HUNGARIAN JOURNAL OF INDUSTRY AND CHEMISTRY Vol. 49(1) pp. 83–88 (2021) hjic.mk.uni-pannon.hu DOI: 10.33927/hjic-2021-11 INFLUENCE OF MINERAL COMPOSITION IN NATURAL GRANITE ROCKS ON MICROHARDNESS ESZTER KELEMEN-CSERTA*1 AND ISTVÁN GÁBOR GYURIKA1 1Research Centre for Engineering Sciences, University of Pannonia, Egyetem u. 10, 8200 Veszprém, HUNGARY Granites are becoming increasingly popular and, as a result, their areas of use are expanding. In addition to their colour and particle size, the surface roughness of the machined material is becoming an important aspect of their application. In order to create a suitable surface roughness, the type of rocks located on the surface, elemental composition and microhardness of the minerals are important, because knowledge of these characteristics can be used to determine the machining parameters. Microhardness is affected by the atomic percentage values of Si, Al and Na. In addition, for some minerals, a correlation can also be established between the Si, Al and Na components. Keywords: natural rocks, granite, microhardness, surface roughness 1. Introduction Granites are one of the most commonly used natural rocks due to their beautiful appearance and the fact that many variations in their colour can be found. Their ap- pearance is special since the grains of the constituent min- erals found in granite are nicely outlined and clearly vis- ible with the naked eye, therefore, are also preferred ar- chitecturally. Zhao et al. [1] studied the effect of weather- ing on samples of granite. For this purpose, the samples were soaked in different concentrations of Na2SO4 and MgSO4 before the wetting–drying cycles. It was found that the increase in wetting–drying cycles greatly influ- ences the physical parameters, namely weight, colour, surface roughness and hardness. In addition, the effect of Na2SO4 on the specimens was greater than that of MgSO4 and the higher salt concentration accelerated weathering of the rock. Although the spectacular grain structure of granite is advantageous in terms of appearance, it is disadvanta- geous in terms of machinability. The particles are com- posed of different minerals that result in a heterogeneous structure. This heterogeneous property makes machining difficult. Ramnath et al. [2] have developed epoxy granite com- posites (EGCs) as a novel alternative material, which exhibit several positive properties, e.g., are stable and lighter, but due to their heterogeneous structure, machin- ing was critical. According to experiments, it was shown that 600 rpm is the optimal spindle speed and 0.09 m/min is the optimal feed rate for machining this material. This *Correspondence: kelemen.cserta.eszter@mk.uni-pannon.hu value can even be used for natural granite rocks. In collaboration with us, many researchers have stud- ied the formation, change and effect of surface roughness. Sanmartín et al. [3] studied the effect of surface treatment on roughness, surface gloss and colour. It was found that the colour of granite affects the gloss of the surface dif- ferently for smooth and rough surfaces. The change in luminosity also depends on the mineral composition of the rock. Shen et al. [4] studied concrete and granite by in- vestigating the relationship between surface roughness and hydrophilicity. In their experiments, it was observed that the bonding strength always increases with the joint roughness coefficient (JRC). Furthermore, the relation- ship between surface roughness and hydrophilicity was determined as well as the importance of knowing the ef- fect of surface adhesion and surface characteristics eval- uated. Aydin et al. [5] machined the surface of granite rocks with diamond saw blades and examined their surface roughness. They concluded that instead of mechanical properties, mineral properties affect surface roughness. Among the mineralogical properties, the particle size was chosen as the primary aspect of surface roughness. Others have studied the hardness of granite from dif- ferent perspectives. Prikryl [6] cut thin sections of the rock samples in which the size, distribution, shape, orien- tation and mineral composition of the grains were anal- ysed. It was found that as the particle size of minerals decreases, their strength increases. Rajpurohit et al. [7] investigated a statistical relation- ship between the Cerchar Hardness Index and this dia- https://doi.org/10.33927/hjic-2021-11 mailto:kelemen.cserta.eszter@mk.uni-pannon.hu 84 KELEMEN-CSERTA AND GYURIKA mond tool. It was concluded that the hardness of the rock greatly influences the wear of the tool. A similar result was obtained by Delgado et al. [8] who studied the sawa- bility of granite as well as determined that its hardness strongly influences the sawability and wear of diamond tools. The Vickers hardness of minerals was measured ex- perimentally and it was determined that a small increase in hardness leads to a large decrease in the sawing speed. A similar result was obtained by Dong et al. [9] who de- veloped an innovative method for sawing hard rocks and demonstrated that the sawing force correlates with the properties of the rock. A similar experiment was performed by Yilmaz [10], who examined the wear of diamond tools as a function of the hardness of the minerals that granite is composed of. The rock hardness index of the specimens was used to determine the hardness according to the percentage oc- currence of the constituent minerals. The results showed that the rock hardness index correlated with the sawblade wear rate (SWR) of the tool. Even though previous stud- ies have attributed the percentage occurrence of quartz to the main reason for sawblade wear, Yilmaz’s results