Afghanite, (Na,K)22Ca10[Si24Al24O96](SO4)6Cl6,[2] 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.[2] 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.[4] It fluoresces a bright orange.

Afghanite
General
CategoryTectosilicates
Formula
(repeating unit)
(Na,K)22Ca10[Si24Al24O96](SO4)6Cl6
IMA symbolAfg[1]
Strunz classification9.FB.05
Crystal systemTrigonal
Crystal classDitrigonal pyramidal (3m)
H-M symbol: (3m)
Space groupP31c[2]
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 hardness5.5–6
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[3][4][5][6]
Afghanite

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 of 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.[3]

It is used as a gemstone.[7]

See also

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References

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  1. ^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. ^ a b c Ballirano, Paolo; Bonaccorsi, Elena; Maras, Adriana; Merlino, Stefanο (1996). "Crystal structure of afghanite, the eight-layer member of the cancrinite-group: Evidence for long-range Si,Al ordering". European Journal of Mineralogy. 9 (1): 21–30. Bibcode:1996EJMin...9...21B. doi:10.1127/ejm/9/1/0021.
  3. ^ a b Mineral Data Publishing 2001
  4. ^ a b Mindat with location data
  5. ^ Webmineral data
  6. ^ "Radioactive Gems : ClassicGems.net".
  7. ^ Tables of Gemstone Identification, By Roger Dedeyne, Ivo Quintens, p.112