Skały i minerały krystalicznego podłoża Niżu Polskiego

Abstract

ROCKS AND MINERALS FROM THE CRYSTALLINE SUBSTRATUM OF THE POLISH LOWLANDThe author presents a survey of the hitherto undertaken geological and petrographical investigations of the crystaline substratum of Northeastern Poland. He stresses the importance of detailed mineralogical studies of some accessory minerals.As an example he points to titanite from alkali-syenite, derived from bore-hole Ełk. At the depth of 62·7m. there occur, in the syenite, titanite crystals of up to 1 mm. diameter. The author has observed the following forms: n (111), r (110), P (100), t (111), y (001), q (010), μ (14B). The results of goniometric measurements are listed in Table 2. The angle of the optical axes, measured in sodium light, is 2Vγ = 25°. Chemically investigated, titanite discloses the presence of iron in the amount of 2.75% Fe2O3, and slight amounts of trace elements, as listed on page 823.The increased content of iron may partly be caused by the presence of biotite and magnetite inclusions in the titanite.As a rule, titanite is older than biotite in igneous rocks. However, biotite or, at least, its older generation, has crystallized out earlier than has titanite in the investigated syenite; biotite is present there, among other components, in the shape of inclusion in the titanite crystals.The properties of titanite prove that it is not a product of crystalization from magma, but has been produced after consolidation of the syenite. Well-formed titanite metacrysts are of metasomatic origin, formed owing to migrations of dissolved chemical constituents.It may be concluded that processes of metasomatism are widely spread in the crystalline substratum.