Formation of monazite-(Ce, La) by fluid-apatite interaction: the Floresta Azul Alkaline Complex, Bahia, Brazil

Jailson Júnior Alves Santos,Maria De Lourdes Da Silva Rosa,Herbet Conceição,Marcel Vinícius Santos Leandro

doi:10.1590/2317-4889201820180069

Jailson Júnior Alves Santos, Maria De Lourdes Da Silva Rosa + Show 2 more

Open Access

https://doi.org/10.1590/2317-4889201820180069

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Abstract

Monazite is a common accessory mineral in the Floresta Azul Alkaline Complex, occurring in three different rock types, which form this batholith: nepheline syenite, granite and fenite. Two compositional types of monazite can be found: monazite-(Ce) and monazite-(La), the latter being found only in syenites. Monazite occurs in close association with apatite as anhedral crystals ranging in size from 0.1–100 μm, and textures indicate different genetic processes. In nepheline syenite, monazite is closely related to ancylite and apatite, as a late crystallizing phase. Monazite occurs in granites as pore fillings in apatite and was formed by the remobilization of rare earth elements (REE) from apatite by late CO 2 — rich fluids. In fenite, monazite appears as acicular crystals, parallel to apatite’s crystallographic c-axis formed by exsolution from apatite by metamorphism. The data show that the monazite genesis was closely related to the activity of fluids associated either with the final stages of crystallization of nepheline syenite and granite or with fenitization of country rock.

Highlights

Monazite is a rare earth element (REE) mineral with varied composition and morphology
We described and discussed the textures, compositional variations and the genesis of monazite in the nepheline syenites, granites and fenites from the Floresta Azul Alkaline Complex (FAAC)
Monazite has been found in three samples of FAAC rocks: nepheline syenite, granite and fenite

Summary

Introduction

Monazite is a rare earth element (REE) mineral with varied composition and morphology. It usually occurs as an accessory mineral in a number of rock types (Deer et al 1992), and due to its high resistance to weathering, it can commonly be found in sedimentary debris. It was discovered in the mountains of Ilmen, in Miass (Russia), being named by Breithaupt in 1829 (Overstreet 1967), that used the Greek word μονάζειν (monazein), that means to be solitary, a clear reference to its rarity in that period. It usually forms solid solution with cheralite-[(Ca, Ce, Th) (P, Si)O4]

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