Zum Temperverhalten von Sanidinen: Die Ausnahmenrolle der Eifelsanidin-Megakristalle
[On the annealing behaviour of sanidines: the exceptional case of Eifel sanidine megacrystals]
Bertelmann, Dieter; Förtssch, Erich; Wondratschek, Hans
Neues Jahrbuch für Mineralogie - Abhandlungen Band 152 Heft 2 (1985), p. 123 - 141
15 references
published: Aug 5, 1985
ArtNo. ESP154015202002, Price: 29.00 €
Abstract
Slabs of sanidine crystals from different localities, but mainly from the Eifel (West-Germany), cut perpendicular to the a-axis were annealed at 750, 850, 950, and 1050 °C for between 1/4 to 2800 hours. The time rate of change of the optic angle 2Vx during annealing depends on the temperature as well as on the thermal history of the specimen. For example, prolonged heating at 650 °C, which in itself produces no visible change in 2Vx, has the effect of reducing the time rate of change in 2Vx when the same sample is subsequently annealed at higher temperatures. A short pre-heating (1/4 hour) at higher temperature (1050 °C) results in a rapid change in 2Vx. Subsequent annealing at lower temperatures reduces the optic angle if the pre-heating was not too long (less than 2 hours at 1050 °C or 8 hours at 950 °C). The changes of the optic angle can be explained by Al/Si order-disorder transformations in the feldspar structure. The diffusion of Al/Si in sanidines of the Eifel is extraordinarily fast. The “equilibrium optic angle” of different Eifel sanidines when plotted as tan2Vx against temperature shows a linear relationship. It is possible to estimate the freezing temperature of the original Al/Si-distribution. Values between 630 °C and 1030 °C are found for different Eifel sanidines. Assuming a linear relationship between tan2Vx and the Or content too, one of the three data 2Vx, Or content, and annealing temperature may be estimated if the other two are known.
Keywords
Sanidine • Al-Si-distribution • order-disorder transformation • optic axial angle • Al/Si-exchange rate • Eifel • Germany