DISSOLUTION OF K-FELDSPAR INTO SI-AL GEL AND CRYSTALLIZATION OF HALLOYSITE IDENTIFIED IN THE KAOLIN DEPOSIT OF SÃO VICENTE DE PEREIRA (PORTUGAL)
Published: Feb 2000
Pages: 49 - 57
Authors: IULIU BOBOS, CELSO GOMES
Abstract: Dissolution of K-feldspar into Si-Al non-crystalline phase was identified in the kaolin deposit of São Vicente de Pereira (Portugal). The gel having a SiO2/Al2O3 ratio approximately equal to the unity, crystallized into halloysite-0.7 nm. SEM investigations carried out on K-feldspar grains show a vacuolar texture characterized by etchpits of various dimensions. The pits exhibit various shapes, either polygonal or circular. The gel was identified by SEM on the K-feldspar surfaces and by TEM in the < 2 ωm clay fractions. TEM images display tubes of halloysite-0.7 nm surrounding flakes of Si-Al gel. Relicts of K-feldspar pseudomorphs submitted to amorphization are identified by TEM. SAED analyses confirmed the non-crystalline state of the gel. Electron microprobe analyses attested a very fast destabilization of the chemical structure of K-feldspar grains.
Keywords: Portugal, kaolin deposit, scanning and transmission electron microscopy, X-ray diffraction, K-feldspar dissolution, Si-Al gel, halloysite-0.7 nm
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Volume 51 no. 1 / February 2000
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