A group of dark-colored ferromagnesian silicate
minerals with the general chemical formula:
A2–3 B5 (Si, Al)8O22(OH)2
where A = Mg, Fe+2, Ca, or Na, and B = Mg, Fe+3, Fe+2or Al.
Amphiboles contain continuous double chains of cross-linked
double silicate tetrahedra. The chains are bound together by
cations such as Ca, Mg, and Fe, which satisfy the negative
charges of the polymerized tetrahedra. Most amphiboles are
monoclinic, but some crystallize in the orthorhombic crystal
system. They have good prismatic cleavage intersecting at 56°
and 124° and typically form columnar or fibrous prismatic
crystals. Amphiboles are very common constituents of metamorphic
and igneous rocks and have a chemical composition
similar to pyroxenes. Some of the common amphibole minerals
include hornblende, tremolite, actinolite, anthophyllite,
cummingtonite, riebeckite, and glaucophane.
Amphibole is a fairly common mineral in intermediate to
mafic igneous rocks such as granodiorite, diorite, and gabbro,
forming up to 25 percent of these rocks in some cases.
Since amphibole is a hydrous mineral, it typically forms in
igneous environments where water is available. Amphibole is
best known, however, as a metamorphic mineral indicative of
medium grade pressure-temperature metamorphism of mafic
rocks. When basalt, gabbro, or similar rocks are heated to
930°F–1,300°F (500°C–700°C) at 3–10 kilobars pressure
(equivalent to 6 to 20-mile or 10 to 30-km depth), the primary
mineral assemblage will commonly turn to an assemblage
of amphibole+plagioclase feldspar. Many field geologists will
call such a rock an “amphibolite,” although this term should
be reserved for a description of the metamorphic conditions
(known as facies) at which these rocks formed.
See also MINERALOGY.
minerals with the general chemical formula:
A2–3 B5 (Si, Al)8O22(OH)2
where A = Mg, Fe+2, Ca, or Na, and B = Mg, Fe+3, Fe+2or Al.
Amphiboles contain continuous double chains of cross-linked
double silicate tetrahedra. The chains are bound together by
cations such as Ca, Mg, and Fe, which satisfy the negative
charges of the polymerized tetrahedra. Most amphiboles are
monoclinic, but some crystallize in the orthorhombic crystal
system. They have good prismatic cleavage intersecting at 56°
and 124° and typically form columnar or fibrous prismatic
crystals. Amphiboles are very common constituents of metamorphic
and igneous rocks and have a chemical composition
similar to pyroxenes. Some of the common amphibole minerals
include hornblende, tremolite, actinolite, anthophyllite,
cummingtonite, riebeckite, and glaucophane.
Amphibole is a fairly common mineral in intermediate to
mafic igneous rocks such as granodiorite, diorite, and gabbro,
forming up to 25 percent of these rocks in some cases.
Since amphibole is a hydrous mineral, it typically forms in
igneous environments where water is available. Amphibole is
best known, however, as a metamorphic mineral indicative of
medium grade pressure-temperature metamorphism of mafic
rocks. When basalt, gabbro, or similar rocks are heated to
930°F–1,300°F (500°C–700°C) at 3–10 kilobars pressure
(equivalent to 6 to 20-mile or 10 to 30-km depth), the primary
mineral assemblage will commonly turn to an assemblage
of amphibole+plagioclase feldspar. Many field geologists will
call such a rock an “amphibolite,” although this term should
be reserved for a description of the metamorphic conditions
(known as facies) at which these rocks formed.
See also MINERALOGY.
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