Naslov
Moissanite (SiC) in Sediments of Pazin Cave (Istrian Peninsula, Croatia)

Autori
Wacha, Lara ; Horvat, Marija ; Brčić, Vlatko

Vrsta, podvrsta i kategorija rada
Sažeci sa skupova, sažetak, znanstveni

Izvornik
6. hrvatski geološki kongres s međunarodnim sudjelovanjem: Knjiga sažetaka - 6th Croatian Geological Congress with international participation: Abstracts Book / Horvat, Marija ; Matoš, Bojan ; Wacha, Lara - Zagreb : Hrvatski geološki institut, 2019, 211-211

Skup
6. hrvatski geološki kongres s međunarodnim sudjelovanjem

Mjesto i datum
Zagreb, Hrvatska, 09.10.2019. - 12.10.2019

Vrsta sudjelovanja
Poster

Vrsta recenzije
Recenziran

Ključne riječi
SiC, moissanite, cave sediment, Pazin cave, Croatia

Sažetak
The Pazin cave was formed within the succession of the upper Cenomanian limestones (NW part of the former Adriatic Carbonate Platform - Milna Formation). The cave is passable to the southeastern part of the Martel Lake (sampling site) and represents a typical "Dinaric" fault zone (northwest-southeast, the average measured discontinuities 55/50). During the cave investigation a sediment sample of clayey silt was further investigated to get insight into the mineralogical composition. A standard light and heavy mineral analysis was performed using sodium polytungstate (SPT ; ϱ= 2.8 g/cm3) to separate the heavy and light mineral fractions. Analysis was performed by determination of 250-300 grains per sample using the ribbon counting method according to MANGE & MAURER (1992). In the light mineral fraction quartz grains predominate, while feldspar, especially plagioclase grains with typical polysynthetic lamellae, and chert grains are present in similar amounts. Mica is present in small quantities, as well. The sample contains 3.9 % of the heavy mineral fraction among which transparent heavy minerals predominate. Opaque minerals are also present. Among the heavy mineral fraction pyroxene grains prevail, mostly clinopyroxenes, with typical prismatic habitus and “hacksaw” cleavage. Some needle-like pyroxene grains are present, too. Pink garnets and some unknown blue glass-like fragments are also abundant in the sample. Rutile, tourmaline, staurolite, amphibole, epidote, and kyanite are present in a negligible amount. The uncommon blue grains detected in the sample show glassy to metallic lustre on thicker grains, conchoidal fracture and a high relief. The interference figure is excellent showing an uniaxial positive pattern. Since such mineral grains are very uncommon in cave sediments or any other sediment in the area, SEM-EDS analysis (JEOL Multi-Purpose scanning microscope (JSM-35) and INCAx-act Oxford Instruments detector) was applied to determine in more detailed the present mineral and to confirm the composition of the cave sediment. SEM image revealed that the blue glass-like fragments show flat surface, without cracks. Nevertheless, some grains show traces probably of mechanical disturbances (scrap marks). EDS spectra showed only two elements ; Si and C. Based on all optical mineralogical properties and confirmed by the EDS spectra it was concluded that it is very likely moissanite (SiC). Beside SiC, EDS spectra distinguished Ca, Mg and Fe peaks for some grains confirming the presence of clinopyroxenes (diopside- hedenbergite series?). Opaque minerals are mostly Fe-oxides and Fe, Mn-oxides (Fe-Mn nodules often found in cave sediments and Quaternary palaeosols and soils – unpublished data). Corundum (Fe-Al-oxides, Fe-Ti-oxides and Al-oxides) and barite were also detected in the investigated cave sediment. Moissanite is very rarely found in sedimentary archives especially in Croatia. Synthetic SiC is produced at high-temperatures (up to 2500°C) and under extremely reducing conditions and is often used as a replacement for diamonds due to its hardness. Furthermore, detected Al-, Fe, Al- and Fe, Ti-oxides in the sample are also most widely used as abrasives due to their hardness. The detected moissanite is probably synthetic and originated or resulted from industrial activity in the area. It is clear that the found grains in the cave sediment of Pazin cave are a result of pollution. Intensive contamination of the Pazin cave is an actual problem since the cave is located on a densely populated area, near the quarries and factories.

Izvorni jezik
Engleski

Znanstvena područja
Geologija

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