Nordmarkite

Photomicrographs

Potassium feldspar – Kfs intergrown with acid plagioclase, most frequently albite, and quartz – Qz. Exsolutions of acid plagioclases in form of tiny lenses, coarser lamellae or structures resembling tiny veinlets originated during slow cooling from initially homogeneous Na-K feldspars. These structures discernible in nordmarkites only under microscope are designated as microperthite. Potassium feldspar is in places replaced by a mixture of clay minerals originating at a low temperature (kaolinization). The alteration is discernible in plan-parallel polarized light as grey-coloured parts in the potassium feldspar. Biotite phenocrysts – Bt and alkalic amphibole – ferro-eckermannite – Fec – occur in intergranular spaces between alkali feldspars and quartz. Ferro-eckermannite displays faint blue-green to green-yellow pleochroism in plan-parallel polarized light. In contrast, another alkalic amphibole – arfvedsonite – has more intense blue-green, blue, violet to brown and brown-orange pleochroism. Fe-Ti oxides – Ti-magnetite or ilmenite – Ilm are opaque accessory phases. Bottom images also display accessoric apatite – Ap, in sections perpendicular to optical axis, that is in an extict position in crossed polars. Widths of all photomicrographs correspond to 2.2 mm.

Normative composition

Nordmarkite is quartz-normative rock with relatively high content of normative albite – ab and orthoclase - or. Depletion in CaO is reflected in a low content of normative anorthite – an. Some nordmarkites do not contain normative anorthite, but normative acmite – ac instead, and also hematite in the case of a Fe³⁺ excess. Nordmarkite, as a variety of alkali feldspar syenite with quartz, exhibits significantly increased content of normative quartz – q compared to that in common syenite.

Comment Normative anorthite – an was created from excess molar proportion of aluminium after creation of normative albite – ab and orthoclase – or.

Chemical composition

Normarkite is characteristic of increased SiO₂ contents compared to those in syenite, ranging around 60 wt. %. It also has lower CaO and MgO contents. Contents of alkalis in syenite and nordmarkite are essentially similar, but that of K₂O in nordmarkite is somewhat higher. F and Cl contents are negligible. The CO₂ content is also low to negligible. The nordmarkite from the Lake Mykle is typical of increased Zr, Th, La and Ce contents compared to syenite.