=Flint.= See Chalcedony. =Garnets.= A series of silicates with the general composition of R_{3}″R_{2}‴(SiO_{3})_{4}, R″ being Ca, Fe″, or Mn, and R‴ being Al or Fe‴. The common red garnet in mica-schists is _Almandine_, Fe_{3}Al_{2}(SiO_{3})_{4}, while that in altered limestones is _Grossularite_, Ca_{3}Al_{2}(SiO_{3})_{4}. =Glauconite.= A hydrous iron potassium silicate, with some aluminium, magnesium, and calcium, formed in marine deposits. =Gypsum.= Hydrous calcium sulphate, CaSO_{4} + 2 H_{2}O. =Hornblende.= See Amphiboles. =Hypersthene.= See Pyroxenes. =Ilmenite.= Titanium iron oxide, _m_ FeTiO_{3} + _n_ Fe_{2}O_{3}. =Iron Pyrites.= Iron disulphide, FeS_{2}. A cubic species, _Pyrite_, and a less common rhombic species, _Marcasite_, occur. =Kaolin.= Hydrous aluminium silicate, H_{4}Al_{2}Si_{2}O_{9}. =Kyanite.= Aluminium silicate, Al_{2}SiO_{5}, crystallised in the triclinic system. See Andalusite. =Labradorite.= See Felspars. =Leucite.= Potassium aluminium silicate, KAl(SiO_{3})_{2}. =Limonite.= Hydrous iron oxide. H_{6}Fe_{4}O_{9}. =Magnetite.= Magnetic iron oxide, Fe_{3}O_{4}. =Marcasite.= See Iron Pyrites. =Micas.= A series of aluminium silicates, with potassium, magnesium, or iron, or all of these. Lithium and sodium sometimes occur. The two marked types are _Muscovite_, rich in aluminium and potassium, the common "alkali mica," H_{2}KAl_{3}(SiO_{4})_{3}, with a silvery aspect, and _Biotite_, the common dark "ferromagnesian" mica, (H, K)_{2}(Mg, Fe″)_{2}(Al, Fe‴)_{2}(SiO_{4})_{3}. =Microcline.= See Felspars. =Muscovite.= See Micas. =Nepheline.= Sodium aluminium silicate, with some potassium, the pure sodium type being NaAlSiO_{4}; the types with potassium contain slightly more silica. =Oligoclase.= See Felspars. =Olivine.= Magnesium iron silicate, (Mg, Fe)_{2}SiO_{4}. =Opal.= Uncrystallised silica, SiO_{2}, with some water. =Orthoclase.= See Felspars. =Pyrite.= See Iron Pyrites. =Pyroxenes.= A series of silicates corresponding in composition to the Amphiboles, but crystallising in prisms which have angles of about 87° and 93°. On the whole, the pyroxenes are richer in calcium than the amphiboles. The formula of _Wollastonite_ is CaSiO_{3}. _Diopside_ consists of Ca(Mg, Fe)(SiO_{3})_{2}. _Augite_, the commonest form, is aluminous, corresponding to Hornblende among the amphiboles; but the change from Augite into Hornblende, which often occurs, may imply a loss of calcium. _Enstatite_ and _Hypersthene_ are species crystallising in the rhombic system; the former consists of MgSiO_{3}, while in Hypersthene iron replaces some of the magnesium. =Quartz.= Silica, SiO_{2}, crystallised in the trigonal system.