Pargasite

Pargasite or pargasitic hornblende is a complex inosilicate mineral of the amphibole group with formula NaCa₂(Mg₄Al)(Si₆Al₂)O₂₂(OH)₂.

Pargasite
Single crystal of pargasite, 1.5 cm long, on a matrix of white marble from Hunza Valley, Pakistan
General
Category	Inosilicates
Formula	NaCa2(Mg4Al)(Si6Al2)O22(OH)2
IMA symbol	Prg[1]
Strunz classification	9.DE.15
Dana classification	66.1.3a.12
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	C2/m
Identification
Color	Bluish green, grayish black, light brown
Crystal habit	Stout prismatic to tabular
Twinning	Simple and lamellar – common
Cleavage	{110} perfect
Fracture	Splintery
Mohs scale hardness	5–6
Luster	Vitreous
Diaphaneity	Translucent, will transmit light on thin edges.
Specific gravity	3.04–3.17
Optical properties	Biaxial (−)
Refractive index	nα = 1.630 nβ = 1.640 nγ = 1.650
Birefringence	δ = 0.020 max.
References	[2][3][4][5][6]

It was first described for an occurrence in Pargas, Finland in 1814 and named for the locality.^[6]

It occurs in high temperature regional metamorphic rocks and in the skarns within contact aureoles around igneous intrusions. It also occurs in andesite volcanic rocks and altered ultramafic rocks.^[3]

Pargasite is the main water-storage site in the uppermost mantle; however, it becomes unstable at depths greater than 90 km (56 mi). This has significant consequences for the water storage capacity, and the solidus temperature of the lherzolite of the upper mantle.^[7]

It is used as a gemstone.^[8][9]

See also

• Anyolite