Das Hoch-Tief-Umwandlungsverhalten mikrokristalliner Quarze

The optical and x-ray investigation of 51 microcrystalline quartz samples show that they are built up of fibrous chalcedony (agates) or finest crystalline, isometric grained quartz (flints, cherts, jasper or Chrysoprase). These samples were examined by standardized d.t.a. — methods in order to get their inversion characteristics.

1. A third of the samples conteins low-temperature cristobalite in addition to quartz (in two samples even as main component); the high-low inversion of this mineral lies between 80 and 245° C according to its defect character. In five samples of jasper low- tridymite is occurring as minor component.
2. In contrary to macrocrystalline quartz crystals microcrystalline quartzes generally show no sharp inversion point. The inversion takes place over an interval of nearly 50° C, to be seen in the d.t.a. curve as a broad, only slightly endothermic effect with hardly visible peak or several small minima. In the curves of 15 samples no inversion could be detected. This broad inversion peak is caused by different defect characters of the crystals or even of parts of the crystals which invert at different temperatures (Flörke, 1955).
3. There is no dependance of t _i on the size of crystallites at least for crystals with diameter greater than 0.05 μ. The temperature of the most prominent inversion minimum allows a division of the microcrystalline quartz crystals analogous the macrocrystalline classification into samples with t _i below and above 570° C. The lowest (518±2° C) and the highest t _i (578,7+0,3° C) were measured at different parts of the same sample of a Chrysoprase.
4. The intensity of the brown and red coloured parts of agate and jasper grows with increasing Fe₂O₃-content, but there is no connection between t _i and chemical composition.
5. As in macrocrystalline quartzes of caverns and veins there are also some t _i -differences between the marginal and the central parts of agates or jaspers. The explanation lies in the mechanism of formation: higher t _i in the margin (e.g. in one globular jasper) points to a formation by silification, whereas higher t _i in the center indicates a formation by the filling up of cavities and veins.
6. The variation in t _i of microcrystalline quartzes and the comparison of these t _i ? with those of associated low-temperature cristobalite shows a connection between temperatures of formation and inversion: samples with low t _i (i.e. with a great number of defects) should be formed at low temperatures, that is by sedimentary or diagenetic processes, and samples with t _i above 570° C (i.e. nearly without defects in the structure) should have crystallized out of hydrothermal solutions. The weak inversion peaks below 570° which in d.t.a. curves often appear besides a main peak at 570° C represent the small amount of diagenetically formed microcrystalline quartz with a large degree of defect character.