Dissolution of K-feldspar into Si-Al gel and crystallization of halloysiteidentified in the kaolin deposit of Sao Vicente de Pereira (Portugal)

Dissolution of K-feldspar into Si-Al non-crystalline phase was identified i n the kaolin deposit of Sao Vicente de Pereira (Portugal). The gel having a SiO2/Al2O3 ratio approximately equal to the unity, crystallized into hallo ysite-0.7 nm. SEM investigations carried out on K-feldspar grains show a va cuolar texture characterized by etch-pits of various dimensions. The pits e xhibit various shapes, either polygonal or circular. The gel was identified by SEM on the K-feldspar surfaces and by TEM in the < 2 mu m clay fraction s. TEM images display tubes of halloysite-0.7 nm surrounding flakes of SI-A I gel. Relicts of K-feldspar pseudomorphs submitted to amorphization are id entified by TEM. SAED analyses confirmed the non-crystalline state of the g el. Electron microprobe analyses attested a very fast destabilization of th e chemical structure of K-feldspar grains.