Paragonite

Paragonite is a mineral, related to muscovite. Its empirical formula is NaAl2[(OH)2|AlSi3O10]. 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.[5] Jadeite bearing pyroxene minerals have suggested Clinozoisite and paragonite are associated and derived from lawsonite releasing Quartz and water via the following reaction:[6]

Paragonite
Paragonite with Garnet
General
CategoryPhyllosilicates
Micas
Formula
(repeating unit)		NaAl2[(OH)2|AlSi3O10]
Strunz classification9.EC.15
Crystal systemMonoclinic
Crystal classPrismatic (2/m)
(same H-M symbol)
Space groupC2/c
Identification
ColorColorless, pale yellow, grayish, grayish white, greenish, light apple-green
Crystal habitmassive, fibrous or scaly
Twinningcommon on the [310] less common on the {001}
CleavagePerfect on the {001}
FractureMicaeous
TenacityElastic
Mohs scale hardness2.5 - 3
LusterPearly
StreakWhite
Diaphaneitytransparent to translucent
Specific gravity2.78
Optical propertiesBiaxial (-)
Refractive indexnα = 1.564 - 1.580 nβ = 1.594 - 1.609 nγ = 1.600 - 1.609
Birefringenceδ = 0.036
Dispersionr < v strong
Ultraviolet fluorescenceNone
References[1][2][3][4]

It was first described in 1843 for an occurrence at Mt. Campione, Tessin, Switzerland.[3] The name derives from the Greek, paragon, for misleading, due to its similar appearance to talc.[4]

References
  1. Mineralienatlas
  2. Mindat
  3. Webmineral
  4. Handbook of Mineralogy
  5. 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.
  6. Deer, William A. (1997). Single-chain Silicates, Volume 2A. Geological Society of London. p. 477.
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