Pargasite

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Pargasite
Pargasite-ww49d.jpg
Single crystal of pargasite, 1.5 cm long, on a matrix of white marble from Hunza Valley, Pakistan
General
Category Silicate mineral
Formula
(repeating unit)
NaCa2(Mg4Al)(Si6Al2)O22(OH)2
Identification
Color Bluish green, grayish black, light brown
Crystal habit Stout prismatic to tabular
Crystal system Monoclinic; 2/m
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 [1][2][3][4]

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

It was first described for an occurrence in Pargas, Finland in 1814 and named for the locality.[4]

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.[1]

Pargasite is the main water-storage site in the uppermost mantle, however it becomes unstable at depths greater than 90 km. This has significant consequences for the water storage capacity, and the solidus temperature of the lherzolite of the upper mantle.[5]

See also[edit]

References[edit]

  1. ^ a b "Pargasite" (PDF). Handbook of Mineralogy (pdf). Mineralogical Society of America. Retrieved 2012-12-17. 
  2. ^ IMA Master List
  3. ^ "Pargasite". mindat.org. Retrieved 2012-12-17. 
  4. ^ a b "Pargasite Mineral Data". webmineral.com. Retrieved 2012-12-17.  (Java plugin required)
  5. ^ Green, D H; Hibberson, W O; Kovacs, Istvan; Rosenthal, A (23 September 2010). "Water and its influence on the lithosphere–asthenosphere boundary". Nature 467 (7314): 448–451. doi:10.1038/nature09369.  (subscription required)


Spinel and Pargasite on Marble, Luc Yen District, Vietnam. Specimen size: 4.5 x 3.5 x 3.5 cm.