showed that other minerals also influence tool wear. In another study, where Yilmaz et al. [11] conducted exper- iments on natural granite, they found that the maximum particle size of quartz, orthoclase and microcline had the greatest effect on SWR. Li et al. [12] studied how the mineral content of rock affects its properties. For this purpose, the mineral con- tent, particle size, abrasion resistance and hardness of 96 samples from 10 provinces in China were measured. The test results showed that the wear and hardness of the rock can be accurately determined based on the type of rock, mineral content and particle size. Diamond is extremely hard, tough and thermally con- ductive, making it an excellent material for the purpose of cutting rocks. The abrasion resistance of rocks varies, so it is important to provide the right metal matrix in which the diamond grains can be placed. During the experiment, 2 materials were investigated, that is, the Co- and Fe- based metal matrices. The results showed that the Co- based metal matrix is suitable for cutting granites, while the Fe-based one is suitable for cutting marbles [13]. This theory is also supported according to a study by Gupta and Pratap [14]. They also studied the metal matrix of the tool and found that the metal matrix composite (MMC) influences the efficiency and service life of the tool. Yan et al. [15] also searched for a way of modifying the machining tool to ensure granite surfaces are properly machined. An ultrahard polycrystalline diamond (UH- PCD) tool was compared to a polycrystalline diamond (PCD) one. The hardness of UHPCD was 105-115 GPa, while that of PCD was 53-57 GPa. The results showed that the lifetime of UHPCD bits was 132.33 m and that of PCD ones was 83.76 m. Using a hammer, Adebayo and Okewale [16] deter- mined the rebound values of the specimens as a function of hardness as well as the mineral compositions of the samples. It was concluded that the sample with the high- est quartz content resulted in the highest Vickers hardness number. A document published by the University of Kiel [17] contains a table presenting the Vickers hardness number of several materials, in which the Vickers hardness num- ber of granite is HVgranite = 850 and that of quartz is HVquartz = 1200. Yusupov and co-researchers [18] studied the hardness of minerals in terms of grindability. It was concluded that the presence of quartz increases the grinding time by 1.5- 2 times. To optimize the process and reduce the amount of energy required, they proposed a method in which the minerals are machined separately. Xie and Tamaki [19] investigated the effect of the micro-hardness distribution in granite on the efficiency of abrasive machining. In their research, it was determined that the efficiency of abrasive machining increases as the microhardness of granite decreases. From the literature, it is also clear that researchers have considered the types of minerals when studying nat- ural rocks. In contrast, in the present paper, the mineral constituents are examined to observe how these affect the hardness of specimens and thus their machinability. Our goal is to draw up a prediction system based on complex analysis to facilitate the specification of the pa- rameters required for surface machining. Alternatively, the parameters can be modified according to the types of minerals present on the surface or the quality of the surface to be created. 2. Materials and methods The granites used in the experiments differ in terms of colour (pink, yellowish, orange, greyish, black and white), grain size (fine, medium and coarse) and com- position. 4 of the 5 samples come from Brazil and 1 from the Hungarian mine in Süttő. During the experiment, an experimental process was developed to investigate the composition, formation and hardness of the rock. It was determined that although all granite samples contained the minerals quartz, feldspar and biotite, the difference was in the type of feldspar min- erals. 2.1 Rock working The surface of the slabs composed of natural rock was machined by a CNC machine manufactured in Italy (Fig. 1). The used equipment was an Italian Prussiani Golden Plus-type CNC machine with a maximum power con- sumption of 15 kW. The machining cutting depth was 1 mm, the cutting width was 40 mm and the feed speed was 0.1 mm/tooth. The cutting speed used during machining was 37.7 m/min. The machining tool in the CNC machine consisted of 22 segments and the face mill was 100 mm in diameter. Although the elemental composition of the face-mill matrix is not specified by the manufacturers, Hungarian Journal of Industry and Chemistry INFLUENCE OF MINERAL COMPOSITION IN NATURAL GRANITE ROCKS ON MICROHARDNESS 85 Figure 1: The CNC equipment and various tools Figure 2: One of the granite samples (orange) scanning electron microscopy (SEM) of the tool showed that Co, Cu, Sn and Ag were also present. 