Fe-Li Micas: A New Approach to the Substitution Series

Sun Shihua and Yu Jie
Research Center for Mineral Resources Exploration, Chinese Academy of Sciences, P.O. Box 9701, A-11 Datun Rd, Beijing 100101, China

Abstract: Over the last 50 years many definitions of element substitution series have been proposed for describing natural Fe-Li micas with complex chemical compositions. In order to compare these definitions and ascertain their reliability, a new geometric frame composed of ideal mica points, mica joins (segments), substitution vectors and substitution planes is constructed to express the entire substitution system of ideal Fe-Al-Li micas. The frame is built in composition space with the coordinate system (0; Si, AlVI AlIV, Fe2+, Li, □VI, K), but has a 3-dimensional analogue, i.e. a visual image.

Using this frame, it has been proved that there are only five possible types of replacement for definitions of Fe-Li micas, and that all possible types have been suggested except the AlIV-constant. Furthermore, it has been proved that the main point in different definitions is that the replacement of Li by Fe2+ needs to be balanced with AlIV, AlVI, or □VI. In order to solve this problem, a set of formulae determining spatial relations between geometric elements in the frame is suggested. With these formulae, the abstract frame is suggested to be a datum system used to ascertain quantitatively the reliability of definition of natural Fe-Li micas.

Keywords: Fe-Li micas • substitution series • substitution vector • substitution plane

Mineralogical Magazine; December 1999 v. 63; no. 6; p. 933-945; DOI: 10.1180/002646199548925
© 1999, The Mineralogical Society
Mineralogical Society (www.minersoc.org)