Manganian andalusite from Manbazar, Purulia District, West Bengal, India

Abstract Manganian andalusite occurs abundantly as porphyroblasts in manganiferous metasediments subjected to contact metamorphism under hornblende hornfels facies at the contact of a picrodolerite dyke near Manbazar, Purulia District, India. The Mn2O3content of andalusite varies from 13.2% to 19.17%, corresponding to 14.8 and 21.14 mole per cent of ‘Mn2SiO5’ respectively. Based on the analysis showing maximum amount of Mn2O3in andalusite, the mineral formula may be represented as follows: Other minerals in the assemblage are muscovite, manganophyllite, spessartine, piemontite, quartz, braunite, hematite and rutile. The manganian andalusite is completely fresh and appears to have formed at the expense of spessartine, piemontite and braunite during contact metamorphism. The manganian andalusite probably formed at about 600°C at around 3 kbar pressure. This is another rare example of andalusite with very high Mn2O3(and Fe2O3) as well as that of an occurrence of abundant manganian andalusite.