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Classification
Crystal System: Monoclinic
Class: Silicate
Sub Class: Inosilicate (double chain)
Group: Amphibole group
Structure and Composition
Composition: (Ca,Mg,Fe)7Si8O22(OH)2
Structure: Amphiboles have a double-chain silicate structure where silica tetrahedra, SiO4, are linked into two parallel rows. These double chains are bonded to layers of octahedrally coordinated cations, magnesium and iron, and interspersed with larger cations, calcium, present within the structure for charge balancing.
In the structure on the right, the red represents oxygen, the tan represents silicon, the orange represents magnesium or iron, and the purple represents calcium.
Identification In Hand Sample
*The underlined portions are the characteristics I consider most diagnostic to identification
Color: Dark green, dark brown, black
Luster: Vitreous to dull, opaque
Hardness: 5-6
Streak: White
Specific gravity: 3.0-3.4
Common Habit: Prismatic or asbestiform
Cleavage: Two directions that meet at 56 and 124 degrees, uneven fracture.
Identification In Thin Section
PPL Properties
Relief: High Positive
Habit/Form: Blades or long slender crystals are typical; diamond-shaped cross-sections are less common.
Color: Green or brown of various tones
Cleavage: Good in two directions at nearly 120° and 60°, and in one direction for the longitudinal section. Sections parallel to (100) display no cleavage since both cleavages are at too acute an angle to the section to be visible.
Pleochroism: Very intense to absent
XPL Properties
Isotropy/Anisotropy: Anisotropic
Interference color: Order II green and bluish near extinction.
Extinction angle: 12°-34°
Twins: Simple and lamellar twins on {100}
Uniaxial/Biaxial: Biaxial (+/-)
Amphibole in XPL (photo by me)
Amphibole in PPL (photo by me)
Geologic Occurrence
Amphiboles are common minerals in many types of intermediate igneous and metamorphic rocks. Amphiboles sometimes form in rocks by the alteration of pyroxenes.
Significance
Ancient jade carvings from Chinese palaces and Maori sites in New Zealand were made using the amphibole mineral Nephrite. Many different amphibole minerals have been used for many different purposes as asbestos, caused by the fibrous habit of some amphiboles. Uses for asbestiform amphiboles included fireproofing, heat resistance, insulation, and more.
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