Energetics of kaolin polymorphs

Journal Article · Thu Apr 01 04:00:00 EST 1999 · American Mineralogist

DOI:https://doi.org/10.2138/am-1999-0404· OSTI ID:343762

Ligny, D de; Navrotsky, A ^[1]

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1. Univ. of California, Davis, CA (United States). Dept. of Chemical Engineering and Materials Science

The enthalpy of formation of kaolin polymorphs at 298 K has been determined by drop-solution calorimetry into molten lead borate at 975 K. Corrections have been made for impurities in the samples. The standard enthalpy of formation from the elements is: kaolinite {minus}4120.2 {+-} 6.6 kJ/mol, dickite {minus}4107.6 {+-} 5.7 kJ/mol, nacrite {minus}4104.0 {+-} 7.6 kJ/mol, and halloysite {minus}4097.5 {+-} 5.6 kJ/mol. Using entropy data from the literature, the standard free energy of formation from the elements at 298 K is /{minus}3799.4 {+-} 6.4 kJ/mol for kaolinite, {minus}3785.1 {+-} 5.6 kJ/mol for dickite, and {minus}3776.8 {+-} 5.8 kJ/mol for halloysite. The effect of crystallinity (Hinckley index ranging from 1.6 to 0.4) on the enthalpy of formation of kaolinite is smaller than 5 kJ/mol, the experimental error. The relative stability of the polymorphs probably does not change significantly with pressure and temperature over their range of occurrence. Thus the geological occurrence of halloysite, nacrite, and dickite, which are metastable phases, must be interpreted in terms of kinetics or as the result of a specific synthesis path, rather than as resulting from changes in the thermodynamically stable phase assemblage.

• Sponsoring Organization: USDOE, Washington, DC (United States)
• OSTI ID: 343762
• Journal Information: American Mineralogist, Journal Name: American Mineralogist Journal Issue: 4 Vol. 84; ISSN AMMIAY; ISSN 0003-004X
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
CRYSTAL STRUCTURE
FORMATION FREE ENERGY
FORMATION HEAT
KAOLIN
MINERALS
PHASE STUDIES
STABILITY