2.2 Hardness measurements Samples with approximate dimensions of 5 mm × 5 mm (Fig. 2) were cut from the machined rock slabs to ensure that they could be easily placed on the tables of various measuring devices. The minerals were separated based on their appear- ance. Biotite, which could be identified by its character- istic dark/black colour, was the first to be examined, fol- lowed by feldspar and quartz. In the case of the latter two, even though it was more difficult to distinguish them from each other, in the following examination of mineral com- position, it was possible to make an accurate distinction between them. The hardness of minerals is usually given according to the Mohs scale. This does not provide an exact value, rather only reflects the relative hardness of the different minerals. Secondly, it is worth mentioning the methods used to measure the hardness of metals, which despite yielding accurate values, are greatly influenced by different com- positions of minerals. Table 1: Parameters of the Wolpert Group hardness tester Eyepiece magnification 10× Resolution 0.1 µm Objective magnification factor 10×, 20×, 40×, 50×, 60× Total magnification 400× (for measurements) 100× (for observations) Measuring range 200 µm Hardness value 5-digit Maximum specimen height 85 mm XY stage dimensions 100 × 100 mm Operating temperature Range: 10 to 38 ◦C (50 to 100 °F) Figure 3: Imprint of the 136° square-based diamond pyra- mid Measurements were performed on a WOLPERT W Group Micro Vickers digital auto turret 402MVD-type hardness tester, the parameters of which are shown in Ta- ble 1. When measurements were taken, a specimen was placed on the measuring table and then the 136° square- based diamond pyramid for measuring the Vickers hard- ness was placed over the relevant mineral. After starting to take the measurements, the pyramid was pressed into the surface of the mineral where it left an imprint (Fig. 3). The peaks of the imprint were marked using a micro- scope, before the machine evaluated the results. 2.3 SEM The chemical composition can be determined by scan- ning electron microscopy (SEM). A Thermo Fischer Sci- entific Aprea SEM, FEI/Philips XL-30 ESEM were used to examine the elemental composition of the sample’s surface in a low vacuum at a resolution of 20 Å. During the evaluation, the equipment took a photo of the surface before evaluating the compositions of the selected areas. Given that the results were given in percentage compo- sition, the types of constituent minerals present could be deduced (Fig. 4). 49(1) pp. 83–88 (2021) 86 KELEMEN-CSERTA AND GYURIKA Figure 4: Elemental composition of biotite Figure 5: Effect of Mg, Al, Si and K constituents on Vick- ers hardness 3. Results During this study, the minerals biotite, feldspar and quartz were examined on the surfaces of 5 different spec- imens. Firstly, biotite was placed under the diamond pyra- mid of the hardness tester. It is known that the content of SiO2 and grain size greatly influence the hardness as well as machinability of the specimens. However, it has not yet been studied whether and how the atomic percentage composition of the particles affects the hardness. Given that the hardness of granites is related to their quartz con- tent, it was examined whether the hardness varies as a function of the Si content. Our measurements showed no correlation between the values. For biotite, no correlation was found between Si content and Vickers hardness. Additional measurements also showed that the atomic percentage composition of biotite did not correlate with the Vickers hardness (HV). During the evaluation, the constituent elements (Mg, Al, K) were examined sep- arately, but no correlation (R2Mg = 0.315, R 2 Al = 0.028, R2Si = 0.3689, R 2 K = 0.6127 where R 2 de- notes the coefficient of determination) was found be- tween changes in any of these constituents and hardness. It is also clear from the diagram (Fig. 5) that the individ- ual constituents do not affect the Vickers hardness. The next group of minerals studied were feldspars. After evaluating the measurements, several correlations were observed. Given that as the Si content increases, the total content of Na and Al decreases (Fig. 6), it can be stated that a decrease in the Si content leads to an increase Figure 6: The correlation between Na+Al and Si contents Figure 7: The correlation between HV and Na+Al content in the Na+Al content in the case of feldspars (R2Al+Na = 0.8445). It also greatly affects the Vickers hardness. The change in Vickers hardness is inversely proportional to an increase in Na+Al content (Fig. 7). As the Na+Al content in the mineral increases, the Vickers hardness decreases (R2HV = 0.9551). The results of our measurements are illustrated in the diagram Fig. 7. In the case of the mineral quartz, the results do not exhibit such a correlation R2HV = 0.2646). In uncontam- inated quartz, where no impurities (C, K, Ca, Fe) were detected, large variations in hardness were observed (Fig. 8), presumably due to the crystallization of quartz. For minerals where other constituents are present in addition to quartz, neither were clear correlations be- tween the changes in Vickers hardness and elemental composition observed. Given the results, changes in the constituents of quartz do not affect the Vickers hardness. Figure 8: The correlation between HV and Si content Hungarian Journal of Industry and Chemistry INFLUENCE OF MINERAL COMPOSITION IN NATURAL GRANITE ROCKS ON MICROHARDNESS 87 4. Conclusions An important aspect when machining natural rocks is the hardness of the minerals they are composed of, be- cause the hardness of the rock surface greatly influences machinability. Based on preliminary measurements, the surface composition of feldspars was examined in order to predict their hardness. With the knowledge obtained during the present study, the parameters of the processing equipment can be adjusted according to the mineral composition on the given surface. In the case of the minerals biotite and quartz, clear values for machining could not be provided, since in- creasing the Si content of either mineral has no effect on the Vickers hardness. Our results suggest that Na+Al constituents do not af- fect either the change in hardness or the Si content. In the case of feldspars, on the other hand, it was ob- served that both the hardness and Si content are inversely proportional to the Na+Al content. 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