Kaszab Zoltán (szerk.): A Magyar Természettudományi Múzeum évkönyve 76. (Budapest 1984)
Noske-Fazekas, G.: Universal stage measurements of plagioclase feldspars originating from gabbroic inclusions of the Bondoró-hegy alkali basalts (Hungary)
was to sec that feldspars in a direct contact or fixed by a couple of \x of anisotropic matter were for the most part rectilinear cystals meeting at an angle of 120° (Plate I: 1 and II: 1). A contact of crystal surfaces with the same angle can be observed in the case of clinopyroxenes of the gabbro as well (Plate I: 2). The extinction of the feldspars is generally uniform, a slight undulatory extinction being manifested quite seldom. Their inclusion content is extremely poor, the presence of some pyroxene, apatite and opaque is quite accidental (Plate II: 2) in them. In some of the examined samples, however, dark glass inclusions 30 to 60 [X size, usually elongated in one direction occur in a dense juxtaposition on the rims of the plagioclase grains. This garland of inclusions is in sharp contrast with the almost transparent inner parts of the feldspar grains (Plate II: 1). Most of the plagioclase feldspar grains are not affected by twinning, a total of a quarter of the feldspars having proved to be twin crystals only. Table 1. Mineral composition of the gabbros Bo-3001 Bo-3002 Bo-3004 Bo-3006 Bo-3008 Bo-3009 Plagioclase 48,7 20.2 25.1 26.5 36.3 32.0 Pyroxene 5.8 43.7 41.4 35.5 31.4 46.2 Ore 3.0 4.2 4.2 4.5 5.1 6.1 Glass 4.9 3.4 Other 37.6 31.9 25.9 33.5 27.2 15.7 Besides the so-called primary generation phases previously discussed, minerals crystallized after secondary remelting are also observable in most of the studied gabbros. Here tht size of feldspars is much smaller (30-150 \x), their shape being elongated, tabular, to lath shaped, and twin crystals are sensibly more frequent (Plate III: 1). The dissimilarities of plagioclases within the two generations will be discussed in the context of universal stage measurements. Measurement of plagioclase crystals on the universal stage A rather typical feature of plagioclases that twinning in the case of these gabbros is limited to only about a fifth of the primary feldspars (Table 2). The twin crystals are composed mainly of one pair of twins, polysynthetic lamellae being less frequently observable. At the Table 2. Frequency of different plagiochlase twin laws and the average A-contents Bo-3001 Bo-3002 Bo-3004 Bo-3006 Bo-3008 Bo-3009 Twinned crystals °/ 0 21 20 19 13 19 46 Untwinned crystals % 79 SO SI 87 SI 54 A-twins % 100 100 100 93 1C0 79 C-twins % 7 21 Average An-content °/ 0 49 49 56 50 49 71
