Chamosite from oolitic ironstones: The necessity of a combined XRD-EDX approach

Chemical compositions of fine flakes of chlorite within chamositic coated g rains from ironstones in regressive and condensed Ordovician and Jurassic s equences in Poland, Germany, Czech Republic and India were established by e nergy-dispersion analysis (EDX). The chemical data are inaccurate, mainly o wing to the presence of intergrowths with other mineral phases. Structural formulae of trioctahedral chlorites calculated from the EDX data show appar ent vacancies in the octahedral site. A more accurate chemical composition was obtained using an X-ray-diffraction (XRD) method. Relationships between d(001) and b and the chemical composition of trioctahedral chlorites enabl ed the determination of their structural formulae. A comparison of the expe rimental powder-diffraction patterns with calculated patterns allows the as signment of chlorites from Pastek to subfamilies C and D, and of chlorites in samples from other localities, to subfamily D. A sample from Paslek was identified as a mixture of two chlorite polytypes: 1A(C)-I-ho (82%) and 1M( D)-II-ho (8%) with quartz (10%). Both polytypes reveal home-octahedral occu pancy of cations, and differ slightly in their chemical composition, which is manifested in different values of door and b. The combination of XRD and EDX methods of determination of chemical composition of chamosite is super ior to EDX analyses alone. From a simple measurement of d(001) and b, one o btains five major chemical components: Si, Al-IV, Al-VI, Mg and Fe2+ not co ntaminated by mineral intergrowths. The EDX approach in addition provides i nformation on specific heavy elements.