International Geological Journal - Official Journal of the Carpathian-Balkan Geological Association

DISSOLUTION OF K-FELDSPAR INTO SI-AL GEL AND CRYSTALLIZATION OF HALLOYSITE IDENTIFIED IN THE KAOLIN DEPOSIT OF SÃO VICENTE DE PEREIRA (PORTUGAL)

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

Download PDF document Show Text Version