A distinguish feature of Val Malenco, Central Alps, Italy, is the remarkable number of recovered minerals (more than 250 species), which have attracted the interest of mineralogists since the end of 18th century, making the valley renowned at international level. Among the various minerals, some have a significant gemological interest and are used in jewelry as gems or ornamental carvings. The most notable and one of the best Italian gem material is the green andradite garnet (Ca3Fe2Si3O12), known with the varietal name "demantoid" (Adamo et al., 2009). Attractive gemstones have been produced from crystal recovered in asbestos mines hosted by serpentinites and located in a small area, where the most famous localities are: Cengiàsc (including the Sferlùn quarry), Dossi di Franscia, Coston d'Acquanegra, Valbrutta, and Al Ross (Bedogné et al., 1993). These asbestos deposits, now closed for safety reasons, are sited at ~1800-2200 m above sea level. The gemological properties, including the characteristic "horse-tail" inclusions (fibrous chrysotile), and the chemical composition (Adr >=98 mol%) are typical of serpentinites-related demantoid. Trace of chromium, together with major iron, are responsible for the green color of this gem. Another interesting gem material is the nephrite jade, discovered at Alpe Mastabia and hosted in a talc-tremolitic orebody, associated with dolomitic marbles (Adamo & Bocchio, 2013). The jade, occurring as intergrowth of fine fibers, is mainly composed of pure tremolite, Ca2Mg5Si8O22(OH)2, and contains a low concentration of iron, which causes its typical pale green color. The occurrence of other constituents, including calcite, molybdenite and galena, influences the color of the jade. Gem-quality rhodonite (Mn5Si5O15) from Val Malenco occurs in the Mn-mineralization at Monte del Forno. Rhodonite has typically a massive aspect, with an attractive pink color (due to Mn). It is always associated with tephroite, spessartine, pyroxmangite, biotite, magnetite, calcite, quartz, and Mn-oxides and hydroxides. "Noble" serpentine, green and yellowish green in color, composed mainly of lizardite and antigorite (with various amount of calcite, dolomite, brucite, and chlorite) was found at Pizzo Tremogge included in forsterite-bearing marbles. In the same area, precious Mn-rich clinozoisite (clinothulite variety) has also mined for its fine pink color. Other interesting gem materials from Val Malenco include diopside (green or, more rarely, blue) and Cr-bearing green grossular.
Val Malenco, Central Alps, Italy: a source of gem materials
2014
Abstract
A distinguish feature of Val Malenco, Central Alps, Italy, is the remarkable number of recovered minerals (more than 250 species), which have attracted the interest of mineralogists since the end of 18th century, making the valley renowned at international level. Among the various minerals, some have a significant gemological interest and are used in jewelry as gems or ornamental carvings. The most notable and one of the best Italian gem material is the green andradite garnet (Ca3Fe2Si3O12), known with the varietal name "demantoid" (Adamo et al., 2009). Attractive gemstones have been produced from crystal recovered in asbestos mines hosted by serpentinites and located in a small area, where the most famous localities are: Cengiàsc (including the Sferlùn quarry), Dossi di Franscia, Coston d'Acquanegra, Valbrutta, and Al Ross (Bedogné et al., 1993). These asbestos deposits, now closed for safety reasons, are sited at ~1800-2200 m above sea level. The gemological properties, including the characteristic "horse-tail" inclusions (fibrous chrysotile), and the chemical composition (Adr >=98 mol%) are typical of serpentinites-related demantoid. Trace of chromium, together with major iron, are responsible for the green color of this gem. Another interesting gem material is the nephrite jade, discovered at Alpe Mastabia and hosted in a talc-tremolitic orebody, associated with dolomitic marbles (Adamo & Bocchio, 2013). The jade, occurring as intergrowth of fine fibers, is mainly composed of pure tremolite, Ca2Mg5Si8O22(OH)2, and contains a low concentration of iron, which causes its typical pale green color. The occurrence of other constituents, including calcite, molybdenite and galena, influences the color of the jade. Gem-quality rhodonite (Mn5Si5O15) from Val Malenco occurs in the Mn-mineralization at Monte del Forno. Rhodonite has typically a massive aspect, with an attractive pink color (due to Mn). It is always associated with tephroite, spessartine, pyroxmangite, biotite, magnetite, calcite, quartz, and Mn-oxides and hydroxides. "Noble" serpentine, green and yellowish green in color, composed mainly of lizardite and antigorite (with various amount of calcite, dolomite, brucite, and chlorite) was found at Pizzo Tremogge included in forsterite-bearing marbles. In the same area, precious Mn-rich clinozoisite (clinothulite variety) has also mined for its fine pink color. Other interesting gem materials from Val Malenco include diopside (green or, more rarely, blue) and Cr-bearing green grossular.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
