Description: The amphibole group of silicates is an important group of rock-forming minerals in igneous and metamorphic rocks. Amphiboles share a similar crystal structure and cleavage pattern, but contain different proportions of sodium (Na), calcium (Ca), iron (Fe) and magnesium (Mg), which substitute for one another in its crystal structure. The difficulty in distinguishing amphibole from pyroxene is reflected in the group’s name. Amphibole comes from the Greek word amfibolos, which means ‘ambiguous’ or ‘doubtful’.
Chemical Formula: A • X2 • Z5 ((Si,Al,Ti)8• O22) • (OH,F,Cl,O)2
A = Li, Na, K, Ca, Pb2+
X = Li, Na, Mg, Fe2+, Mn2+, Ca
Z = Li, Na, Mg, Fe2+, Mn2+, Zn, Co, Ni, Al, Fe3+, Cr3+, Mn3+, V3+, Ti, Zr
Crystal Form: Long prismatic, acicular, or fibrous crystal habit
Crystal System: Monoclinic
Color: Dark, commonly black to green, but can be colorless to white, green, brown, black, blue, or lavender.
Streak: White to gray
Luster: Vitreous (glassy) to dull or opaque.
Fracture: Uneven
Cleavage: Strong, two directions that meet at 56 and 124 degrees, uneven fracture
Hardness: 5 to 6 (harder than glass)
Density/Specific Gravity: 3 - 3.4 g/cm3
Magnetism: N/A
Taste: N/A
Hydrochloric acid: N/A
Radioactivity: N/A
Fluorescence: Fluorescent amphiboles have been noted, from Franklin, NJ. Greenish blue, long wave UV.
Distinguishing Physical Properties:
Cleavage - Distinctive, 2 directions at 120 or 60 degrees
Color: dark-colored (Black, dark green)
Hardness: hard (5-6)
Difficult: unfortunately, amphiboles are so similar in appearance that often they can only be distinguished from one another by examination under an optical microscope. They are important components of many intermediate igneous and metamorphic rocks, but are easily confused with pyroxene minerals that exhibit the same hardness and dark coloration.
Photograph Attribution: Amphibole specimen photograph by Sean C. Murphy, 2020.
Photograph Attribution: Another amphibole specimen photograph by Sean C. Murphy, 2020.