Dissolution enthalpies of magnesian calcites

被引:16
作者
Bischoff, WD [1 ]
机构
[1] Wichita State Univ, Dept Geol, Wichita, KS 67260 USA
关键词
magnesian calcite; enthalpy; entropy; dissolution; calorimetry; carbonates;
D O I
10.1023/A:1009684214945
中图分类号
P3 [地球物理学]; P59 [地球化学];
学科分类号
0708 ; 070902 ;
摘要
A series of synthetic and biogenic magnesian calcites was dissolved in weak acetic acid solutions to measure the enthalpies of dissolution at 25 degrees C. For the synthetic phases, heat released was 33.5 kJ/mol for calcite, decreasing to 33 kJ/mol for a phase of 2 mol% MgCO3, and increasing to 35 kJ/mol for a phase of 15 mol%. Values of excess enthalpies, Delta H-xs, calculated using calcite and magnesite end-members, average about 1 kJ/mol for the synthetic phases. Total entropies of solid solution formation, Delta S-SS, also were calculated using available data on Gibbs free energies of formation and these excess enthalpies. Values of Delta S-SS range from -2 J/(mol-K) at 2 mol% to -5 J/(mol-K) at 15 mol% MgCO3. These negative values of Delta H-xs and Delta S-SS suggest that some form of ordering (cation?) is obtained in the synthetic phases, and that vibrational entropies of the solid solution are diminished in comparison to the end-members. In contrast, biogenic samples generally have positive values of Delta H-xs, increasing from +1 kJ/mol at 5 mol% to +3 kJ/mol at 20 mol% MgCO3. Most values of Delta S-SS are equal (within errors) to values expected from configurational enthalpy alone. Thus, in most biogenic materials cation ordering probably is not obtained, and most phases are more typical of equivalent-site solid solutions.
引用
收藏
页码:321 / 336
页数:16
相关论文
共 35 条
[1]  
BISCHOFF WD, 1983, AM MINERAL, V68, P1183
[2]   STABILITIES OF SYNTHETIC MAGNESIAN CALCITES IN AQUEOUS-SOLUTION - COMPARISON WITH BIOGENIC MATERIALS [J].
BISCHOFF, WD ;
MACKENZIE, FT ;
BISHOP, FC .
GEOCHIMICA ET COSMOCHIMICA ACTA, 1987, 51 (06) :1413-1423
[3]   DIAGENETIC STABILIZATION PATHWAYS OF MAGNESIAN CALCITES [J].
BISCHOFF, WD ;
BERTRAM, MA ;
MACKENZIE, FT ;
BISHOP, FC .
CARBONATES AND EVAPORITES, 1993, 8 (01) :82-89
[4]  
BISCHOFF WD, 1985, AM MINERAL, V70, P581
[5]  
BURTON BP, 1987, AM MINERAL, V72, P329
[6]   THERMODYNAMICS OF MAGNESIAN CALCITE SOLID-SOLUTIONS AT 25-DEGREES-C AND 1-ATM TOTAL PRESSURE [J].
BUSENBERG, E ;
PLUMMER, LN .
GEOCHIMICA ET COSMOCHIMICA ACTA, 1989, 53 (06) :1189-1208
[7]  
CHAI L, 1995, GEOCHIM COSMOCHIM AC, V59, P939, DOI 10.1016/00167-0379(50)00119-
[8]   ASPECTS OF THE BIOGEOCHEMISTRY OF MAGNESIUM .1. CALCAREOUS MARINE ORGANISMS [J].
CHAVE, KE .
JOURNAL OF GEOLOGY, 1954, 62 (03) :266-283
[9]   A SOLID SOLUTION BETWEEN CALCITE AND DOLOMITE [J].
CHAVE, KE .
JOURNAL OF GEOLOGY, 1952, 60 (02) :190-192
[10]   OBSERVATIONS ON SOLUBILITY OF SKELETAL CARBONATES IN AQUEOUS SOLUTIONS [J].
CHAVE, KE ;
GARRELS, RM ;
THOMPSON, ME ;
DEFFEYES, KS ;
WEYL, PK .
SCIENCE, 1962, 137 (3523) :33-&