Gemmology
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Zoisite

Class: Silicates
Group: Epidote

Varieties | Names:

Typically in regionally metamorphosed crystalline schists formed from igneous, sedimentary, or metamorphic rocks relatively high in calcium; in eclogites (Anthony et al., 2001—2005).
Zoisite — specimen 0035
Zoisite — specimen 0035, photo © NMNHS

Original description: Jameson, R., 1841. Zoisite — System of Mineralogy, Bell and Bradfute, Edinburgh: 597—598 [view in ‘Library’].

Type locality: Prickler Halt, Saualpe, Carinthia, Austria.

Type material: unknown.

Etymology: named after Sigmund Zois, Baron von Edelstein, Austrian supporter of mineralogy.

Distribution: Austria: Saualpe, Carinthia, and in the Zillertal, Tirol; Canada: from Asbestos, Quebec; Norway: near Trondheim; Pakistan: Shigar; Tanzania: exceptional gem material from the Merelani, Lelatema Mountains, Manyara; USA (Anthony et al., 2001—2005).

Chemistry

Ca2Al3(SiO4)(Si2O7)(O,OH)

Essential elements: hydrogen (H), oxygen (O), aluminium (Al), silicon (Si), calcium (Ca).

Crystal data

Crystallography: orthorhombic — dipyramidal. Crystal habit: crystals prismatic, to 10 cm, typically deeply striated and poorly terminated; columnar to compact, massive (Anthony et al., 2001—2005).

Physical properties

Cleavage: {010}, perfect; {100}, imperfect (Anthony et al., 2001—2005). Fracture: uneven, conchoidal (Anthony et al., 2001—2005). Tenacity: brittle (Anthony et al., 2001—2005). Hardness: 6—7 (Lazzarelli, 2012). Density: 3.2—3.5 g/cm3 (Lazzarelli, 2012). Luminescence: none.

Optical properties

Colour: white, grey, greenish brown, greenish grey, pink, blue, purple; colourless to pink in thin section (Anthony et al., 2001—2005). Diaphaneity: transparent to translucent (Anthony et al., 2001—2005). Lustre: vitreous (Anthony et al., 2001—2005). Refractive index: 1.685—1.707 — anisotropic [biaxial (+)] (Lazzarelli, 2012). Birefringence: 0.006—0.013. Dispersion: 0.0196. Pleochroism: X = pale pink to red-violet; Y = nearly colourless to bright pink or deep blue; Z = pale yellow to yellow-green (Anthony et al., 2001—2005).

Material from ‘Repository’

8 specimens: 0035 — 1.11 ct, Afghanistan; 0387 — 0.47 ct, Afghanistan; 0235 — 1.20 ct, Tanzania, Manyara, Lelatema Mountains, Merelani; 0363 — 1.12 ct, Tanzania, Manyara, Lelatema Mountains, Merelani; 0383 — 0.54 ct, Tanzania, Manyara, Lelatema Mountains, Merelani; 0220 — 1.13 ct, Tanzania, Manyara, Lelatema Mountains, Merelani; 0281 — 1.03 ct, Tanzania, Manyara, Lelatema Mountains, Merelani; 0460 — 1.06 ct, Tanzania, Manyara, Lelatema Mountains, Merelani.


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