Coesite(alpha)

Formula : SiO2

Family: 
Oxides
Space group: 
Monoclinic - Prismatic H-M Symbol (2/m) Space Group: C 2/c
Dana class: 
75.1.4.1 (75) Tectosilicate Si Tetrahedral Frameworks(75.1) SiO2 with [4] coordinated Si (75.1.4) Dana Group
High temperature: 
Coesite is a high presure mineral its stability is limited by stishovite (an other silica polymorph) at about 65 kbar.
Geological context: 
Formed at pressures in excess of 20 kbar, as fine-grained intergrowths with quartz and silica glass in impact metamorphosed rocks and tektites. A primary phase in grospydite (kyanite-garnet-clinopyroxene) xenoliths in kimberlite, and intergrown with quartz as inclusions in garnet and clinopyroxene within eclogites of high-grade metamorphic terranes.

Dernière mise à jour le 25.01.2021

THERMODYNAMICS

Individual properties (298.15 K)
M
g·mol-1
 V°
cm3·mol-1
 ΔGºf
J·mol-1
 ΔHºf
J·mol-1
 Sº
J·mol-1·k-1
 CP
J·mol-1·K-1
60.084 20.641 (Ref.: 78hel/del) Helgeson, H.C., Delany, J.M, Nesbitt, H.W., and Bird, D.K., 1978. Summary and Critique of the Thermodynamic Properties of Rock Forming Minerals: Amer. J. Sci., 278A, 229 pp. -851577 (Ref.: Internal calculation) -906313 (Ref.: 78hel/del) Helgeson, H.C., Delany, J.M, Nesbitt, H.W., and Bird, D.K., 1978. Summary and Critique of the Thermodynamic Properties of Rock Forming Minerals: Amer. J. Sci., 278A, 229 pp. 40.376 (Ref.: 78hel/del) Helgeson, H.C., Delany, J.M, Nesbitt, H.W., and Bird, D.K., 1978. Summary and Critique of the Thermodynamic Properties of Rock Forming Minerals: Amer. J. Sci., 278A, 229 pp. 43.54 (Ref.: 78hel/del) Helgeson, H.C., Delany, J.M, Nesbitt, H.W., and Bird, D.K., 1978. Summary and Critique of the Thermodynamic Properties of Rock Forming Minerals: Amer. J. Sci., 278A, 229 pp.
CP(T) = a + b·T + ·c·T-2 (T in Kelvin)
a
J·mol-1·K-1
 b * 103
J·mol-1·K-2
 c * 10-5
J·K·mol-1
 Maier Kelley Ref.
46.02 34.31 -11.30 (Ref.: 78hel/del) Helgeson, H.C., Delany, J.M, Nesbitt, H.W., and Bird, D.K., 1978. Summary and Critique of the Thermodynamic Properties of Rock Forming Minerals: Amer. J. Sci., 278A, 229 pp.
Reaction properties

Coesite(alpha) + 2H2O ⇌ H4SiO4

298.15 K, 1 bar log10K: -2.91 ΔH°r:19112 J·mol-1 (Ref.: 78hel/del) Helgeson, H.C., Delany, J.M, Nesbitt, H.W., and Bird, D.K., 1978. Summary and Critique of the Thermodynamic Properties of Rock Forming Minerals: Amer. J. Sci., 278A, 229 pp.
T(°C) 0 25 60 100 150 200 250 300
log10K -3.26 -2.91 -2.59 -2.32 -2.06 -1.85 -1.68 -1.55
log10K(T) = A + B·T + C·T-1 + D·log10(T) + E·T−2 (T in Kelvin)
A B C D E
-352.703096 -0.041818 21950.589760 123.864743 -1587368.679096