Exsoluções Em Cromo-Espinélios

Abstract

EXSOLUTION IN CHROME-SPINEL Chrome-spinel composition is largely used for petrogenetic interpretations. Most mafic/ultramafic rock occurrences underwent metamorphism which may impose modification to the original spinel chemical composition. Internally consistent thermodynamic calculations show large miscibility gaps for (Fe,Mg)(Al,Fe +3 , Cr) 2 O 4 spinels in between 500°C and 600°C. Exsolution in natural chromian spinels agrees well with the calculated model. The two most common exsolved phases are Mg(Al,Cr> 2 O 4 and Fe(Fe,Cr> 2 O 4 which occur as intergrown lamellae, blebs, rods or composite grains. In more evolved systems both phases may form discrete crystals. Ilmenite may be produced by oxidation exsolution in ferrian chrome-spinel crystals or in the Fe(Fe,Cr) 2 O 4 exsolved phases. Under metamorphic conditions of clinochlore stability field, the Mg(Al,Cr) 2 O 4 component may enter silicate reactions and be entirely consumed. Above the clinochlore stability field MgAl 3 O 4 may be produced. Whenever petrogenetic interpretations are based on chromian spinel composition from metamorphosed rocks it is necessary to investigate the assemblage evolution. Magmatic interpretations may not be possible.

Ferricromita : Revisão E Implicações Petrogenéticas

“…É feita aqui uma revisão das ocorrências descritas bem como uma avaliação crítica das hipóteses genéticas e suas implicações petrogenéticas. O presente trabalho é parte de um projeto mais amplo que vem sendo desenvolvido pelos autores (Candia 1983, Candia & Gaspar 1996a, 1996b, 1997, Szabó 1996, visando uma compreensão mais aprofundada dos processos subsolidus que envolvem espinélios cromíferos.…”