Sepiolite

Formula : Mg4Si6O15(OH)2:6H2O

Space group: 
Orthorhombic - Dipyramidal H-M Symbol (2/m 2/m 2/m) Space Group: Pnan
Dana class: 
74.3.1b.1 (74) Phyllosilicate Modulated Layers (74.3) with joined strips (74.3.1b) Palygorskite-sepiolite group (Sepiolite subgroup)
High temperature: 
Stable in supergene environment
Geological context: 
As sedimentary clay mineral but also as secondary mineral associated with serpentine. Can precipitate from alkali saline waters in arid environments. This mineral is often present in weathering profile of rocks above basic rocks or from magnesium-bearing silicates (olivine, pyroxene).

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
647.830 285.500 (Ref.: 88sto) Stoessell, R. K. (1988) 25oC and 1 atm dissolution experiments of sepiolite and kerolite: Geochimica Cosmochimica Acta, 52, 365-374. -9260556 (Ref.: 88sto) Stoessell, R. K. (1988) 25oC and 1 atm dissolution experiments of sepiolite and kerolite: Geochimica Cosmochimica Acta, 52, 365-374. -10109552 (Ref.: Internal calculation) 670.000 (Ref.: 88sto) Stoessell, R. K. (1988) 25oC and 1 atm dissolution experiments of sepiolite and kerolite: Geochimica Cosmochimica Acta, 52, 365-374. 736.41 (Ref.: 88sto) Stoessell, R. K. (1988) 25oC and 1 atm dissolution experiments of sepiolite and kerolite: Geochimica Cosmochimica Acta, 52, 365-374.
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.
778.00 268.00 -108.00 (Ref.: 88sto) Stoessell, R. K. (1988) 25oC and 1 atm dissolution experiments of sepiolite and kerolite: Geochimica Cosmochimica Acta, 52, 365-374.
Reaction properties

Sepiolite + 8H+ + H2O ⇌ 4Mg+2 + 6H4SiO4

298.15 K, 1 bar log10K: 31.42 ΔH°r:-225784 J·mol-1 (Ref.: Internal calculation)
T(°C) 0 25 60 100 150 200 250 300
log10K 34.86 31.42 27.19 23.37 19.77 17.03 14.77 12.66
log10K(T) = A + B·T + C·T-1 + D·log10(T) + E·T−2 (T in Kelvin)
A B C D E
-3471.484334 -0.447145 219304.225124 1231.816529 -13101111.366018