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O objetivo desse trabalho foi caracterizar os defeitos pontuais relacionados aos grupos OH e H2O em quartzos e ametistas, crescidos em cavidades do tipo geodo em basalto da Formação Serra Geral (RS), usando a espectroscopia infravermelha (IV) a -175°C. Em um dos cristais, os espectros foram realizados separadamente nos setores de crescimento r{10<IMG SRC="/img/revistas/rem/v62n3/a050i01.gif" ALIGN="absmiddle">1} e z{01<IMG SRC="/img/revistas/rem/v62n3/a050i01.gif" ALIGN="absmiddle">1}. Na faixa entre 3800 e 3000 cm-1, os espectros apresentaram várias bandas de absorção superpostas a uma intensa banda larga. As bandas atribuídas ao defeito &#91;H4O4&#93;0 foram observadas independentemente da coloração dos cristais, enquanto que as bandas atribuídas ao centro &#91;AlO4/H&#93;0 ocorreram nos quartzos translúcido e levemente amarelado (citrino). Além da intensidade da banda larga, os espectros desses cristais se caracterizaram pela existência da banda a 3595 cm-1. Ao contrário do reportado por outros autores, verificou-se que a concentração de OH e H2O e a coloração violeta da ametista são maiores nos setores z do que nos setores r. Essa inversão foi discutida em função da não uniformidade das condições de crescimento, confirmada pelo tamanho das faces romboédricas do cristal. Concluiu-se que a intensidade da coloração da ametista não é afetada pelas altas concentrações de defeitos OH e H2O. The aim of this study is to characterize OH-related defects in natural quartz and amethyst grown in geodic cavities formed during basaltic flow. For this, infrared (IR) spectroscopy at low temperature (-175°C) was carried out in amethyst and quartz crystals taken from two deposits located in the State of Rio Grande do Sul in Brazil. For one specimen, IR spectra were recorded directly upon the growth sectors r{10<IMG SRC="/img/revistas/rem/v62n3/a050i01.gif" ALIGN="absmiddle">1} and z{01<IMG SRC="/img/revistas/rem/v62n3/a050i01.gif" ALIGN="absmiddle">1} appearing in (0001) plates. IR spectra recorded from 3800 to 3000 cm-1 showed an intense broad band superposed upon sharp bands assigned to &#91;H4O4&#93;0 point defects. These bands were observed in all samples and growth sectors independently from the intensity of its violet color. On the other hand, sharp bands assigned to &#91;AlO4/H&#93;0 defects were observed only in translucent and pale citrine quartz and also in the colorless regions of r and z growth sectors in amethyst. Contrarily to previously reported data, it was observed that z growth sectors have the highest content of OH-related defects and the darkest violet color when compared to r sectors. This reversal relation was discussed taking into account the malformed habitus of the amethyst specimen probably caused by the anisotropic solution flow inside the geode. It was concluded that the amethyst color is independent of the concentration of OH-related defects. Besides the intense broad OH-band, the IR spectra of such crystals were characterized by the presence of a sharp band near 3595 cm-1

Espectroscopia infravermelha à baixa temperatura em quartzos e ametistas com altas concentrações de OH e H2O