Inorganic substance that is (1) naturally occurring (but does not have a biologic or man-made origin) and formed by physical (not biological) forces with a (2) defined chemical composition of limited variation, has a (3) distinctive set of of physical properties including being a solid, and has a (4) homogeneous Click on Term to Read More generally found in terrestrial Rocks that have recrystallized in a solid state due to changes in temperature, pressure, and chemical environment. Click on Term to Read More rocks, although igneous examples are not uncommon. Garnet is a significant reservoir of Al in the Earth’s upper Main silicate-rich zone within a planet between the crust and metallic core. The mantle accounts for 82% of Earth's volume and is composed of silicate minerals rich in Mg. The temperature of the mantle can be as high as 3,700 °C. Heat generated in the core causes convection currents in Click on Term to Read More.
The garnet structure consists of isolated SiO4 tetrahedra bound to two Positively charged ion. Click on Term to Read More sites. The A site holds relatively large divalent cations (Ca2+, Mg2+, Fe2+, Mn2+); the 6-fold B site holds smaller trivalent cations (Al3+, Cr3+, Fe3+). The BO6 octahedra share corners with tetrahedra forming a continuous network and the A sites are surrounded by 8 oxygens that form an irregular polyhedron.
The general formula of garnet is A3B2(SiO4)3. However, Ca2+ cation is appreciably larger than the Mg2+, Fe2+, and Mn2+ cations (100 pm vs. 72 to 83 pm), resulting in separate mineralogical series of Ca-rich and Ca-poor garnets.
Ca-rich garnet comprise the ugrandite series; Ca-poor garnet form the pyralspite series. The ugrandite series gets is name from the main minerals in it: uvarovite, Ca3Cr2(SiO4)3, grossular, Ca3Al2(SiO4)3, and andradite, Ca3Fe3+2(SiO4)3. Site occupancies are: A = Ca2+, B = Al3+, Cr3+, Fe3+. The names based upon the B site occupancy. Grossular-andradite forms a complete solid Compositional variation resulting from the substitution of one ion or ionic compound for another ion or ionic compound in an isostructural material. This results in a mineral structure with specific atomic sites occupied by two or more ions or ionic groups in variable proportions. Solid solutions can be complete (with. The pyralspite series name is also based upon its minerals: pyrope, Mg3Al2(SiO4)3, almandine, Fe3Al2(SiO4)3, and spessartine, Mn3Al2(SiO4)3. Site occupancies are: A = Mg2+, Fe2+, and Mn2+, B = Al3+. The names based upon the A site occupancy. Pyrope-almandine and almandine-spessartine form complete solid solutions.
There is limited solid solution between the pyralspite and ugrandite series as shown below.
There are many types of synthetic garnets. The gadolinium gallium garnet (GGG), Gd3Ga2(GaO4)3 is used in magnetic bubble memories, and the yttrium aluminum garnet (YAG), Y3Al2(AlO4)3 isd used for synthetic gemstones and as lasing medium in lasers (doped with Nd3+).
Some or all content above used with permission from J. H. Wittke.