Estudio comparativo de las morfologías de crecimiento teórica y experimental de cristales de cotunita

Resumen

Cotunnite crystals have been grown in a silica gel by diffusion of Pb(N03)2 (0.125, 0.25, 0.5 and 1M). Silica gel was prepared by acidification of a Na2S i0 3 solution with IN HC1. During the preparation of the described silica gel, NaCl formed by the polymerisation reaction of the silicates as a soluble byproduct gives the so formed gel an “active” character (Henisch, 1970). Cotunnite cristalizes by reaction between Pb2+ ions from the reactive and Cl' ions from the interstitial solution. Cotunnite shows a moderate solubility (0.99 g. per 100 cc. of water at 20 C) that explains the small range of metastability of its solutions which makes it easier to obtain idiomorphic crystals. On the other hand, the employed technique can be described like an “open system”. As a result, crystal show a morphological evolution (space and time) that reflects growth, dissolution, and dissolution recrystallization processes. The interaction between gel and reactive also leads to formation of other solids phases like phosgenite (Pb,CO,Cl,) due to the presence of the presence of atmospheric C 02 that will not be studied in this paper. The growth morphologies obtained are compared with the theoretical growth morphology of cotunnite, defined by Woensdregt and Hartman (1988).