The chemical composition of coexisting illites and chlorites was investigated in Al-saturated metapelites of very-low- to low-metamorphic grade (temperature 160-350 degrees C; pressure: 2-7 kbar). The main chemical characteristics of illites consist of a deficiency in interlayer cations, an important celadonitic substitution and a slight deviation of the octahedral sheet from dioctahedral to trioctahedral character. Interlayer cation deficiency appears to be controlled by K XII +Al IV --> SiIV+[] XII substitution and by the entry of some (Mg+Fe) VI into the octahedral layer in excess of a full dioctahedral occupancy. The first substitution represents the main mechanism of interlayer cation deficiency control: it appears to be mostly dependent on metamorphic temperature. The small amounts of (Mg+Fe) VI producing deviation from a perfect dioctahedral occupancy only balance about 0.2 vacancies in the interlayer sites: this mechanism appears more dependent on bulk rock chemistry than on metamorphic conditions. Most of the Mg and Fe cations present in the octahedral layer result from celadonitic substitution, which is essentially related to the metamorphic pressure. Use of Sigma (Mg+Fe) as a measure of the celadonite content is shown to be much more reliable than Sigma Si, which is also influenced by interlayer occupancy. The main chemical characteristics of chlorites are: a) an ubiquitous octahedral cation deficiency, which appears to decrease systematically with increasing metamorphic grade; this trend is coupled with an increase of (Mg+Fe) VI and a decrease of Al VI ; b) a conspicuous substitution of Al IV for Si, which is seen to increase systematically with increasing metamorphic grade; c) a largely constant value of Fe/(Mg+Fe) ratio, which seems to be independent of the metamorphic grade. The octahedral cation deficiency is dependent on rock composition, as well as on temperature and pressure conditions. Thus it can not be proposed as a reliable geothermometer. On the contrary, the Si IV content can provide a reliable geothermometer at constant host-rock composition.

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