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

Abstract

This paper discusses the morphological vs compositional variation of pyrophyllite from the schistose
rocks exposed near Keonjhar. Morphologically, pyrophyllite in the study area can be grouped into six
categories such as: Flaky, Laminated, Bladed, Fibrous, Platy and Granular. The first three types are enriched
in alumina (Al2O3-31 to 33%) and poor in silica (SiO2-49 to 56%) content while the other three
varieties 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 between
these micro-morpho types are established through in-situ analysis by the SEM-EDAX system. A pristine
pyrophyllite exhibits acicular radiating, anhedral crystalline, globular, massive, monoclinic crystalline and
banded forms. Pyrophyllite containing Fe-impurities (6.9 to 18% Fe) appear in needle, rod and prismatic
form. 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 pyrophyllite
mineral in the schistose rock. Growth of pristine type or the one having impurities like Fe, Mg, K or Na
depends upon their protolithic composition. The clean variety is most likely formed through solution and
precipitation without any contamination. The Fe-containing pyrophyllite appears to have iron minerals in
their inter-granular space, which might have been derived from the breaking down of biotite. Similarly, both
Mg and Fe + Mg-containing pyrophyllite are derived from phologopite or biotite. Both K and Na-containing
pyrophyllite have probably formed at the expense of orthoclase / mica and plagioclase respectively.