Occurrences of Tourmalines in Quixeramobim, Ceará

Abstract

Tourmaline is a group of minerals that have the same crystalline structure, but vary in chemical composition, color and other properties. In tourmalines, the general chemical formula XY3Z6B3Si6 (O, OH) 30 (OH, F) is used. The ions in the Y position (Boron) are surrounded by oxygen ions and octahedrically arranged; the three octahedrons surround the ternary axis and each tetrahedron shares an edge with each of the other two equivalent neighboring tetrahedrons. Deposits of this mineral group are found especially in pegmatites and alluvial deposits. This work aims at the gemological characterization of specimens that are difficult to acquire tourmaline from the municipality of Quixeramobim (CE) that have the potential to be used as gems. Tourmaline in red and bicolor colors (blue and green) from Ceará are seldom used in Ceará commerce because there are few gemological specimens and little known in academia. For the characterization of Quixeramobim tourmalines, a refractometer, hydrostatic balance, dichroscope, spectroscope, ultraviolet lamp (UV) and gemological microscope were used. The red and bicolor specimens mostly have the same gemological characteristics as tourmalines from other regions, but with a variation in pleochroism and absorption spectrum of some specimens. The minerals have a good degree of color in accordance with the rules of the “Price List for Diamonds and Color Gems”, and noticeable fluid inclusions with the aid of optical equipment. Therefore, through the listed gemological instruments and accurate analyzes, it was possible to perform an excellent characterization and compilation of mineral data from this region, in addition to proving that the specimens have good to excellent characteristics to be used as gems.