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
CategoryTectosilicates
Formula
(repeating unit)		(Na,K)22Ca10[Si24Al24O96](SO4)6Cl6
Strunz classification9.FB.05
Crystal systemTrigonal
Crystal classDitrigonal pyramidal (3m)
H-M symbol: (3m)
Space groupP31c[1]
Unit cella = 12.796, c = 21.409 [Å]; Z = 1
Identification
Colorlight blue, dark blue
Crystal habitLath shaped crystals, rounded grains
CleavagePerfect {1010}
FractureConchoidal
Mohs scale hardness5​1⁄2 - 6
LusterVitreous
Streakwhite
DiaphaneityTransparent
Specific gravity2.55 - 2.65
Optical propertiesUniaxial (+)
Refractive indexnω = 1.523 nε = 1.529
Birefringenceδ = 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]

References
  1. https://www.schweizerbart.de/papers/ejm/detail/9/83316/Crystal_structure_of_afghanite_the_eight_layer_member_of_the_cancrinite_group_evidence_for_long_range_Si_Al_ordering
  2. Mineral Data Publishing 2001
  3. Mindat with location data
  4. Webmineral data
  6. Tables of Gemstone Identification, By Roger Dedeyne, Ivo Quintens, p.112
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