Petrographic and Mechanical Analyses of Selected Granitic Rock Deposits in Ado-Ekiti, South-West Nigeria.

In this study, the petrographic and mineralogy of selected granitic rocks on their physical and mechanical strength were evaluated. Granitic rock samples sourced from seven locations were analyzed to investigate their mineral micro-texture and compositions using petrographic examination and X-ray diffraction (XRD) analysis respectively. Mechanical properties including compressive strength and hardness of the examined rocks were performed following ASTM D7012–10 and EN 1534 standards. Their physical properties in term of specic gravity, porosity and water absorption were examined following ASTM C97/C97M. Results show that the predominant minerals in rock samples are quartz and kaolinite. Stress-strain curve displays plastic fragmentation after initial fracture in most granitic rock samples with rock samples from Olorunda Zone 9 and Olorunda Zone 2 exhibiting superior plastic deformation over a wide strain elongation. Their physical properties were within the acceptable range for construction applications.


Introduction
The study of petrographic, physical and mechanical properties of rocks is crucial when selecting rocks for geotechnical applications (Fahimifar and Soroush, 2007). Petrographic analysis of a rock sample involves the use of a petrographic microscope to identify the structure of its interlocking minerals.
Distortion in a granitic rock's physical, structural as well as mechanical properties due to variations in environmental conditions including varying temperatures, different rates of heating and cooling, variation in mechanical load, weathering, etc, can result in substantial detrimental changes in their performances (Sajid et al., 2016a). Several related studies have been conducted on rocks from selected locations from different countries focusing on the interaction between their petrographic and mechanical properties (Tugřul and Zarif, 1999 The correlation between the petrographic characteristics and mechanical behaviours of different granites from the lower Himalayan regime, Pakistan, was studied by (Sajid et al., 2016b). They found that the predominant texture features in uencing the mechanical properties are modal concentration and grain sizes of constitute minerals, rock mean grain size and grain size distribution within a rock. (Aladejare, 2020) reported the petrographic and chemical composition analyses as well as the physical and mechanical studies carried out on gabbro and granites rocks sample from Otanmaki, Finland. He concluded that though gabbro and granite rock samples are igneous rocks, their mineralogy differs.
(Gunes Yilmaz, Goktan and Kibici, 2011) evaluated the in uence of petrographic attribute and physicomechanical properties on diamond tool blade wear performance. They established that shore hardness, volume percentage and grain size of individual minerals in the granitic rock could be used as an indicator for diamond saw blade wear performance. The petrographic and physicomechanical properties of alkali granites, alkali quartz syenite and nepheline syenite from Ambela, Pakistan was investigated by (Arif et al., 2013) to ascertain their application for construction applications. They proposed that all the rocks are moderately strong and their physical properties including speci c gravity and absorption values are within the permissible range. The correlation between petrographic characteristics of clastic rocks to their strength and bursting potential was by (Meng and Pan, 2007).
Their results show that the petrographic features of a clastic rock have an intrinsic in uence on its strength and failure duration. (Ündül, 2016) assessed the effect of constituent minerals and their petrography on the petrophysical properties, elastic behaviours, and strength of volcanic rocks. He concluded that the mineral mass fractions have an impact on their speci c gravity and loss-on-ignition (LOI) values whereas the petrographic features in uence the uncon ned compressive strength (UCS) and elastic properties of the rock. It is clear from previous research efforts that the mineralogy of different granitic rock vary and the petrographic features in uences the rock mechanical properties and eventually their performance. Therefore, there need to assess the petrographic properties and mechanical properties of selected granitic rock deposits that are considered construction materials in Ado-Ekiti, West Nigeria.

Sample collection
Representative granitic rock samples of 50mm and 40mm from seven locations of the selected granitic rock deposits were identi ed. Rock samples were collected from each granite deposit with the aid of a geological hammer. Rock samples were labelled and kept in a sample bag. All of the block samples were collected fresh. The positions in terms of the coordinates and elevations where granitic rock samples were sourced were recorded using a global positioning system (G.P.S) and presented in Table 1.

Sample preparation
The granitic rock samples were washed with distilled water and then dried with cleaned clothes, then were cut into required sizes with the aid of a rock cutting machine for physical and mechanical analyses, and some part of the rock samples were polished with the use of corundum powder for XRD and petrographic analyses.

Petrographic analysis
Petrographic examination of granitic samples was performed using Brunel petrographic microscopes equipped with a mounted DVC camera linked with a computer for the examination of prepared samples' thin section.

Xray diffraction analysis
X-ray Diffraction (XRD) analysis of the samples was performed by a Shimadzu XDS 2400H diffractometer with Cu anode, λ1 CU = 1.5406 [A°], attached to a digitized computer along with graphical assembly on uncompressed powders to collect the maximum of the diffraction lines and better identi cation of the phases.

Compressive strength analysis
The compressive strength analysis on the granitic rock samples was performed according to ASTM D7012-10 (ASTM, 2010). Respective samples were cut into a cube of 5cm to determine the compressive strength and loaded gradually, one at a time till the rst crack appears in the test specimen indicating the beginning of failure, on the base of a Universal Testing Machine (UTM).

Hardness analysis
Brinell hardness evaluation of the samples was measured according to British standard Brinell EN 1534 (15) using Llyod testing machine. A steel ball of 10mm diameter was pressed on the surface of the samples to reach the maximum load of 3KN in 15secs. The load was maintained for 25secs and then the load will steadily be relieved to zero within 15secs. The indentation diameter was measured with a Brinell microscope. The mean value of hardness for seven representative samples was determined and used in the study.

