gsminmag Mineralogical Magazine Mineralogical Magazine 0026-461X Mineralogical Society 10.1180/002646199548367 Crystal Chemistry and Significance of Cation Ordering in Mg-Al Rich Spinels from High-Grade Hornfels (Predazzo-Monzoni, NE Italy) Princivalle F. * Della Giusta A. ** De Min A. * Piccirillo E. M. * Dipartimento di Scienze della Terra, Università di Trieste, Via E. Weiss 8, I-34127 Trieste, Italy Dipartimento di Mineralogia e Petrologia, Università di Padova, C.so Garibaldi 37, I-35100 Padova, Italy 4 1999 63 2 257 262 6 8 1997 27 7 1998 Copyright © 1999, The Mineralogical Society 1999 Abstract

Two Mg-Al spinels (FAS1 and FAS2) in high-grade hornfels from the Toal de Mason skarn (Predazzo Monzoni, NE Italy) were investigated by means of single crystal X-ray diffraction and electron microprobe analysis in order to evaluate the last intra-crystalline closure temperature.

The evidence provided by crystal chemical data from the investigated spinels, compared with that for two other spinels from a websterite dyke and a chlorite schist (TS2 and SP78a, respectively) with similar chemical characteristics, yielded a new empirical geothermometer. This allowed estimation of intra-crystalline temperatures as low as c. 400°C and indicates that the closure temperature of FAS1 and FAS2 is c. 500°C i.e. 150–200°C lower than the spinel crystallisation temperature.

 Keywords Mg-Al spinel crystal-chemistry intra-crystalline temperature