Ortho-, Ring-, and Chain Silicates

Abstract

The silicate minerals summed up in this chapter only show thermal effects below 900° C, measurable in conventional DTA, if they contain water (e.g. the minerals lawsonite, hemimorphite, dioptase, chrysocolla). Generally silicate structures are very stable. So even the dehydration of OH ⁻ -containing silicates (with the exception of the sheet silicates) will take place at very high temperatures. This was demonstrated by Hunziker when he described the dehydration temperatures of epidotes lying between 960 and 1200° C, or by Wittels, Peters (1963), Van der Plas and Hügi reporting the dehydration of amphiboles lying at temperatures about 1000° C. So do the vesuvianite (Peters, 1961) and the minerals of the group of tourmalines (see Fig. 34). The DTA characteristics of all these silicates have to be studied in a high-temperature DTA apparatus running up to temperatures of 1500° C. Schwab, Schwab and Jablonski investigated the decomposition and transformation behaviour of some pyroxenes. Dietrich published the data of the mineral ilvaite, and garnets can also be investigated by means of DTA if they contain some (OH) instead of (SiO₄); e.g. see Heflik and Zabinski, who reported on the data of hydrogrossular. Thermogravimetric investigations of Freeman and of Peters (1963) lead us to suppose that DTA is a good possibility of classification for amphiboles and epidotes similar to the method described in the present study for chlorites (compare with III-4): Freeman found out increasing dehydration temperatures of amphiboles (>980° C) with increasing contents of magnesium.