Synthetic high-pressure micas compositionally intermediate between the dioctahedral and trioctahedral mica series |
| |
| Authors: | T H Green |
| |
| Institution: | (1) School of Earth Sciences, Macquarie University, 2113 North Ryde, NSW, USA |
| |
| Abstract: | Intermediate-composition micas with octahedral occupancy  2.5 have been crystallized experimentally from natural phengite, 50% phengite+50% biotite, and synthetic basalt compositions in the pressure range 20–35 kb and temperatures of 800–1,000° C. Their compositions suggest a complete range of micas with octahedral occupancy between 3.0 and 2.5, but a very restricted range between 2.0 and 2.5. These 2.5-octahedral micas lie close to the new mica series proposed by Seifert and Schreyer (1965, 1971), with one end-member composition of K Mg2.5 (Si4O10) (OH2) which is extended by the present results into alumina-bearing members of the series (e. g. K Mg1.5 Al1.0 (Si3Al1.0O10) (OH)2). However, the possibility of interlayering of dioctahedral and trioctahedral micas to give an apparently intermediate composition cannot be ruled out. X-ray powder diffraction data on the critical 060 reflection for the phengite mix suggest a transitional change from a single phengite field, through a 2-phase phengite — 2.5-octahedral mica field to a single phase 2.5-octahedral mica field.Natural micas of similar composition have not so far been identified, due probably to the unlikelihood of obtaining a mineralogical record of an appropriate composition at the restricted pressure and temperature conditions apparently needed to stabilize the 2.5-octahedral mica phase. Nevertheless, such a phase may have an important role in mineral assemblages and melting reactions in the deep continental crust, subducted oceanic crust and in the upper mantle; evidence of its existence may be removed by later, lower-pressure reactions. |
| |
| Keywords: | |
| 本文献已被 SpringerLink 等数据库收录！ |
|
