Zoisite

Zoisite
Yellow zoisite crystal (1.7 x 1 x 0.8 cm)
General
Category	Sorosilicate - epidote group
Formula (repeating unit)	Ca2Al3(SiO4)(Si2O7)O(OH)
Strunz classification	9.BG.10
Dana classification	58.2.1b.1
Crystal system	Orthorhombic
Crystal class	Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m)
Space group	Pnma
Identification
Color	White, gray, greenish brown, greenish gray, pink, blue, purple
Crystal habit	Prismatic crystals with striations; massive to columnar
Cleavage	Perfect {010} imperfect {100}
Fracture	Uneven to conchoidal
Mohs scale hardness	6 to 7
Luster	Vitreous, pearly on cleavage surfaces
Streak	White or colorless
Diaphaneity	Transparent to translucent
Specific gravity	3.10–3.36
Optical properties	biaxial positive
Refractive index	nα = 1.696 - 1.700 nβ = 1.696 - 1.702 nγ = 1.702 - 1.718
Birefringence	0.006-0.018
Pleochroism	X = pale pink to red-violet; Y = nearly colorless to bright pink or deep blue; Z = pale yellow to yellow-green
References
Major varieties
Tanzanite	Gem-quality zoisite, blue-purple
Thulite	Pink

Zoisite, first known as saualpite, after its type locality, is a calcium aluminium hydroxy sorosilicate belonging to the epidote group of minerals. Its chemical formula is Ca₂Al₃(SiO₄)(Si₂O₇)O(OH).

Zoisite occurs as prismatic, orthorhombic (2/m 2/m 2/m) crystals or in massive form, being found in metamorphic and pegmatitic rock. Zoisite may be blue to violet, green, brown, pink, yellow, gray, or colorless. It has a vitreous luster and a conchoidal to uneven fracture. When euhedral, zoisite crystals are striated parallel to the principal axis (c-axis). Also parallel to the principal axis is one direction of perfect cleavage. The mineral is somewhat higher than 6 in hardness, and its specific gravity ranges from 3.10 to 3.38, depending on the variety. It streaks white and is said to be brittle. Clinozoisite is a more common monoclinic polymorph of Ca₂Al₃(SiO₄)(Si₂O₇)O(OH).

Transparent material is fashioned into gemstones while translucent-to-opaque material is usually carved. A metamorphic rock known as anyolite consists of green zoisite with black tschermakite and ruby crystals. These rubies themselves are not of gem quality, but nevertheless their color provides a striking contrast to the green zoisite, and greatly enhances the decorative pieces that are carved from the rock.