LIST OF THE COMMON MINERALS THAT FORM ROCKS
Actinolite. See [Amphiboles].
Albite. See [Felspars].
Amphiboles. A series of silicates with the general formula RSiO3, where R is magnesium, iron or calcium; in many, such as the common species Hornblende, molecules occur in addition in which aluminium and triad iron are introduced. Hornblende thus consists of m (Mg, Fe″, Ca) SiO3. n (Mg, Fe″) (Al, Fe‴)2 SiO6. Actinolite is a non-aluminous amphibole occurring in needle-like prisms. The amphiboles crystallise in prisms having angles of about 56° and 124°. See [Pyroxenes].
Anatase. See [Rutile].
Andalusite. Aluminium silicate, Al_{2}SiO_{5}, crystallising in the rhombic system. Sillimanite consists also of Al2SiO5 and is rhombic, but crystallises with different fundamental angles.
Anorthite. See [Felspars].
Apatite. Calcium phosphate, with fluorine, or sometimes chlorine, (CaF)Ca4(PO4)3 = 3Ca3(PO4)2. CaF2.
Aragonite. Calcium carbonate, CaCO3, crystallising in the rhombic system, with a specific gravity of 2·93. See [Calcite].
Augite. See [Pyroxenes].
Biotite. See [Micas].
Calcite. Calcium carbonate, CaCO3, crystallising in the trigonal system, with a specific gravity of 2·72. See Aragonite.
Chalcedony. Crystalline silica, SiO2, in fibrous and often mammillated forms. Flint or Chert is a concretionary form, in which some interstitial opal may be present.
Chert. See [Chalcedony].
Chlorites. Hydrous aluminium magnesium iron silicates, resembling green micas, but softer and with non-elastic plates.
Chromite. Iron chromium oxide, FeCr2O4. Magnesium may replace part of the dyad iron, and aluminium and triad iron some of the chromium.
Diallage. An altered augite with a shimmery submetallic lustre.
Diopside. See [Pyroxenes].
Dolomite. Magnesium calcium carbonate, MgCa(CO3)2.
Enstatite. See [Pyroxenes].
Epidote. Calcium aluminium iron silicate, Ca2(AlOH)(Al,Fe‴)2(SiO4)3.
Felspars. A series of silicates of aluminium with potassium or sodium or calcium, or all of these. Orthoclase, KAlSi3O8, and the corresponding sodium form, Albite, NaAlSi3O8, lie at one end of the series, and the calcium felspar Anorthite, CaAl2(SiO4)2, at the other. While Orthoclase crystallises in the monoclinic system, a triclinic form, Microcline, with the same composition, is also common. All the other felspars are triclinic, and, with microcline, are often styled plagioclases. The principal felspars between Albite and Anorthite are Oligoclase, the "soda-lime felspar," and Labradorite, the "lime-soda felspar."
Flint. See [Chalcedony].
Garnets. A series of silicates with the general composition of R3″R2‴(SiO3)4, R″ being Ca, Fe″, or Mn, and R‴ being Al or Fe‴. The common red garnet in mica-schists is Almandine, Fe3Al2(SiO3)4, while that in altered limestones is Grossularite, Ca3Al2(SiO3)4.
Glauconite. A hydrous iron potassium silicate, with some aluminium, magnesium, and calcium, formed in marine deposits.
Gypsum. Hydrous calcium sulphate, CaSO4 + 2 H2O.
Hornblende. See [Amphiboles].
Hypersthene. See [Pyroxenes].
Ilmenite. Titanium iron oxide, m FeTiO3 + n Fe2O3.
Iron Pyrites. Iron disulphide, FeS2. A cubic species, Pyrite, and a less common rhombic species, Marcasite, occur.
Kaolin. Hydrous aluminium silicate, H4Al2Si2O9.
Kyanite. Aluminium silicate, Al2SiO5, crystallised in the triclinic system. See [Andalusite].
Labradorite. See [Felspars].
Leucite. Potassium aluminium silicate, KAl(SiO3)2.
Limonite. Hydrous iron oxide. H6Fe4O9.
Magnetite. Magnetic iron oxide, Fe3O4.
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," H2KAl3(SiO4)3, with a silvery aspect, and Biotite, the common dark "ferromagnesian" mica, (H, K)2(Mg, Fe″)2(Al, Fe‴)2(SiO4)3.
Microcline. See [Felspars].
Muscovite. See [Micas].
Nepheline. Sodium aluminium silicate, with some potassium, the pure sodium type being NaAlSiO4; the types with potassium contain slightly more silica.
Oligoclase. See [Felspars].
Olivine. Magnesium iron silicate, (Mg, Fe)2SiO4.
Opal. Uncrystallised silica, SiO2, 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 CaSiO3. Diopside consists of Ca(Mg, Fe)(SiO3)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 MgSiO3, while in Hypersthene iron replaces some of the magnesium.
Quartz. Silica, SiO2, crystallised in the trigonal system.
Rock-Salt. Sodium chloride, NaCl.
Rutile. Titanium dioxide, TiO2, crystallised in the tetragonal system. Anatase has the same composition, and is tetragonal, but has different fundamental angles.
Serpentine. Hydrous magnesium iron silicate, H4(Mg, Fe)3Si2O9.
Siderite. Iron carbonate, FeCO3.
Sillimanite. See [Andalusite].
Talc. Hydrous magnesium silicate, H2Mg3(SiO3)4.
Tourmaline. A borosilicate of aluminium with various other elements, R′9Al3(BOH)2Si4O19. R represents H, Na, Al, Mg, Fe.
Tridymite. Silica, SiO2, crystallised in doubly refracting six-sided plates. Its specific gravity is 2·3, that of Quartz being 2·65.
Wollastonite. See [Pyroxenes].
Zeolites. A series of hydrous aluminium silicates, with potassium, sodium, calcium, and sometimes barium.
Zircon. Zirconium silicate, ZrSiO4.