Dwight R. Soares

Chemical composition of gahnite and degree of pegmatitic fractionation in the Borborema Pegmatitic Province, northeastern Brazil

Gahnite electron probe microanalyses from three pegmatites (Mirador, Capoeira and Quintos) of the Borborema Pegmatitic Province in northeastern Brazil allowed to determine molar compositions ranging from 86.22 to 95.41 mol% gahnite (ZnAl2O4), 1.51 to 6.62 mol% hercynite (Fe 2+ Al2O4), 0.11 to 5.89 mol% spinel (MgAl2O4), 1.48 to 2.74 mol% galaxite (MnAl2O4), and Zn/FeTotal atomic ratios ranging from 11.53 to 60.87. These data compared with gahnite compositions from albite subtype pegmatites from Cap de Creus, Spain (Zn/FeTotal between 10 and 60) and from the source granite of the Separation Rapids pegmatite field, Ontario, Canada (Zn/FeTotal =3.5) indicate a high degree of fractionation of the studied pegmatites. This interpretation agrees with conclusions based on compositional data of tourmaline (vacancy in X-site 0.42 to 0.49), white mica (K/Rb between 8 and 33), feldspar (K/Rb between 8 and 109), garnet (Mn/(Mn+Fe 2+ ) between 0.93 and 0.97) and in Nb-Ta oxides (Mn/(Mn+Fe 2+ ) > 0.5) in the same pegmatites.

Mineral chemistry of tantalate species new in the Borborema Pegmatitic Province, Northeast Brazil

Tantalate samples, supposedly of the columbite group, were collected in the Borborema Pegmatitic Province, aiming to test the Mn/(Mn+Fe) and Ta/(Ta+Nb) ratios as geochemical indicators of pegmatite fractionation. Surprisingly, preliminary microprobe data allowed recognizing some species, so far unknown in the Province, namely titanian ixiolite, fersmite, brannerite, struverite, natrobistantite, plumbo- and stibiomicrolite, plumboand uranpyrochlore. The identification of these exotic tantalates with unusual composition, in addition to its distribution in several pegmatites, far from the classical Alto do Giz and Seridozinho pegmatites, indicate that the elevated degree of fractionation is not restricted to these two occurrences but may be reached in other pegmatite areas of the Province. It indicates also that this degree of fractionation may be very variable between pegmatites in small areas. The zoning patterns observed in the titanian ixiolite, with Ti and Nb enrichment at the borders at expense of Ta enriched in the core, are also quite unusual and reverse in comparison with the normal trend of progressive Ta and Mn enrichment in tantalates with the degree of fractionation. A similar ‘‘reverse’’ trend was observed in titanian wodginite of petalite/pollucite bearing pegmatites of the Separation Rapids Province in Ontario, Canada.

Chemical and mineralogical characterization of elbaites from the Alto Quixaba pegmatite, Seridó province, NE Brazil

The Alto Quixaba pegmatite, Seridó region, northeastern Brazil, is a 60 degrees/80 degrees SW-trending subvertical dike discordantly intruded into biotite schists of the Upper Neoproterozoic Seridó Formation. It has three distinct mineralogical and textural zones, besides a replacement body that cuts the pegmatite at its central portion and in which occur, among other gem minerals, colored elbaites. Elbaites usually occur as prismatic crystals, elongate according to the c-axis, with rounded faces and striations parallel to this axis. Optically, crystals are uniaxial negative with strong pleochroism; refractive index extraordinary axis = 1.619-1.622 and ordinary axis = 1.639-1.643, birefringence between 0.019 and 0.021, average relative density of 3.07, and the following unit cell parameters: ao = 15.845 A, co = 7.085 A and V = 1540.476 A. There is alkali deficiency in the X site of 12-17%. The elbaites are relatively enriched in MnO (1.69 to 2.87%) and ZnO (up to 2.98%).

VARIEDADES GEMOLÓGICAS DE MINERAIS DA PROVÍNCIA PEGMATÍTICA DA BORBOREMA, NE DO BRASIL: UMA SÍNTESE

Pegmatitos graniticos sao fonte de numerosos minerais com elevado potencial gerador de variedades de boa qualidade gemologica, com destaque para berilo, varias especies do super-grupo da turmalina, topazio, espodumenio, quartzo, feldspatos, alem de outros minerais menos comuns {p. ex. granadas, euclasio, gahnita, tantalita-(Mn), microlitas}. Na Provincia Pegmatitica da Borborema (PPB), alvo deste estudo, destacam-se as turmalinas, incluindo a famosa “turmalina Paraiba, berilo, quartzo, espessartita, gahnita, euclasio, como minerais de bom potencial gemologico. A formacao destas gemas na PPB, assim como na maioria das Provincias Pegmatiticas mundiais, e localizada em cavidades situadas preferencialmente na transicao entre a zona intermediaria interna e o nucleo de quartzo de pegmatitos heterogeneos mais diferenciados. A descricao das principais ocorrencias de gemas da PPB sera objeto deste trabalho.

REPLICAÇÃO DE FÓSSEIS DE FORAMINIFERA UTILIZANDO REAPROVEITAMENTO DE “ISOPOR” COM ALUNOS DO IFPB-CG

Os fosseis sao unidades paleontologicas preciosas e dificeis de serem coletadas para pesquisas ou principalmente para aulas praticas sobre o passado geologica da Terra. Desta forma, lancam-se mao de tecnicas alternativas para se fazer replicas dessas unidades. O presente artigo vem apresentar atividades de sistematica paleozoologica, utilizando o “isopor” como materia prima para confeccao de modelos de fosseis em aulas praticas para a comunidade escolar do Instituto Federal de Educacao, Ciencia e Tecnologia da Paraiba, campus de Campina Grande. Dessa forma, o instituto esta iniciando a construcao de um acervo visual 3D alternativo de fosseis guia do grupo dos Foraminifera, em especifico, os bioindicadores de petroleo, alem de contribuir para reutilizacao de residuos solidos poluidores ao meio ambiente.