AMPHIBOLE

 

Amphibole is a group of silicate minerals.  The garden-variety type of amphibole is hornblende (~(Ca,Na)2-3(Mg,Fe,Al)5Si6(Si,Al)2O22(OH)2 - calcium sodium magnesium iron hydroxy-aluminosilicate).  Hornblende has a nonmetallic luster, jet black to very dark green color, pale greenish-gray streak, a hardness of about 5.5 on the Mohs Hardness Scale, and two cleavage planes at ~60º and ~120º.  Many specimens, even at a microscopic scale, show a splintery fracture network.  Hornblende is a common mafic mineral in granites & granodiorites.

 

Hornblende (from left to right: 2.4 cm across, 1.4 cm across, 1.2 cm across).

 

 

Photo gallery of hornblende

 

 

Tremolite (field of view ~2 cm across) - tremolite schist from Balmat, New York, USA.  Tremolite is a whitish amphibole having the formula Ca2Mg5Si8O22(OH)2 - calcium magnesium hydroxysilicate.  It often forms small needle-like crystals.

 

 

Tremolite (field of view ~5.6 cm across) - nice spray of tremolite needles in dolomitic marble.

Geologic context: glacial clast derived from Upper Pleistocene till

Locality: Granville, Ohio, USA.

Collected & owned by Ryan Martyn.

 

 

Photo gallery of tremolite

 

 

Actinolite (8.1 cm across) from California, USA.  Actinolite is a greenish amphibole having the formula Ca2(Mg,Fe)5Si8O22(OH)2 - calcium magnesium iron hydroxysilicate.  Unlike tremolite, actinolite has a significant iron component, which causes the dark coloration.  Actinolite often forms radiating clusters of elongated blades.  It forms a continuous chemical spectrum with tremolite.

 

 

 

Photo gallery of actinolite

 

 

Cummingtonite - this Paleozoic metamorphic rock from Massachusetts consists of sprays of brownish cummingtonite amphibole needles, quartz, garnet, and magnetite.

Locality: at or near West Chesterfield, western Hampshire County, western Massachusetts, USA.

Cummingtonite is a brownish to grayish to dark greenish amphibole ((Mg,Fe)7Si8O22(OH)2) that occurs in some metamorphic rocks and occasionally in some igneous rocks.    It has a silky luster and usually forms radiating masses of acicular (needle-shaped) crystals or irregular masses of fibers.

Specimen owned by James Cheshire.

 

 

Photo gallery of cummingonite

 

 

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