Vesuvianite

Vesuvianite, also known as idocrase, is a green, brown, yellow, or blue silicate mineral. Vesuvianite occurs as tetragonal crystals in skarn deposits and limestones that have been subjected to contact metamorphism.[2] It was first discovered within included blocks or adjacent to lavas on Mount Vesuvius, hence its name. Attractive-looking crystals are sometimes cut as gemstones. Localities which have yielded fine crystallized specimens include Mount Vesuvius and the Ala Valley near Turin, Piedmont.[4]

Vesuvianite
Vesuvianite from the Jeffrey Mine in Asbestos, Quebec
General
Category	Sorosilicate
Formula (repeating unit)	Ca10(Mg, Fe)2Al4(SiO4)5(Si2O7)2(OH,F)4
Strunz classification	9.BG.35
Crystal system	Tetragonal
Crystal class	Ditetragonal dipyramidal (4/mmm) H-M symbol: (4/m 2/m 2/m)
Space group	P4/nnc
Unit cell	a = 15.52 Å, c = 11.82 Å Z = 2
Identification
Color	Yellow, green, brown; colorless to white, brown-black, light green, emerald green, violet, blue-green to blue, pink, purple, red, black, commonly zoned
Crystal habit	Short pyramidal to long prismatic crystals common, massive to columnar
Twinning	Fine twin domains observed
Cleavage	Poor on {110} and {100} very poor on {001}
Fracture	Sub conchoidal to irregular
Tenacity	Brittle
Mohs scale hardness	6–7
Luster	Vitreous to resinous
Streak	White
Diaphaneity	Subtransparent to translucent
Specific gravity	3.32–3.43
Optical properties	Uniaxial (−)
Refractive index	nω = 1.703–1.752 nε = 1.700–1.746
Birefringence	0.004–0.006
Pleochroism	slight in colored varieties
Solubility	Vesuvianite is virtually insolouble in acids
Other characteristics	striated lengthwise
References	[1][2][3]

The specific gravity is 3.4 and the Mohs hardness is ​6 ¹⁄₂. The name "vesuvianite" was given by Abraham Gottlob Werner in 1795, because fine crystals of the mineral are found at Vesuvius; these are brown in color and occur in the ejected limestone blocks of Monte Somma. Several other names were applied to this species, one of which, "idocrase" by René Just Haüy in 1796, is now in common use.[4]

A sky bluish variety known as cyprine has been reported from Franklin, New Jersey and other locations; the blue is due to impurities of copper in a complex calcium aluminum sorosilicate. Californite is a name sometimes used for jade-like vesuvianite, also known as California jade, American jade or Vesuvianite jade. Xanthite is a manganese rich variety. Wiluite is an optically positive variety from Wilui, Siberia. Idocrase is an older synonym sometimes used for gemstone-quality vesuvianite.