Degrees of ordering and origin of K-feldspar phenocrysts in a mineralized porphyry of the Yulong porphyry copper deposit,Tibet.

X-ray powder diffraction and infrared spectrum analyses of K-feldspar phenocrysts from a mineralized porphyry at Yulong provide degrees of ordering of 0.457～0.597(X-ray diffraction)and 0.64～0.72(infrared spectrometry)and triclinities of 0.675～0.775(X-ray diffraction)and 0.11～0.44(infrared spectrometry)respectively,as well as structural parameters of 0.78～0.813. These data suggest that K-feldspar phenocrysts are andesine-microcline with structural temperatures of 370～442℃.Furthermore,the origin of the microcline is discussed from the thermodynamics and crystal structure.The authors think that the transformation between triclinic microcline and monoclinic orthoclase is most likely to be an irreversible process in the geologic history.Microcline may be transformed into orthoclase by heating,while orthoclase cannot be spontaneously transformed into microcline on cooling.Therefore the structural temperatures obtained by microcline measurements should be the highest temperatures at which k-feldspar forms.Thus it is concluded the k-feldspar phenocrysts in the Yulong mineralized porphyry is not the product of magmatic crystallization but the product of hydrothermal activity.