Light lithophile elements in metamorphic rock-forming minerals
Hawthorne, Frank C.
European Journal of Mineralogy Volume 7 Number 3 (1995), p. 607 - 622
published: May 19, 1995
manuscript accepted: Jan 24, 1995
manuscript received: Aug 11, 1994
ArtNo. ESP147050703014, Price: 29.00 €
Abstract The light lithophile elements (H,Li,Be) are not detectable by electron-microprobe analysis, and it is only recently that B can be determined by this method. This situation has led to the neglect of these constituents in most rock-forming minerals. However, crystal-structure refinement, ion-microprobe analysis, and bulk chemical techniques have shown that the light lithophile elements play hitherto unrecognized but important roles in several rock-forming metamorphic minerals. Staurolite may be written as A4B4C18D4T8O40X8 where A = Fe2+,Mg,□(vacancy); B = Fe2+,Zn,Co,Mn, Al,Mg,Li,□; C = Al,Fe3+,Mg,Ti; D = Al,Mg,□; T = Si,Al; X = OH,F,O2. Until recently, the two principal chemical variables, H and Li, were assumed to be constant and absent, respectively. As a consequence, the chemical formula was wrong, activity models were inappropriate, and conflicting results from experimental and field studies were common. This unsatisfactory situation has now been resolved with our understanding of the role of H and Li in the staurolite structure. Vesuvianite may be written as X19Y13Z18T5O68W10 where X ~ Ca; Y ~ Al,Mg,Fe2+, Fe3+; Z = Si; T = B,Al,□; W = OH,F,O2-. Crystal-structure refinement, electron-microprobe analysis, infrared spectroscopy and bulk analysis for H2O indicate that B must be replacing H in the structure. Vesuvianite can contain up to ~ 4 B apfu (~ 4 wt% B2O3) and is a potentially significant sink for B in metamorphic rocks. The general formula for alkali amphiboles may be written as AB2C5T8O22X2 where A = Na,KP; B = Na,Ca,Mn2+,Fe2+; C = Mg,Fe2+,Al,Fe3+,Li; T = Si,Al; X = OH,F,O2-. Crystal-structure refinement, bulk AA and ion-microprobe analysis showed high Li values (Li2O ~ 1 wt%) from alkali amphiboles from several localities, and several new amphibole species have been described recently. In geochemically suitable environments, many common rock-forming minerals can incorporate major amounts of light lithophile elements into their structures. Many of these substitutions are invisible to the electron microprobe, and emphasize the importance of using crystal-structure refinement and new microprobe methods (e.g., SIMS, PIGE) for adequate characterization of metamorphic minerals.