Paragonite

Paragonite
Paragonite with garnet
General
Category	Phyllosilicates Micas
Formula (repeating unit)	NaAl2(AlSi3O10)(OH)2
IMA symbol	Pg
Strunz classification	9.EC.15
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	C2/c
Identification
Color	Colorless, pale yellow, grayish, grayish white, greenish, light apple-green
Crystal habit	massive, fibrous or scaly
Twinning	common on the [310] less common on the {001}
Cleavage	Perfect on the {001}
Fracture	Micaeous
Tenacity	Elastic
Mohs scale hardness	2.5–3
Luster	Pearly
Streak	White
Diaphaneity	transparent to translucent
Specific gravity	2.78
Optical properties	Biaxial (−)
Refractive index	nα = 1.564 – 1.580 nβ = 1.594 – 1.609 nγ = 1.600 – 1.609
Birefringence	δ = 0.036
Dispersion	r < v strong
Ultraviolet fluorescence	None
References

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. Jadeite bearing pyroxene minerals have suggested clinozoisite and paragonite are associated and derived from lawsonite releasing quartz and water via the following reaction:

${\ce {4CaAl2Si2O8(H2O)2 + NaAlSi2O6 <=> 2Ca2Al3Si3O12(OH) + NaAl3Si3O10(OH)2 + SiO2 + 6H2O}}$

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