Chemical composition of metapelitic chlorites: X-ray diffraction and optical property approach
European Journal of Mineralogy Volume 9 Number 4 (1997), p. 829 - 842
published: Jul 23, 1997
manuscript accepted: Jan 27, 1997
manuscript received: Nov 21, 1995
ArtNo. ESP147050904014, Price: 29.00 €
Abstract The crystallographic parameters, intensity relationships of basal reflections, and optical properties of chlorites from the low-grade metamorphic metapelites of the Alpujárride Complex (Betic Cordillera, SE Spain) have been compared with EMPA data obtained from areas selected from backscattered electron images. The Rausell-Colom et al. (1991) formula gives the best results for the calculation of A1IV from d0oi, corrected according to the Fe content, which may be obtained from the new equation b0 = 9.2216 + 0.0293 (Fe+Mn). This relationship gives intermediate results between those obtained by the Brindley (1961) and Von Engelhardt (1942) equations. Poor results are obtained by intensity relationship of basal reflection methods, due to uncertainty in the conversion of intensities to structure factors. Sign of elongation and anomalous interference colours depend exclusively on the Fe/(Fe+Mg) ratio and are independent of the AlIV content. The joint use of these two optical properties allows the classification of chlorite and the recognition of coarse compositional differences between various chlorites of the same thin section. The proposed methods also allow two of the three mean compositional vectors of chlorite defined by Laird (1988) to be determined. Dioctahedral substitution has been found to be low and fairly constant but significant, with an average value of Σoct = 5.9 a.f.u. and a standard deviation of 0.03.