37. Overlooked experimental evidence for K-replacements of plagioclase and origin of microcline in granite plutons

Experimental studies show that K replacement of Na in triclinic plagioclase under hydrothermal conditions results directly in triclinic microcline without forming intermediate monoclinic orthoclase. These experiments bring laboratory and field evidence into agreement for the K-metasomatic origin of granites which contain exclusively microcline as the potash feldspar. Such replacements of a more mafic rock can be on a plutonic scale and range from barely perceptible to being so extensive that almost no trace of an original relatively-mafic rock remains. Microcline, as the exclusive K-feldspar, and myrmekite may be the only clues to the metasomatism.