Ocorrência e Variabilidade de Composição dos Fosfatos do Grupo da Monazita em Carbonatitos

Abstract

Monazite structural group includes arsenates, phosphates, and silicates, with general formulae A B O4, (A = Bi, Ca, Rare Earth Elements, Th, U and B = As5+, P5+, Si4+). Among them, the phosphate monazite is the most abundant independent Rare Earth Element mineral, occurring in several geological sites. It presents a variable composition and morphology and has strong chemical and physical stability. This paper presents a review about this mineral, particularly for monazite associated with carbonatite complexes. Detailed study of the composition of monazite will improve geochemical and petrological interpretations. In the economic field, monazite is, with bastnaesite, an ore mineral present in the main deposits in the world; its morphological and composition characteristics have influence in ore quality and in efficiency of concentration processes. Since the seventhies, its importance reachs the environmental field and his highly stable structure has been investigated as model for a synthetic phase to long-term storage of radionuclides from nuclear waste. The most important monazite ore deposits in Brazil are alluvional. Monazite occurs also in other materials, mainly in carbonatites and their weathering products, which may constitute important source of REE. In these rocks, because its high stability and possibility of weathering genesis, monazite is the most important among the phosphate minerals. However, researches about monazite associated with Brazilian carbonatitic materials have shown mainly unfavourable morphological characteristics for economic profit.