This fact results in progressively more sodic sanidines and anorthoclase overgrowing around corroded plagioclase.
Essential minerals are phenocrysts and glomerocrysts of anorthoclase, sanidine up to 1,5 cm and anorthoclase-mantled plagioclase; accessory minerals are red biotite, and hastingsite while trace minerals are augite, zircon, sphene and magnetite.
Hand samples are white to red dense and welded pyroclastic rocks, poorly sorted, with up to 40 % of anorthoclase (Kfs) and sanidine (Sa) phenocrystals in a vitric-clastic ash matrix.
Juvenile crystals are the same as in unit 1.1 samples, with dominating sanidine crystals, anorthoclase glomerocrysts and poikilocrysts (Figure 10).
The peak position, half-width, and intensity were calculated for four components: quartz, K-sanidine, and two K-Na sanidines. The K-Na sanidine peak position was calculated in relation to quartz or K-sanidine, depending on which of these minerals dominated in a specimen.
In the present study we used the composition of magmatic sanidine to recognize coeval volcanic beds in different sections.
Coarse fractions contain euhedral and broken magmatic quartz, biotite, and K-Na sanidine. The estimated content of magmatic phenocrysts was 1-10% of the bulk sediment.
The cleaned coarse fraction still contained, besides magmatic minerals (K-Na sanidine, quartz, biotite, zircon), also authigenic K-sanidine, pyrite or chalcopyrite.
The mentioned studies used magmatic sanidine composition of bentonites (altered volcanic ashes) analysed by X-ray diffractometry as a correlation criterion.
Magmatic sanidine phenocrysts (K,Na,Ca)Al[Si.sub.3][O.sub.8] were analysed in coarse fractions (0.04-0.1 mm) separated from 2 g of bentonite using the method described in detail in Kiipli et al.
Silurian bentonites of Lithuanian cores are mostly characterized by wide and very wide XRD 201 reflections of the main sanidine component (Table 1, Fig.
Most of these bentonites are characterized by a wide or weak sanidine reflection (Figs 2, 4, Table 1), while in some bentonites sanidine is absent at all.