Amphibole
“Amphibole”
refers not to a single mineral, but to a group of 57 silicate minerals.
Amphibolite is a metamaporthic rock composed of amphibole. It is very hard
and takes a high polish. The combination of its ability to be polished, its dark
color and its texture have made amphibolite a popular dimension stone in
construction. It is used as paving stones and as a veneer or facing on buildings
(both for interior and exterior use).
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.