LIST OF THE COMMON MINERALS THAT FORM ROCKS =Actinolite.= See Amphiboles. =Albite.= See Felspars. =Amphiboles.= A series of silicates with the general formula RSiO_{3}, 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) SiO_{3}. _n_ (Mg, Fe″) (Al, Fe‴)_{2} SiO_{6}. _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 Al_{2}SiO_{5} and is rhombic, but crystallises with different fundamental angles. =Anorthite.= See Felspars. =Apatite.= Calcium phosphate, with fluorine, or sometimes chlorine, (CaF)Ca_{4}(PO_{4})_{3} = 3Ca_{3}(PO_{4})_{2}. CaF_{2}. =Aragonite.= Calcium carbonate, CaCO_{3}, crystallising in the rhombic system, with a specific gravity of 2·93. See Calcite. =Augite.= See Pyroxenes. =Biotite.= See Micas. =Calcite.= Calcium carbonate, CaCO_{3}, crystallising in the trigonal system, with a specific gravity of 2·72. See Aragonite. =Chalcedony.= Crystalline silica, SiO_{2}, 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, FeCr_{2}O_{4}. 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(CO_{3})_{2}. =Enstatite.= See Pyroxenes. =Epidote.= Calcium aluminium iron silicate, Ca_{2}(AlOH)(Al, Fe‴)_{2}(SiO_{4})_{3}. =Felspars.= A series of silicates of aluminium with potassium or sodium or calcium, or all of these. _Orthoclase_, KAlSi_{3}O_{8}, and the corresponding sodium form, _Albite_, NaAlSi_{3}O_{8}, lie at one end of the series, and the calcium felspar _Anorthite_, CaAl_{2}(SiO_{4})_{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."