The general formula of chemical composition is A3B2 [SiO2] 3. Wherein, a represents divalent cations Mg2+, Fe2+, Mn2+, Ca2+, Y+, K+, Na+, etc. B represents trivalent cations Al3+, Fe3+, Cr3+, V3+, Ti4+, Zr4+, etc. According to the relationship between cations, this group of minerals can be divided into two complete isomorphic series (aluminum series and calcium series) (table 19-4), and incomplete isomorphic substitution can also occur between the two series. The actual minerals are mixed crystals with different end-member compositions, and sometimes some new varieties can be formed, such as hausmannite CA3 (Fe3+, Ti) 2 [SiO _ 4] 3 and hausmannite (the content of TiO2 _ 2 is 4.6% ~ 16.44%). Emerald garnet Ca3(Fe3+, Cr) 2 [SiO2] 3; Hydrogrossular ca3al2 [sio4] 3-x (oh) 4x (H2O content can reach 8.5%). The chemical composition of garnet is an important genetic type. Table 19-5 reflects the relationship between the main components of garnet and its occurrence. Trace and trace elements are also important typomorphic characteristics: rare and rare earth elements are abundant in alkaline rocks and granite; It is rich in yttrium, lithium, beryllium and phosphorus in pegmatite; Garnet Cr/(Cr+Al) > 0. 1 in diamond-rich kimberlite. The element distribution between garnet and biological minerals can be used as an effective mineral thermometer.

Table 19-4 chemical composition and structural characteristics of garnet group minerals

Table 19-5 main physical properties and genetic occurrence of garnet minerals

Figure 19- 12 The island structure of garnet The isolated [SiO 4] tetrahedron is connected by [[AO8]] twisted cube and [[BO6]] octahedron.

Equiaxed crystal system with crystal structure; Island structure (figure19-12); Space group-ia3d; A0 = 1. 1459 ~ 1.248nm； Z=8. The radius and a0 value of 8-coordinated cation in garnet are closely related to the mineral formation pressure (Table 19-4 and Table 19-5). It is known that the radii of Ca2+(0. 1 12nm), Mn2+(0.096nm), Fe2+(0.092nm) and Mg2+(0.089nm) decrease in turn. According to the coordination theory, the larger Ca2+ needs less pressure when it is coordinated with 8, while the smaller Mn2+, Fe2+ and Mg2+ need more pressure when it is coordinated with 8. Therefore, calcium garnet is formed in magmatic rocks, contact metamorphic rocks and low-pressure hydrothermal veins, with a relatively high a0 value. However, aluminum-based garnet, almandine and magnesium-almandine are formed in low, middle and high regional metamorphic rocks and kimberlite with slightly higher pressure, respectively, and a0 value is small.

Morphological symmetry m3m, common rhombic dodecahedron, quadrangular octahedron and their poly (figure 19- 13). The rhombic dodecahedron crystal plane often has the long diagonal of parallelogram. Sometimes we can see the sensing surface of adjacent particles after separation. Aggregates are usually dense particles or blocks.

Fig. 19- 13 garnet crystal

Rhombic dodecahedron d {110}; Tetrahedral octahedron n {2 1 1}

Red, yellow and green with different physical properties (table 19-5), white or slightly yellowish brown stripes; Glass luster, fracture grease luster; Transparent-translucent. The hardness is 6.5 ~ 7.5; No cleavage; Brittle (crack development). The relative density is 3.5 ~ 4.2, which increases with the increase of iron, manganese and titanium content.

Genetic occurrences are widely distributed in various geological products (Table 19-5). Because of its stable nature, it is very common in placer mines. After hydrothermal alteration and strong weathering, it can be transformed into chlorite, sericite and limonite.

Identification features equiaxed crystal, oily luster, no cleavage and high hardness. The identification of mineral species requires X-ray diffraction or electron probe analysis.

Its main purpose is to use its high hardness as grinding material. Coarse particles (> 8 mm, green ones can be as small as 3mm) and beautiful and transparent colors can be used as gemstone raw materials.