Date of Award

1996

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Geology and Geophysics

First Advisor

Gary R. Byerly

Abstract

Presented are three papers on the aspects of crystal chemistry, structure, and formation conditions of IIb trioctahedral chlorite and actinolite (tremolite) of low-grade metamorphism and metasomatism in the Archean Barberton greenstone belt. A fourth paper reports the pressure effect on the Na-K geothermometer, which results from the study of fluid-rock interaction during low temperature geological processes. The chemical compositions of the two Fe-Mg minerals show strong associations with the MgO$\sb{\rm Rock}$ because MgO$\sb{\rm Rock}$ was the least available stoichiometric component during the crystallization of the minerals. All major cation abundances in both minerals are strongly correlated with each other. A complex exchange vector explains over 90% of the chlorite compositional variation: $\rm Mg\sb4SiFe\sp{2+}\sb{-3}Al\sp{VI}\sb{-1}Al\sp{IV}\sb{-1}$, and a complex substitution expresses the actinolite variation: 3 parts of edenite substitution-5 parts of tschermakite substitution-2 parts of riebeckite/glaucophane substitution. A chemical composition limitation is observed, 1.5-9.2 Mg pfu and 1.0-3.2 Al$\rm\sp{IV}$ for the chlorite and 2.5-4.5 Mg for the actinolite. The chlorite chemical variation pattern is required for the dimensional fit between the tetrahedral and octahedral sheets, and between the talc-like and brucite-like layers. This is consistent with the calculation of b-dimensions and dimensional changes upon the chemical variation of the chlorite. The tetrahedral rotation $\alpha$ is not important in fitting the "free" structural sheets in trioctahedral chlorite. The dimensional fit also plays a role in the ordering of octahedral cations and in the limitation for the amounts of Mg and Si in chlorite. The actinolite chemical variation relates to various cation substitution mechanisms involving local charge balance and dimensional fit between the silica double chain and octahedral strip. Molal volume data can be used to derive the InK-T-P relation in considering the pressure effect for chemical geothermometers. For the Na-K exchange reaction:$$\eqalign{&\log K\sb2(T,P)=\cr &\log K\sb1(T,P\sb1){-}{{(9.064+\overline{V}\sp\circ\sb{Na\ {+}}-\overline{V} \sp\circ\sb{K+})(P-P\sb1)10\sp{-1}m\sp3{\cdot}bar}\over{2.303\times 8.314(J/K{\cdot}mol)T(K)}}\cr}$$The pressure factor is positive on equilibrium temperature within the temperature range of 0$\sp\circ$ to 300$\sp\circ$C and the pressure has an effect of 9$\sp\circ$C at an equilibrium temperature of about 260$\sp\circ$C between 1 bar and 1000 bar.

ISBN

9780591133912

Pages

183

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