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: Măgureaua 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 Măgureaua 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) Å at Oraviţa, from 7.683(4) to 7.735(1) Å at Măgureaua Vaţei, and from 7.684(3) to 7.733(1) Å at Cornet Hill, whereas c varies from 5.043(3) to 5.065(3) Å at Oraviţa, from 5.040(1) to 5.070(3) Å at Măgureaua Vaţei, and from 5.044(1) to 5.067(4) Å at Cornet Hill. It is not possible to quantify the variations in the crystallographic parameters by considering only the åkermanitic substitution because the "Na-melilite" and "Fe-åkermanite" substitutions also play an important role. The cell parameter a remains practically constant within experimental errors, in spite of the increasing åkermanite-for-gehlenite substitution, due to the opposite influence of the "Na-melilite" component. The åkermanite 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.

Mineralogy of a High-Temperature Skarn, in High CO2 Activity Conditions: The Occurrence from Măgureaua Vaţei (Metaliferi Massif, Apuseni Mountains, Romania)

“…Analyses of material from Măgureaua Vaţei were done using a combination of electron microprobe analysis (EMPA), scanning electron microscopy (SEM), X-ray powder diffraction (XRD), single crystal X-ray diffraction (SCXRD), optical microscopy and cathodoluminescence (CL). More detailed descriptions of analytical procedures used for EMPA, XRD and CL were given by [6,15,17].…”

“…EMP analyses were performed using the same devices and operating conditions as described by [6,15]. Data were reduced using the method described by [28].…”

“…Everywhere along the intrusive contact the sedimentary rocks are metamorphosed to calc-silicate hornfels and medium-to coarse-grained marble with intercalated bodies of skarn, usually garnet-, wollastonite-and vesuvianite-bearing. A small area of unusual gehlenite-bearing skarn, occurring at the southern central side of the body, on Cerboaia Valley, was studied in greater detail [4][5][6]8].…”

“…As stated before, gehlenite was one of most studied minerals in the high-temperature skarns from Măgureaua Vaţei (e.g., [4][5][6]8]). The mineral occurs in practically monomineralic blocks, in the outer endoskarn zone from Cerboaia Valley contact.…”

“…The mineral occurs in practically monomineralic blocks, in the outer endoskarn zone from Cerboaia Valley contact. In the classical outcrop described by [5,6], the crystals, with a short prismatic to tabular habit, may reach up to 8 cm across. A macroscopic image of a gehlenite aggregate from this famous outcrop is given in Figure 3, whereas a microscopic image is given in Figure 4A.…”