Afghanite

Afghanite, (Na,K)₂₂Ca₁₀[Si₂₄Al₂₄O₉₆](SO₄)₆Cl₆,[1] is a hydrous sodium, calcium, potassium, sulfate, chloride, carbonate alumino-silicate mineral. Afghanite is a feldspathoid of the cancrinite group and typically occurs with sodalite group minerals. It forms blue to colorless, typically massive crystals in the trigonal crystal system. The lowering of the symmetry from typical (for cancrinite group) hexagonal one is due to ordering of Si and Al.[1] It has a Mohs hardness of 5.5 to 6 and a specific gravity of 2.55 to 2.65. It has refractive index values of nω=1.523 and nε=1.529. It has one direction of perfect cleavage and exhibits conchoidal fracture.[3] It fluoresces a bright orange.

Afghanite

Afghanite
Afghanite on marble, Sar-e-Sang, Badakhshan, Afghanistan. Size: 3.4 x 3.1 x 2.6 cm.
General
Category	Tectosilicates
Formula (repeating unit)	(Na,K)22Ca10[Si24Al24O96](SO4)6Cl6
Strunz classification	9.FB.05
Crystal system	Trigonal
Crystal class	Ditrigonal pyramidal (3m) H-M symbol: (3m)
Space group	P31c[1]
Unit cell	a = 12.796, c = 21.409 [Å]; Z = 1
Identification
Color	light blue, dark blue
Crystal habit	Lath shaped crystals, rounded grains
Cleavage	Perfect {1010}
Fracture	Conchoidal
Mohs scale hardness	5​1⁄2 - 6
Luster	Vitreous
Streak	white
Diaphaneity	Transparent
Specific gravity	2.55 - 2.65
Optical properties	Uniaxial (+)
Refractive index	nω = 1.523 nε = 1.529
Birefringence	δ = 0.006
Other characteristics	Radioactive 2.28% (K)
References	[2][3][4][5]

It was discovered in 1968 in the Lapis-lazuli Mine, Sar-e-Sang, Badakhshan Province, Afghanistan and takes its name from that country. It has also been described from localities in Germany, Italy, the Pamir Mountains, Tajikistan, near Lake Baikal in Siberia, New York and Newfoundland. It occurs as veinlets in lazurite crystals in the Afghan location and in altered limestone xenoliths within pumice in Pitigliano, Tuscany, Italy.[2]

It is used as a gemstone.[6]