Crystal Chemistry and Significance of Cation Ordering in Mg-Al Rich Spinels from High-Grade Hornfels (Predazzo-Monzoni, NE Italy)

F. Princivalle*, A. Della Giusta**, A. De Min* and E. M. Piccirillo*
* Dipartimento di Scienze della Terra, Università di Trieste, Via E. Weiss 8, I-34127 Trieste, Italy
** Dipartimento di Mineralogia e Petrologia, Università di Padova, Garibaldi 37, I-35100 Padova, Italy

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

Mineralogical Magazine; April 1999 v. 63; no. 2; p. 257-262; DOI: 10.1180/002646199548367
© 1999, The Mineralogical Society
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