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.