GEHLENITE FROM THREE OCCURRENCES OF HIGH-TEMPERATURE SKARNS, ROMANIA: NEW MINERALOGICAL DATA

Abstract

Gehlenite is a common mineral in three high-temperature calcic skarns in Romania: Magureaua Vaţei and Cornet Hill in Apuseni Mountains and Oraviţa in Banat. In all three occurrences, a gehlenite zone occurs within the contact between dioritic or monzodioritic bodies of Upper Cretaceous age and carbonaceous sequences of Mesozoic age. The melilite solid-solutions vary from Ak 34.1 to Ak 51.2 (mean Ak 41.2 ) at Oraviţa, from Ak 30.4 to Ak 42.9 (mean Ak 38.3 ) at Magureaua Vaţei, and from Ak 24.3 to Ak 41.7 (mean Ak 32.9 ) at Cornet Hill, respectively. The content of “Na-melilite” is low, from up to 1.96 mol.% at Cornet Hill to up to 3.60 mol.% at Oraviţa. The cell parameter a ranges from 7.679(3) to 7.734(3) A at Oraviţa, from 7.683(4) to 7.735(1) A at Magureaua Vaţei, and from 7.684(3) to 7.733(1) A at Cornet Hill, whereas c varies from 5.043(3) to 5.065(3) A at Oraviţa, from 5.040(1) to 5.070(3) A at Magureaua Vaţei, and from 5.044(1) to 5.067(4) A at Cornet Hill. It is not possible to quantify the variations in the crystallographic parameters by considering only the akermanitic substitution because the “Na-melilite” and “Fe-akermanite” substitutions also play an important role. The cell parameter a remains practically constant within experimental errors, in spite of the increasing akermanite-for-gehlenite substitution, due to the opposite influence of the “Na-melilite” component. The akermanite substitution prevails over the opposite influence of “Na-melilite” in increasing the cell volume, and is additive to the influence of “Na-melilite” in decreasing the cell parameter c . The record in the infrared-absorption spectra of bands located at ~855 cm −1 and ~670 cm −1 , respectively, which may be tentatively assigned to Al–O–Al stretching, is indicative of the presence of gehlenite-rich members of the melilite group in all the three occurrences.