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de Campos

Amostra n. 598 – Lamina n, 864

Exame macroscopico - Contem-aeleolita, crystaes tabulares de orthoclasio com orientação sub-parallela, agulhas de eagyrita e um mineral côr de bonina allotriomorpho. Holocrystallina.

Exame microscopico - Orthoclasio, apresentando a macla de Carlsbad e inclusões aciculares verdes de eagyrita (acmita) mais raras do que inclusões aciculares incolores, talvez do mesmo pyroxenio.

Crystaes idiomorphos, tabulares, com dimensões approximadas e médias de 9mm×2mm.

Nephelita - Crystaes idiomorphos e hypidiomorphos, apresentando na placa, frequentemente, secções hexagonaes. As inclusões guttiformes são menos frequentes do que as de eagyrita e zirconita (?).

Alguns crystaes estão parcialmente alterados e poucos inteiramente alterados em sodalita. Maiores dimensões: 7mm×7mm. Eudialyta - Inteiramente allotriomorpha (côr de vinho claro, em lamina delgada) com abundantes inclusões de pyroxenio (aegyritaacmita), notavelmente pleocroica.

Raramente apresenta secções incompletas.

Aegyrita-acmita - Pyroxenio titanifero em crystaes aciculares verde carregado (verde garrafa), apresentando frequentemente a terminação da acmita. Sendo o primeiro elemento a se crystallizar no magma, todos os outros elementos da rocha apresentam inclusões deste mineral ás vezes de tamanho regular (1mm).

Os crystaes maiores formam concentrações em torno dos grandes crystacs de aeleolita e orthoclasio.

Elementos accessorios - Apatita, zirconita (?) e zeolithas em inclusões na aeleolita.

Segundo o systema americano a textura é phanerocrystallina, fabric poikilitico, domoikic com pequenos hadacrystacs. Orthoclasioe ealeolita euhedral e sub-cuhedral, aegyrita euhedral. Esta rocha é um Lujaurito.

A determinação directa das porcentagens dos elementos essenciaes bascada sobre a differença de solubilidade nos acidos chlorhydrico e fluorhydrico deu:

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Reduzindo as porcentagens a relações molleculares poderemos

calcular as porcentagens dos elementos normativos:

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(1) No teor em alumina entra uma porcentagem pequena de terras raras que não foram separadas.

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Como o exame microscopico não revelou existencia dos elementos zirconita em abundancia, magnetita, diopsidio e wollastonita, parece que os elementos chimicos relativos a esses mineraes entraram na composição do mineral muito abundante na rocha, a cudialyta. Quanto á albita, substitue como elemento normativo a nephelita.

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Q = 4,26 Z = 4,18 F = 39,14 L = 0,51 P = 43,07 M = 6,20

A = 1,81

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(1) A correcção devido ás terras raras é desprezivel.

5898

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