Structural state of K-feldspar in some Hercynian granites from Iberia; a review of data and controlling factors

Several hundred concentrates of K-feldspar from the Hercynian granites and associated aplites and pegmatites of central Portugal were studied by XRD to assess their structural state and to make inferences about their petrogenesis. Their structural state was indicated by the diffraction pattern in the 2theta region 29-31 degrees using CuKalpha radiation, the ordering index Delta (sub SM) and Al-Si distribution derived from cell parameters; a range of values were obtained, though aplites and pegmatites showed less variability. In the great majority of samples from the granites, > 80% of Al is in the T1 site of the K-feldspar. It is concluded that the cooling rate of the granite was the factor controlling the ordering process of these K-feldspars; rapid unroofing of the older granites enhanced the cooling rate, retarding ordering.