Morpho-chemistry of Pyrophyllite in the Schistose Rock in a part of the Singhbhum Craton, Odisha, India

This paper discusses the morphological vs compositional variation of pyrophyllite from the schistoserocks exposed near Keonjhar. Morphologically, pyrophyllite in the study area can be grouped into sixcategories such as: Flaky, Laminated, Bladed, Fibrous, Platy and Granular. The first three types are enrichedin alumina (Al2O3-31 to 33%) and poor in silica (SiO2-49 to 56%) content while the other threevarieties are poor in alumina (Al2O3-20 to 27%) but contain higher amount of silica (SiO2-58 to 73%).Pyrophyllite shows different habits under scanning electron microscope. Compositional difference betweenthese micro-morpho types are established through in-situ analysis by the SEM-EDAX system. A pristinepyrophyllite exhibits acicular radiating, anhedral crystalline, globular, massive, monoclinic crystalline andbanded forms. Pyrophyllite containing Fe-impurities (6.9 to 18% Fe) appear in needle, rod and prismaticform. Similarly, pyrophyllite having Mg-impurities (platy & flaky) and Mg+Fe impurities (sub-hedral crystalline)show different habits. The K-containing pyrophyllite (K2O: 4.6 to 8%) reveals different forms like:inter-grown, porous, spotted, bedded and ooloidal. A spongy / mesh structure is shown by pyrophyllite having~ 11% Na as contaminant.The study demonstrates the inter-dependance nature between different habits and composition of pyrophyllitemineral in the schistose rock. Growth of pristine type or the one having impurities like Fe, Mg, K or Nadepends upon their protolithic composition. The clean variety is most likely formed through solution andprecipitation without any contamination. The Fe-containing pyrophyllite appears to have iron minerals intheir inter-granular space, which might have been derived from the breaking down of biotite. Similarly, bothMg and Fe + Mg-containing pyrophyllite are derived from phologopite or biotite. Both K and Na-containingpyrophyllite have probably formed at the expense of orthoclase / mica and plagioclase respectively.