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Mineral speciesFormula
 Enstatite(alpha)MgSiO3 
Mineralogy
Family: Pyroxene Group 
Space group: Orthorhombic - Dipyramidal H-M Symbol (2/m 2/m 2/m) Space Group: P bca 
Dana class: 65.1.2.1 (65) Inosilicate Single-Width Unbranched Chains, W=1 (65.1) with chains P=2 (65.1.2) Orthopyroxenes 
Geological context
High temperature: Enstatite is stable above around600C. 
Geological context: Occurs in pyroxenites, peridotites, and dunites; in ultramafic inclusions in alkalic olivine basalts and kimberlite; in mafic volcanics, rarely in felsic volcanics. Characteristic of charnockites and the granulite metamorphic facies, in regionally metamorphosed rocks and metagabbros.Common in chondrite, achondrite, and stony-iron meteorites. Occurs also in slags from metal smelting. 
Thermodynamics
Individual properties
Molar weight: 100.389 g/mol  ΔGºf-1,459,877 J/mol ref.  Sº: 67.78 J/mol/K ref. 
  ΔHºf-1,546,766 J/mol ref.  Cp: 79.07 J/mol/K ref. 
Reaction properties
Chemical reaction:  Enstatite(alpha) + 2H+ + H2O = Mg+2 + H4SiO4 
log10(Keq) = A + B.T + C.T-1 + D.log10(T) + E.T−2 (with T expressed in Kelvin)
A B C D E
 -3.79087333e+2   -5.76590049e-2   2.44693105e+4   1.36462840e+2   -1.03306588e+6 
Van t'Hoff
@ 25°C, 1 bar: log 10(Keq)11.84
ΔH°r-95,598 J/mol ref.
T (ºC) : 0 25 60 100 150 200 250 300
log 10(Keq):  13.38   11.84   10.09   8.52   6.99   5.77   4.74   3.81 
Maier Kelley
Cp(T) = a + b*10-3T + C*105T-2 (with T expressed in Kelvin)  ref.
a:102.72 J/mol/K 
b:19.83 J/mol/K2 
c:-26.28 J.K/mol 

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