were reflected in the separated structure of the central and outer fracture regions (Fig. 1b). These crystal regions are clearly distinguished in the distribution of the main chemical elements (Fig. 1b). Stable intensity of the silicon line shows the absence of topographic artifacts. If the scanned region surface is tilted, the horizontal position of the analyzed region is achieved using a five-axis electron microscope stage. According to linear scanning data, the change in Fo% is recorded in the crystal rim. The crystal core has a constant composition. It has a smooth, flat fracture surface. The crystal rim has a stepwise fracture pattern; Fo% increases in this zone. The boundary between these areas is visible only in the fracture topography. This boundary indicates a break in olivine crystallization. This boundary is not observed in polished sections; zoning is visible only by the average atomic number (Fig. 1a). However, in polished sections, submicron inhomogeneities in the composition can appear more clearly. The fracture topography can limit the compositional contrast mode capabilities.
Grains with direct zoning were previously studied with compositional contrast in polished sections (Gembitskaya et al. 2020). At high resolution, they reveal a thin zone between the core and the rim with a higher Fo% and the presence of nickel.
The micron and submicron crystal morphology is fully and clearly manifested on the fracture surface. This distinguishes a fracture from the polished surface of random cuts (sections) of crystals. Pores and inclusions are not filled with epoxy resin when fractured. This enables us to study their composition at high magnification. Composition analysis requires lateral scanning. The polished section surface (Fig. 2a) shows submicron bright areas containing calcium and phosphorus. Based on the obtained data for this sample, the composition corresponds to apatite. The study of open areas of pores and fractures on the fracture surface enabled the confirmation of the mineral diagnosis. In a fracture, apatite is observed in the form of needle-shaped hexagonal crystals (Fig. 2b, d) with a thickness of the order of a micron or less. Submicron crystals of apatite and orthopyroxene crystallizing at the olivine-plagioclase boundary are also observed on the fracture surface (Fig. 2e). Apatite crystals measuring 1–3 µm in size are observed as phenocrysts (inclusions) in augite and plagioclase (Fig. 2f) much more often in the fractured sample compared to the polished counterpart. The bulk morphology of submicron phases is clearly visible on the fracture surface, so the mineral diagnosis becomes more reliable.
apatite and covellite (E); e – apatite crystallizing on olivine (T); f – titanomagnetite and apatite inclusions in feldspar (T)
In lava samples from Etna and Tolbachik, the fracture exposes a large number of mineral phases with copper, zinc, nickel, and iron in oxide and sulfide forms. Some of these mineral phases are accessory minerals in melt inclusions and pores. For example, Fig. 2f shows well-faceted individual titanomagnetite crystals in feldspar. Chromium-bearing titanomagnetite phases were found in the clinopyroxene matrix (Fig. 3a). Figure 3b shows chalcopyrite with nickel as an inclusion in a silicate melt. In a pore space of the Etna basaltic lava sample, associations of elongated pyroxene crystals with 10-µm chalcopyrite and covellite crystals are observed (Fig. 2c, d). In the oxide form, copper appears as well-faceted cuprite crystals (Fig. 3c), and also as micron-sized inclusions in olivine (Fig. 3d) and silicate matrix (Fig. 3f) (Lanzirotti et al. 2019). A micron-sized phase with a predominant copper and nickel content was found on a columnar elongated pyroxene crystal (Fig. 3e). Finding copper in various forms helps to determine its formation mechanism (Sharygin et al. 2018). Direct observation of the crystal habit of copper-bearing minerals enables us to estimate the sulfur deficiency in lava. It is believed that sulfur was lost from the lava as a gas during the extrusion and crystallization (Cornwall 1956; Kamenetsky et al. 2019).
A zinc-dominated phase is clearly observed in Fig. 3e. The distribution of submicron oxide zinc inclusions in the silicate matrix is shown in Fig. 3g.
clinopyroxene (T); b – chalcopyrite with nickel admixture in a silicate melt (E); c – cuprite (T); d – cuprite in olivine (T); e – copper and nickel (left) + zinc (right) (T); f – copper in oxide form in a silicate matrix (T); g – zinc in oxide form in a silicate matrix (T)
Morphological features of micron and submicron pyroxene crystal associations, oxides and sulfides of copper, zinc, iron, and nickel in lava samples from Etna and Tolbachik are presented in Fig. 2b-f, 3a-g. It is clearly seen, that the diversity of accessory minerals is revealed much more representatively and clearly on the fracture surface than on polished ones. It is very hard to obtain these sub-micron grains on the polished surface, and even harder to determine composition. Chemical and morphological features of grains in the pores and fracture surfaces provide additional information on rock-forming conditions. A generalized diagram for comparing a polished surface and a fracture is shown in Fig. 4.
crystal growth along grain boundaries; 6 – submicron inclusions; 7 – crystals in pore
Studying fracture surfaces doesn’t allow precise determination of the chemical composition of main rock-forming minerals, but it has several important advantages. The main advantage is the conservation of all submicron-size grains in porous and macro-grain interfaces. It is also a way of highlighting submicron inclusions in macro grains.