New Insights for Gem-Quality Mn-Bearing Diopside-Omphacite, Violane Variety, from Saint Marcel (Val D’Aosta, Italy): Its Trace Elements and Spectroscopic Characterization

Abstract

This study proposes new data on the rare pyroxene, variety violane, sampled from its type locality, Praborna manganese deposit, near Saint Marcel (Val d’Aosta, Italy). Violane is very appreciated as a gemstone for its different hues of violet-blue color and is characterized by its diopsidic or omphacitic composition. To assess the possible causes of color, electron-probe microanalysis (EMPA) and laser-ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) were used to establish the chemical composition. Raman and Fourier transform infrared (FTIR) spectroscopy, ideal for the non-destructive analysis, identified the different present phases directly on gemstone. Raman and FTIR spectra highlighted the presence, in the same sample, of diopside and omphacite showing almost inappreciable violet hue difference. The two minerals were easily differentiated by microprobe analyses and showed a compositional heterogeneity not linked to the different colors. The best way to detect the color-causing elements in microcrystalline violanes resulted in the analysis of trace elements and their quantification. An enrichment of Ti and Li characterizes darker violet omphacite and that of V and rare-earth elements (REE) the lilac-lavender or light violet-blue diopside. In general, our results led us to say that the color changes, previously proposed as due to Mn both in divalent and trivalent oxidation state, may be controlled by trace elements or by concentration of minor elements, such as Fe, and their oxidation state. REE patterns showed a negative anomaly of Ce that could be ascribed to the variation of the oxygen chemical potential occurring in the ore. The new data, combined with previous results, may provide new constrains on the processes that generated the Mn-rich deposit of Praborna.

An Attractive Blue Diopside from Sissone Valley, Western Alps, Italy

“…Reactions (1) and (2), instead, do not need an additional source of silica such as the infiltration of silica-rich aqueous fluids. The EMP analyses of the main elements have established that the blue pyroxene of the Sissone valley can be classified as almostpure diopside; the omphacitic component, present in the "violane" from the Praborna mine in Italy [7], is absent in this case. XRPD and Raman analyses showed the mineralogical association of the diopsidebearing rock is made up by Mg-calcite, phlogopite, lizardite, clinochlore, dolomite, forsterite and rare tremolite; this composition is in agreement with theliterature [15].…”

“…Blue diopside with polycrystalline texture has been found in three locations in Russia: the first in the Khakassia region of Eastern Siberia [46,47], while the second and third are near Lake Baikal [48,49]. Another type of diopside with similar properties is the aforementioned "violane", which is extracted from the Praborna mine in the Aosta Valley, Italy [7]. The "violane" is also polycrystalline but, unlike the diopside of the Sissone valley, the color is markedly purple with some shades of blue; the chromophore elements are represented by Mn and Fe, while V is practically absent.…”

“…"Violane" is a variety of diopside rich in manganese, extracted mainly from the Praborna manganese mine near St. Marcel in Aosta Valley (Italy). It is a very rare mineral with a color ranging from violet to bluish purple to lilac, depending on the content of the chromophore elements, which are mainly the Mn 2+ , Mn 3+ , Fe 2+ , V 4+ and Ti 4+ ions [1][2][3]7].…”

Gemology, Mineralogy, and Spectroscopy of Gem-Quality Diopside from Pakistan and Russia: New Insights for the Chromogenic Mechanism and Possible Origin

“…In this paper, gem-quality diopsides from Pakistan and Russia were selected (Figure 1), and gemological, spectroscopic, and major and trace element analyses were conducted to determine the gemological and compositional characteristics of these crystals. Meanwhile, we also collected the data of gem-quality blue diopsides from Italy and more Russian diopsides from previous studies, aiming to explore the mechanism of coloration and genetic differences of different diopsides [15,16]. This study can enrich the theoretical, gemological, and mineralogical knowledge of diopside and provide a better understanding of the geological background producing these different diopsides.…”

The Importance of Multidisciplinary Analytical Strategies to Solve Identification and Characterization Challenges in Gemology: The Example of the “Green Stones”

“…Considering the second technique, a spot quantitative chemical analysis is possible. It is interesting to note that with LA-ICP-MS, not only the heavier but also the lighter elements can be detected and quantified [20,81,82,167]. Both techniques can supply very important information, complementary to the ones obtained by ED-XRF.…”