Paragonite

NaAl2(AlSi3O10)(OH)2IMA symbolPg[1]Strunz classification9.EC.15Crystal systemMonoclinicCrystal classPrismatic (2/m)
(same H-M symbol)Space groupC2/cIdentificationColorColorless, pale yellow, grayish, grayish white, greenish, light apple-greenCrystal habitmassive, fibrous or scalyTwinningcommon on the [310] less common on the {001}CleavagePerfect on the {001}FractureMicaeousTenacityElasticMohs scale hardness2.5–3LusterPearlyStreakWhiteDiaphaneitytransparent to translucentSpecific gravity2.78Optical propertiesBiaxial (−)Refractive indexnα = 1.564 – 1.580 nβ = 1.594 – 1.609 nγ = 1.600 – 1.609Birefringenceδ = 0.036Dispersionr < v strongUltraviolet fluorescenceNoneReferences[2][3][4][5]

Paragonite is a mineral, related to muscovite. Its empirical formula is NaAl2(AlSi3O10)(OH)2. A wide solvus separates muscovite from paragonite, such that there is little solid solution along the vector Na+K+ and apparent micas of intermediate composition is most commonly a microscopic (or even sub-microscopic) intergrowth of two distinct micas, one rich in K, and the other in Na. Paragonite is a common mineral in rocks metamorphosed under blueschist facies conditions along with other sodic minerals such as albite, jadeite and glaucophane. During the transition from blueschist to greenschist facies, paragonite and glaucophane are transformed into chlorite and albite.[6] Jadeite bearing pyroxene minerals have suggested clinozoisite and paragonite are associated and derived from lawsonite releasing quartz and water via the following reaction:[7]

4 CaAl 2 Si 2 O 8 ( H 2 O ) 2 + NaAlSi 2 O 6 ↽ − − ⇀ 2 Ca 2 Al 3 Si 3 O 12 ( OH ) + NaAl 3 Si 3 O 10 ( OH ) 2 + SiO 2 + 6 H 2 O {\displaystyle {\ce {4CaAl2Si2O8(H2O)2 + NaAlSi2O6 <=> 2Ca2Al3Si3O12(OH) + NaAl3Si3O10(OH)2 + SiO2 + 6H2O}}}

It was first described in 1843 for an occurrence at Mt. Campione, Tessin, Switzerland.[4] The name derives from the Greek, paragon, for misleading, due to its similar appearance to talc.[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. ^ Mineralienatlas
  3. ^ Mindat
  4. ^ a b Webmineral
  5. ^ a b Handbook of Mineralogy
  6. ^ Deer, W. A.; et al. (2006). Rock-forming minerals, Volume 3A Micas (2 ed.). Geological Society of London. p. 302. ISBN 978-1-86239-142-0.
  7. ^ Deer, William A. (1997). Single-chain Silicates, Volume 2A. Geological Society of London. p. 477.


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