Color and genesis of californite from Pakistan: insights from μ-XRF mapping, optical spectra and X-ray photoelectron spectroscopy

Abstract

Four californite samples from Pakistan with yellowish-green, green and reddish-brown colors were investigated by combining the methods of μ-XRF mapping, XRD, Raman spectra, optical spectra, EPMA and XPS. The results show that the californite is composed mainly of microcrystalline vesuvianite and smaller amounts of clinochlore. Based on the distribution of the clinochlore, the californite can be divided into three types. The gem-quality californite is composed of microcrystalline vesuvianite and has a translucent appearance. The ordinary-quality californite contains microcrystalline vesuvianite as well as clinochlore, and it has an opaque appearance. The transitional-type has properties that are intermediate between those of gem- and ordinary-quality californite. Octahedrally coordinated iron and chromium in the clinochlore reduce the transparency and contribute to the opaque green and yellowish-green colors of the californite. At sites where there is no clinochlore, Cr3+ in the octahedrally coordinated site Y3 of the vesuvianite is mainly responsible for the green tone of the californite, Fe3+ and Mn3+ at the Y3 site contribute mainly to the yellowish-green and reddish-brown colors, respectively. The Fe2+ → Fe3+ charge transfer also occurs in vesuvianite and partly influences the appearance of the californite. The actual color of californite that lacks clinochlore is due to the synergy of Cr3+, Fe3+ and Mn3+ crystal field transfers at the octahedral site Y3 as well as the Fe2+ → Fe3+ charge transfer in the vesuvianite. Vesuvianite in the californite can be assigned to the P4/n space group, and the occurrence of clinochlore reflects the fact that the californite from Pakistan formed under medium-grade metamorphic conditions at temperatures of ~300–500 °C. The content of clinochlore provides a basis for grading the quality of the californite.