Abstract
Potassium feldspar (Kfeldspar), along with albite, is one of the leading minerals of alkaline ore (rare-metal-rare-earth) fenites of the Kosyu ore cluster of the Middle Timan. In the studied potassium feldspars, the data on an insignificant admixture of sodium oxide (0.19-0.38 wt.%) according to the results of microprobe analysis are confirmed by X-ray diffraction studies, which also indicate a minimal structural admixture of the albite component in KF. As determined by X-ray diffraction, potassium feldspar of ore fenites is a microcline. Several of its varieties have been established: the maximum microcline (Oktyabrskoe ore field), the low microcline (Novobobrovskoe and Verkhne-Shchugorskoe ore fields), and the intermediate microcline (Nizhne-Mezenskoe ore field). Kfeldspars of fenites were formed under the action of alkaline solutions on quartzite sandstones in a fairly wide temperature range from 375° to 510°C. The lowest temperature (375°C) is the most ordered maximum microclines of the Oktyabrskoe ore field. For low microclines, temperatures of 420°C and 455°C have been determined for the Novobo¬brovskoe and Verkhne-Shchugorskoe fields, respectively. The least ordered intermediate microcline of the Nizhne-Mezenskoe ore field was formed in the range of 485-510°C.
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