Lepidocrocite

Formula : FeOOH

Family: 
Hydroxides
Space group: 
Orthorhombic - Dipyramidal HM symbol (2/m 2/m 2/m) Space Group: Amam
Dana class: 
6.1.2.2VI - 6 Hydroxides and Oxides Containing Hydroxyl ; 6.1 Hydroxides and Oxides Containing Hydroxyl where X+++O OH ; 6.1.2
High temperature: 
In the presence of pure water lepidocrocite is stable at all temperatures between OC to 100C. Lepidocrocite gives a d.t.a. curve similar to goethite with an endothermic peak at a temperature of 350C to 390C and an small exothermal peak at 370C to 500C .
Geological context: 
Lepidocrocite is a very common mineral typically occurring as a weathering product of iron-bearing minerals such as siderite, magnetite, pyrite, etc. It is normally formed under oxidizing conditions.

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
88.854 22.213 (Ref.: 63wyc) Wyckoff, R.W.G., 1963, Crystal Structures, Second edition, tome 4, Interscience Publishers, New York, p. 320-322. -480017 (Ref.: Internal calculation) -549400 (Ref.: 03maj/gre) Majzlan., J., Grevel, K.-D., Navrotsky, A., (2003) Thermodynamics of iron oxides. H. Enthalpies of formation and relative stability of goethite (a-FeOOH), lepidocrocite (y-FeOOH), and maghemite (y-Fe2O3). American Mineralogist 88, 855-859. 65.100 (Ref.: 03maj/gre) Majzlan., J., Grevel, K.-D., Navrotsky, A., (2003) Thermodynamics of iron oxides. H. Enthalpies of formation and relative stability of goethite (a-FeOOH), lepidocrocite (y-FeOOH), and maghemite (y-Fe2O3). American Mineralogist 88, 855-859. 69.11 (Ref.: 03maj/gre) Majzlan., J., Grevel, K.-D., Navrotsky, A., (2003) Thermodynamics of iron oxides. H. Enthalpies of formation and relative stability of goethite (a-FeOOH), lepidocrocite (y-FeOOH), and maghemite (y-Fe2O3). American Mineralogist 88, 855-859.
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.
59.76 60.52 -7.73 (Ref.: 03maj/gre) Majzlan., J., Grevel, K.-D., Navrotsky, A., (2003) Thermodynamics of iron oxides. H. Enthalpies of formation and relative stability of goethite (a-FeOOH), lepidocrocite (y-FeOOH), and maghemite (y-Fe2O3). American Mineralogist 88, 855-859.
Reaction properties

Lepidocrocite + 3H+ ⇌ Fe+3 + 2H2O

298.15 K, 1 bar log10K: 1.85 ΔH°r:-71260 J·mol-1 (Ref.: 03maj/gre) Majzlan., J., Grevel, K.-D., Navrotsky, A., (2003) Thermodynamics of iron oxides. H. Enthalpies of formation and relative stability of goethite (a-FeOOH), lepidocrocite (y-FeOOH), and maghemite (y-Fe2O3). American Mineralogist 88, 855-859.
T(°C) 0 25 60 100 150 200 250 300
log10K 2.99 1.85 0.54 -0.64 -1.82 -2.81 -3.69 -4.55
log10K(T) = A + B·T + C·T-1 + D·log10(T) + E·T−2 (T in Kelvin)
A B C D E
-478.325657 -0.076622 27558.431790 171.640110 -1255540.488338