Amphibole
The amphibole group comprises a complex group of 57 silicate minerals that, although falling in both the orthorhombic and monoclinic systems, are closely related in crystallography and other
physical properties as well as in chemical composition. A general formula of members of the amphibole group is W(0-1)X2Y5Z8O22(OH,F,CI)2, in which
Briefly, the amphiboles can be categorized in four groups: (1) the iron-magnesium-manganese group, which includes orthohombic anthophyllite, gedrite, and holmquistite and the monoclinic cummingtonite series; (2) the calcic amphibole group, which includes, among others, the tremolite-actinolite series, magnesio- and ferro-hornblende, and hastingsite; (3) the sodic-calcic group; and (4) the alkali-amphibole group, which includes the glaucophanes and riebeckites.
On the basis of composition, the most frequently encountered amphiboles may be conveniently grouped as follows:
Orthorhombic
Monoclinic
Alkali Amphibole Group
Members of the anthophyllite series occur largely, if not wholly, in metamorphic rocks. Members of the cummingtonite series are also more-or-less restricted in occurrence to metamorphic rocks. The tremolite-actinolite series is also most common in metamorphic rocks. The series that we refer to as the hornblende series is more correctly called the magnesiohornblende-ferrohornblende series. Hornblende is the name applied to the dark gray or essentially black-to-greenish black amphiboles that occur in many igneous rocks. The alkali-amphibole group includes three series whose individual members are relatively common in either metamorphic or alkalic igneous rocks. p>