Abstract
This paper reviews on crystal chemistry of plagioclases and geochemical roles of their megacrysts including native coppers and hydrocarbons, discussed in two parts.Part I. Structural classification of feldspar compositions can be allocated into four types; feldspar, paracelsian, hexacelsian and hollandite structures. Though the phase relation between the first and the second types has been open to question, recent probing of their isotropic temperature factors and bond-valence theory to the M-sites intensifies that feldspars are in the wide stability field from low to high temperatures under low pressures, and that high pressure and low pressure are the preference field for paracelsians. Though very common in north-east part of Japanese island and Izu-Bonin arcs, anorthite megacrysts have never been found in other districts in Japan and have been rare in world-wide occurrences. Native coppers, verified by chemical shift method of EPMA and X-ray microdiffraction, are found to be included by anorthite megacrysts from Japanese island arc and by labradorite ones from the continental margin of North America. These anorthites in the basaltic lavas are of approximately 1 to 3 cm size and often contain several corroded Mg-olivines scattering as single crytals less than a few mm long. The plagioclase megacrysts show no chemical zoning, but the anorthites with the Al/Si ordered state have partings developed in places whereas the labradorites are of the AUSi disordered structure indicative of high temperature type. Each role in their plagioclase structure establishes Mg, Fe, Al and Si cations as a few minor endmembers of CaFeSi3O8, CaMgSi3O8, AlAl3SiO8 and_??_Si4O8 for the solid solutions. Furthermore micro-inclusions of native zinc, copper and brass scattering in the red-clouded anorthite megacryst frozen by the supercooling environment could have an implication for a precusor to Kuroko deposits. Review (1) discusses both structural tolerance of plagioclases and their megacrysts including native coppers.
