In thin section (Figure 14) they are classified as vitric and vitric-crystalline flow tuffs (Figure 5) with occasional plagioclase, anorthoclase and sanidine free phenocrysts in a vitroclastic matrix.
(2000 vv e know that a caldera resurgence that began during the SEE with conduit opening and alkaline trachytic to rhyolitic magma emission near the caldera margins, initial volcanism was not explosive in nature: magma could reach the surface after complete volatile release through conduit permeable walls, promoting anorthoclase and sanidine nucleation rate increment and amphibole and mica stability reduction, which generated corrosion and opatization borders in mafics.
Under different nucleation, diffusion and growing rates, augite-aegirine, sphene, zircon, oligoclase, anorthoclase and sanidine could crystallize in the intratelluric environment.
From the separated fraction the 20 [bar.1] reflection of sanidine was measured with maximum accuracy.
The content of NaAl[Si.sub.3][O.sub.8] in mol% in K-Na sanidine was calculated according to Orville (1967), who established that the position of the 20 [bar.1] reflection almost linearly depends on the composition of sanidine solid solution.
K-Na sanidine of various composition was found, containing 20-47 mol% NaAl[Si.sub.3][O.sub.8].
For example, in the Mustjala core two beds, at 115.87 and 117.80 m, can be well characterized by their sanidine compositions, but in other cores beds with a similar composition have opposite succession.
Several volcanic beds of the Aizpute core with wide reflections are correlated only tentatively with other sections as their sanidine composition does not allow us to prove or refute correlations.
On the basis of the characteristic XRD reflections of the sanidine and graptolite zonation, three Lithuanian bentonites can be correlated with Estonian and Lithuanian ones (Figs 2, 4, Table 1).
The riccartonensis to belophorus graptolite biozones include mostly bentonites with wide and very wide sanidine reflections and correlations are therefore often provisional (Fig.
Among these the Siupyliai-69 1076.0 m bentonite can be correlated with Latvian sections according to the sharp sanidine reflection and Na + Ca component of 30.4 mol%.
A 0.1 cm thick bentonite with a very strong and sharp sanidine reflection and the Na + Ca component 23.4 mol% was found in the lower part of the lundgreni Biozone at the beginning of the Homerian Stage at a depth of 1052.0 m in the Siupyliai-69 section.