Pargasite

Amphibole, double chain inosilicate mineral From Wikipedia, the free encyclopedia

Pargasite

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

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Pargasite
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Single crystal of pargasite, 1.5 cm long, on a matrix of white marble from Hunza Valley, Pakistan
General
CategoryInosilicates
FormulaNaCa2(Mg4Al)(Si6Al2)O22(OH)2
IMA symbolPrg[1]
Strunz classification9.DE.15
Dana classification66.1.3a.12
Crystal systemMonoclinic
Crystal classPrismatic (2/m)
(same H-M symbol)
Space groupC2/m
Identification
ColorBluish green, grayish black, light brown
Crystal habitStout prismatic to tabular
TwinningSimple and lamellar – common
Cleavage{110} perfect
FractureSplintery
Mohs scale hardness5–6
LusterVitreous
DiaphaneityTranslucent, will transmit light on thin edges.
Specific gravity3.04–3.17
Optical propertiesBiaxial (−)
Refractive indexnα = 1.630 nβ = 1.640 nγ = 1.650
Birefringenceδ = 0.020 max.
References[2][3][4][5][6]
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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

References

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