NATURAL STRONTIUM-BARIUM FELDSPARS

Abstract

There are reports of a potassic igneous rock complex that is new for the region west of Lake Ladoga and includes two facies - intrusive (hypabyssal) and diatreme-forming, with similar chemical and mineralogical composition. The alkalic rocks constitute a natural series comprising potassic pyroxenite, potassic gabbro and alkali-feldspar syenite (tonsbergite). Feldspars are the most noteworthy feature of these rocks. Their concentration ranges from 1 to 10 precent in nevoite but reaches 80 percent in tonsbergite. They have specific chemical compositions, which are unusual for natural feldspars and, therefore, have apparently not yet been found in other rocks. Our 35 analyses showed that, besides K2O (2.87 to 10.21 percent), Na2O (2.25 to 8.82) and CaO (0.2 to 1.72), they also contain 0.37 to 5.8 percent BaO and 0.73 to 5 percent SrO.