Frankamenite

Frankamenite is the fluorine-dominate variation of the rare mineral canasite with a general formula of K3Na3Ca5(Si12O30)[F,(OH)]4·(H2O).[2]

Frankamenite
General
CategoryInosilicates
Formula
(repeating unit)		K3Na3Ca5(Si12O30)[F,(OH)]4·(H2O)
IMA symbolFkm[1]
Strunz classification9.DG.90
Crystal systemTriclinic
Crystal classPedial (1)
(same H-M symbol)
Space groupP1
Identification
ColorGreenish blue, bluish gray, greenish gray, lilac gray
Crystal habitLath, columnar
CleavageGood on (010) and (100)
TenacityBrittle
Mohs scale hardness5.5
LusterVitreous
StreakPale blue
DiaphaneityTransparent to translucent
Specific gravity2.68
Optical propertiesBiaxial positive
Refractive indexnα = 1.536 nβ = 1.539 nγ = 1.542
Birefringence0.0060
Dispersionr < v strong
References[2][3]

Frankamenite belongs to the triclinic crystal system, with the bases of its structure containing Ca-Na mixed octahedra joined by octagonal tubes SiO4 of the composition (Si12O30). Frankamenite has six Ca-Na mixed positions distributed amongst these octahedra, reflecting its varying compositions.[4]

Frankamenite was named for the Russian mineralogist-crystallographer V. A. Frank-Kamentsky (1915–1994), who discovered the mineral.[3]

Frankamenite occurs in association with the rare mineral charoite, which is found only in the Murun Massif of the Olyokma-Chara Plateau, Aldan Shield, Sakha Republic, Yakutia, Siberia, Russia. Here, metasomatism enriches a syenite massif with potassium when it comes into contact with a limestone at around 200–250 °C. This metamorphic process produces a potassium feldspar metasomatite, the typical geological environment for canasite and, therefore, frankamenite. Frankamenite and charoitein are exclusive to the Sakha Republic in this sort of environment, as mineralogists have yet to discover the minerals elsewhere.[5]

References
  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. Mindat.org
  3. Webmineral data
  4. Rozhdestvenskaya, I.V. and Nikishova, L.V. (1996) The Crystal Structure of Frankamenite. Mineralogical Magazine; 60; 897-905.
  5. Charoite on Webmineral


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