Rythmic changes in crystal chemistry of trioctahedral Cr-chlorites and Cr entrapment: a SEM, EM and Raman study

Abstract

Back-scattered scanning electron microscopy (SEM) images of Cr-chlorite crystals from Erzerum (Turkey) reveal that the crystals are chemically inhomogeneous and display complex but well defined crystal zoning characterized by growth bands with contrasting chemical features. The chemical zoning has been investigated at the micron scale using an integrated approach, combining BSEM images, in situ chemical analysis by electron microprobe and Raman spectroscopy. Enrichment in Cr, due to octahedral Al substitution, reaches up to 0.7 atoms per half formula, especially in bands where the Mg content is depleted. These substitutions are also depicted at the micron scale on Raman spectra by changes in the n(OH) band intensities and positions that correlate with the Cr content. The Cr-enrichment occurs thus during specific stages of crystal growth, probably in response to changes in the fluid chemistry controlling the relative availability of Cr, Mg and Al in solution.