Mohsite is a discredited mineral, and is now recognized as a member (chrichtonite) of the sénaite-chrichtonite series, but the history of its description and identification is of notable interest for the mineralogy of the Susa Valley. The authors had the opportunity to examine and investigate the samples of "mohsite" described by Colomba, an Italian eminent mineralogist, in 1902. These samples were lost during years and found again some years ago, after more than a century, in the historical mineralogical collection of the Turin University, now in the Piedmont Council Museum of Natural Science of Turin. These new analysis proves that the mohsite of Colomba has a high Sr content, and should thus be identified as chrichtonite. Mohsite was first reported by Lévy in 1827, on material probably from the Dauphiné Region, in France. The chemical composition was unknown, and the description was mainly morphological. The original samples of Lévy was never traced again. In 1901 Lacroix found some other samples in the Haute Alpes, again in the Dauphiné. This material was not chemical analyzed because of its very small amount but, being these samples morphologically similar, they became neotype. In 1902 Colomba found similar material from near the Dora Riparia, Oulx, Italian Alps, some ten km from the previous locality but in the Italian sector of the mountain chain. Some qualitative analysis performed at that time shoved that the samples contained titanium and iron, as well as small quantities of Ca and Mg, and for this reasons Colomba identified these small black crystals as mohsite. Kelly et al (1979) obtained a sample of Lacroix's specimen from the Museum National d'Historie Naturelle de Paris, and they found that the main cation was Sr and so identifying the samples as chrichtonite, and in the same time discrediting the name "mohsite" due to temporal priority of the name "chrichtonite" (De Bournon, 1813). The authors, as a consequence of the rediscover of the Colomba's samples, submitted these 1902's specimens to SEM-EDS and micro-XRF analysis, and found that the small crystals were made of a complex oxide of titanium, iron and vanadium with Sr as main bivalent cation, followed by Y and small amount of U and Yb. The formula of this oxide as mean of many analysis could be written, as (Sr0.9 Y0.7U0.2Yb0.1)(Ti13.7Fe7.3V0.3)O38 on the basis of 38 oxigens. This formula is close matching with the one of chrichtonite (Sr,Y,U,Yb)2(Ti,Fe,V)21O38, the Sr-bearing term of the group (the other important terms being senaite if Pb is the main large cation, davidite for REE, landauite for Na and loveringite for Ca). Thus, the mohsite of Colomba is in fact a sample of chrichtonite, and probably Colomba may have misidentified Ca with Sr in the old analysis, or these analysis were performed on dirt samples with interference from others minerals.
THE HISTORY OF THE DISCREDITED MOHSITE (OF COLOMBA)
COSTA, Emanuele;
2011-01-01
Abstract
Mohsite is a discredited mineral, and is now recognized as a member (chrichtonite) of the sénaite-chrichtonite series, but the history of its description and identification is of notable interest for the mineralogy of the Susa Valley. The authors had the opportunity to examine and investigate the samples of "mohsite" described by Colomba, an Italian eminent mineralogist, in 1902. These samples were lost during years and found again some years ago, after more than a century, in the historical mineralogical collection of the Turin University, now in the Piedmont Council Museum of Natural Science of Turin. These new analysis proves that the mohsite of Colomba has a high Sr content, and should thus be identified as chrichtonite. Mohsite was first reported by Lévy in 1827, on material probably from the Dauphiné Region, in France. The chemical composition was unknown, and the description was mainly morphological. The original samples of Lévy was never traced again. In 1901 Lacroix found some other samples in the Haute Alpes, again in the Dauphiné. This material was not chemical analyzed because of its very small amount but, being these samples morphologically similar, they became neotype. In 1902 Colomba found similar material from near the Dora Riparia, Oulx, Italian Alps, some ten km from the previous locality but in the Italian sector of the mountain chain. Some qualitative analysis performed at that time shoved that the samples contained titanium and iron, as well as small quantities of Ca and Mg, and for this reasons Colomba identified these small black crystals as mohsite. Kelly et al (1979) obtained a sample of Lacroix's specimen from the Museum National d'Historie Naturelle de Paris, and they found that the main cation was Sr and so identifying the samples as chrichtonite, and in the same time discrediting the name "mohsite" due to temporal priority of the name "chrichtonite" (De Bournon, 1813). The authors, as a consequence of the rediscover of the Colomba's samples, submitted these 1902's specimens to SEM-EDS and micro-XRF analysis, and found that the small crystals were made of a complex oxide of titanium, iron and vanadium with Sr as main bivalent cation, followed by Y and small amount of U and Yb. The formula of this oxide as mean of many analysis could be written, as (Sr0.9 Y0.7U0.2Yb0.1)(Ti13.7Fe7.3V0.3)O38 on the basis of 38 oxigens. This formula is close matching with the one of chrichtonite (Sr,Y,U,Yb)2(Ti,Fe,V)21O38, the Sr-bearing term of the group (the other important terms being senaite if Pb is the main large cation, davidite for REE, landauite for Na and loveringite for Ca). Thus, the mohsite of Colomba is in fact a sample of chrichtonite, and probably Colomba may have misidentified Ca with Sr in the old analysis, or these analysis were performed on dirt samples with interference from others minerals.
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