Sellaite

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Sellaite
Sellaite-39033.jpg
Sellaite crystal from Serra das Éguas, Brazil (size: 4.2 x 2.4 x 2 cm)
General
CategoryHalide mineral
Formula
(repeating unit)
MgF2
IMA symbolSel[1]
Strunz classification3.AB.15
Crystal systemTetragonal
Crystal classDitetragonal dipyramidal (4/mmm)
H-M symbol: (4/m 2/m 2/m)
Space groupP42/mnm
Unit cella = 4.6213(2)
c = 3.0519(1) [Å]; Z = 2
Identification
ColorColorless to white
Crystal habitPrismatic crystals; fibrous, radial, spherulitic
TwinningOn {011}
CleavagePerfect on {010} and {110}
FractureConchoidal
TenacityBrittle
Mohs scale hardness5–5.5
LusterVitreous
DiaphaneityTransparent
Specific gravity3.15
Optical propertiesUniaxial (+)
Refractive indexnω = 1.378 nε = 1.390
Birefringenceδ = 0.012
References[2][3][4]

Sellaite is a magnesium fluoride mineral with the formula MgF2. It crystallizes in the tetragonal crystal system, typically as clear to white vitreous prisms. It may be fibrous and occur as radiating aggregates. It has a Mohs hardness of 5 to 6 and a specific gravity of 2.97 to 3.15. Refractive index values are nω = 1.378 and nε = 1.390.

Discovery and occurrence[edit]

Sellaite was first described in 1868 and named for Italian mining engineer and mineralogist Quintino Sella (1827–1884). Its type locality is the glacier de Gébroulaz [fr] in France, where it occurred inside bitumen-bearing dolomite-anhydrite clasts within a moraine deposit. It has been reported in an evaporite deposit at Bleicherode; within volcanic ejecta and fumaroles at Vesuvius; in a metamorphic magnesite deposit at Serra das Éguas [pt]; and in sodic alkali granite near Gjerdingen [no].[2][3]

References[edit]

  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 Mineral Data Publishing: Handbook of Mineralogy
  3. ^ a b Mindat with location data
  4. ^ Webmineral data
  • Palache, C., H. Berman, and C. Frondel (1951) Dana’s system of mineralogy, (7th edition), v. II, pp. 37–39