Chemical, structural and optical characterisation of gemological tourmalines from pegmatites of the pegmatitic province of Borborema - batalha, capoeiras, quintos and bulandeira - in the districts of Rio Grande do Norte and Paraíba, NE of Brazil

CHEMICAL, STRUCTURAL AND OPTICAL CHARACTERISATION OF GEMOLOGICAL TOURMALINES FROM PEGMATITES OF THE PEGMATITIC PROVINCE OF BORBOREMA - BATALHA, CAPOEIRAS, QUINTOS AND BULANDEIRA - IN THE DISTRICTS OF RIO GRANDE DO NORTE AND PARAÍBA, NE OF BRAZIL

SANDRA B. BARRETO

Dept. de Engenharia de Minas, UFPE, 50740 Recife, PE.

Presented by ALCIDES N. SIAL

The aims of this work are to characterize the chemical, structural and optical features of gemological tourmalines from four pegmatites in the Pegmatitic Province of Borborema, districts of Rio Grande do Norte and Paraíba, Northeast Brazil. Special emphasis has been devoted to colour and its generation.

The studies were based on several techniques, namely EPMA, ICP-MS, ICP-AES, X-Ray diffraction of single crystal, Raman Spectroscopy, DTA-TG, Visible and IR absortion Spectroscopy, ESCA and refractive indexes determination through an Able refractometer.

The studied elbaites showed different colours which were classified as green-blue, red and colourless. A close relationship between minor and trace chemical composition - Fe, Cu, Zn, Mn, Ca, Ti and Mg - and colour has been determined. Red colour has been shown to be related to Mn^{3 +} and possibly to electron trap colour centres. The colour of green and blue elbaites is related to Fe^{2 +} d-d transitions and/or charge transfer Fe^{2 +} - Fe^{3 +} and Zn^{2 +} - Fe^{3 +} interactions. The purple colour has been related to Cu^{2 +} d-d transitions.

The thermal treatment revealed that the colour of elbaites changes at 600^oC. The red ones become colourless due to the loss of the Mn^{3 +} absorption. The purple ones become turquoise-green due to the loss of Mn^{3 +} absorption while those of Cu^{2 +} remain. Those green and blue elbaites become yellowish by the loss of Mn^{3 +} absorption and the generation of Fe^{3 +} absorption and centres Fe^{3 +} - Fe^{3 +}. A common feature for all the samples is an endothermic transformation which corresponds to the loss of weight (3.6%) when the dehydroxilation occurs.

All the samples analyzed do not present important differences in the XRD patterns and Raman spectra. Regarding the Raman spectra, the most pronounced differences were found in the low-frequency region where the greatest influence of the minor/trace elements occur. (May 19, 2000)

Publication Dates

• Publication in this collection
  05 Jan 2001
• Date of issue
  Dec 2000