Physical properties analysis
The physical properties of granitic rock samples that were investigated including speci c gravity, porosity, and water absorption were determined according to ASTM C97/C97M (ASTM C97/C97M, 2018). Ayedun Quarter appear from medium to coarse with their shape ranging from subhedral to anhedral. The main minerals contents are plagioclase feldspar, microclinic feldspar orthoclase feldspar and quartz depending on their rock formation.  Figure 3 shows a plot of the results of the compressive stress against the compressive strain of the granitic rocks while their uncon ned compressive strengths are presented in Table 3. The plot in Fig. 3 shows that all the rock samples display a similar prolonged brittle deformation as the initial compressive load increases up to the compressive strain value of 0.043. Beyond this initial brittle fragmentation, the response of their compressive stress to rise in compressive strain show substantial variations. The rock samples designated as Basiri Road, Oke Ureje, Ayedun Quarters, Iyana Emirin and Odo Ado produce a substantial increase in compressive stress with a corresponding increase in strain. While Basiri Road, Oke Ureje, Ayedun Quarters and Odo Ado give high compressive stress-strain readings, they fail within a strain range of 0.07 and 0.09, showing that the rocks are suitable for applications that require short-range elongation. Rock sample sourced from Iyana Emirin shows good compressive stress-strain curve but a short-range while rock samples designated as Olorunda Zone 9 and Olorunda Zone 2 display the longest strain and superior stress-strain curve.

Hardness
The hardness values obtained for the seven rocks samples are shown in   Table 3 presents the physical properties which include speci c gravity, porosity, and water absorption as well as the uncon ned compressive strength of the granitic rock aggregates under the study. The speci c gravity of a given rock aggregate depends on the density of its minerals and the total void volume present in the bulk materials (Bell, 2007;Afolagboye, Talabi and Akinola, 2016). The speci c gravity of aggregates ranges between 2.63 and 2.71 with a standard variation of 0.03. Owing to an estimated variation of 0.03, it is clear from data that the changes in speci c gravity quantity not signi cant for the various locations. The standard variation gure is consistent with the mineral composition that is presented in Table 2. The speci c gravity readings of the aggregates are re ections of minerals that made up a substantial proportion of the bulk materials including albites, quartz, mica, kaolinite, and calcites with an average density from 2.61-2.88 g/cm 3 . Since rock aggregates with a speci c gravity that exceed the lower limit of 2.55 are recommended to be suitable for heavy-duty construction applications, selected rock aggregates in this study are suitable for building and road constructions which is the main motivation for their extraction (Afolagboye, Talabi and Akinola, 2016).

Porosity
The porosity of a rock depends on the togetherness of the interlocking of its constituent mineral grains (Bell, 2007) From Table 3, the porosity of the selected granitic rocks ranged from 0.62-1.11%. Sample Iyana Emirin has the highest porosity while sample Basiri Road has the lowest porosity value. Other samples have the following porosity -Oke Ureje (1.01%), Odo Ado (0.98%), Olorunda Zone 9 (0.89%), Olorunda Zone 2 (0.81%) and Ayedun Quarters (0.74%). The mean porosity is 0.88% and the standard deviation is 0.17. The samples with higher porosity exhibit a lower level of water absorption and vice versa, this further con rmed that the porosity of a given rock does not necessarily indicate the amount of water that could be absorbed by it. Although, Bell concluded that rock with higher porosity would retain a greater amount of water (Bell, 2007).

Correlation between porosity and uncon ned compressive strength
Previous works have shown that an increase in the apparent porosity of rock would result in to increase in the amount of water it can hold thereby increasing the susceptibility of the rock to fail mechanically due to chemical and microbial attacks (Dearman, Baynes and Irfan, 1978; Al-Harthi, Al-Amri and Shehata, 1999; Bell, 2007). The relationship between porosity and the uncon ned compressive strength of the rock under study is presented in Fig. 4. From the graph, rock samples from all locations tend to follow the predictions reported in past research. Samples in Iyana Emirin, Oke Ureje, Odo Ado and Olorunda Zone 9 have very high porosity readings with a corresponding low uncon ned compressive strength value. As the porosity values reduce from left to right, for Olorunda Zone 2 and Ayedun Quarter locations, there was a dramatic rise in strength than expected but still consistent with past research ndings.

Water absorption
It is essential to estimate the water absorption characteristics of the aggregates since their strength and colour can be attributed to their cement content which depends on the porosity and therefore water absorption (Bell, 2007

Conclusions
The following conclusions are deducted from this study: The major minerals present in rock samples are quartz and kaolinite with a trace amount of muscovite, biotite, and plagioclase.
There were prolong initial brittle fragmentation in all granitic rock samples followed by short-range plastic deformation. However, the Olorunda Zone 9 and Olorunda 2 sample exhibits the most superior stress-strain behaviours.
The density of the granitic rocks studied ranges between 2.63-2.71 g/cm 3 . Water absorption of rock aggregates are within the acceptable range for construction applications and range from 2.59-3.04%. The petrographic and mechanical properties of the selected granitic rocks are within the acceptable limit for application as construction materials.
There exists an inverse relationship between porosity and moisture content when compare with uncon ned compressive strength for most of the rock samples.
Declarations Figure 1 The study area map and coordinates. Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors. Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors. Porosity and uncon ned compressive strength data of granite rocks.

Figure 5
Water absorption and uncon ned compressive strength data of granite rocks.