Serpentine-group minerals (chrysotile, lizardite and antigorite) are common rock-forming hydrous magnesium phyllosilicates, with ideal chemical formula Mg3Si2O5(OH)4. They form under a wide range of temperatures, including Earth surface conditions and hot hydrothermal temperatures (Evans et al., 2013). When characterized by a compact microstructure and fine and uniform colors (generally green), they are used in gemology for ornamental carvings and gems, often referred as "noble" or "precious" serpentine (O'Donoghue, 2006). In Italy, gem-quality serpentine occurs in various deposits in the Alps and the Apennines. This study aims to provide a gemological and mineralogical characterization of serpentine from Pizzo Tremogge, Val Malenco, Central Alps, which is one of the most known Italian source for "noble" serpentine (Bedogné et al., 1993). At Pizzo Tremogge serpentine minerals are included in forsterite-bearing marbles Paleozoic in age. The suite of rough and cut gem-quality samples from this locality, kindly provided by Mr. P. Nana (Sondrio, Italy), were investigated by means of gemological analyses, X-Ray Powder Diffraction, EMP chemical analyses, and Raman spectroscopy. The gemological properties of serpentine from Val Malenco are in the range of typical gem-quality serpentine. In particular, the specimens have all a massive and opaque aspect, with a color ranging from green to yellowish green, sometimes with white veins and black spots. The refractive index, measured by the distant vision method, is about 1.55, whereas the specific gravity ranges from 2.49-2.67, with variations related to the occurrence of other phases. The samples are inert to UV (254 and 366 nm) radiations and have a Mohs hardness of ~4. Four different samples have been analyzed by X-ray powder diffraction and Raman spectroscopy, which are considered the more efficient methods for identifying different species of serpentine. The identified phases are: lizardite, antigorite, calcite, dolomite, brucite and chlorite. In addition EMP analyses allowed us to evidence chemical differences between lizardite and antigorite that mainly arise from the extent of cationic substitution: lizardite is always Al and Fe -enriched (0.10-0.18 and 0.11-0.16 a.p.f.u., calculated on the basis of seven anhydrous oxygens respectively) in respect to antigorite (0.02-0.08 and 0.04- 0.08 a.p.f.u, respectively). Minor amounts of Ti (0.001-0.008 a.p.f.u.) and trace amounts of Ca, Cr, Ni, and Zn have been also detected. The compact and fine-grained microstructure and the typical green color confer to the serpentine from Pizzo Tremogge an agreeable aspect and make it noteworthy as gemological material.
"Noble" serpentine: a case study from Val Malenco, Central Alps, Italy
2014
Abstract
Serpentine-group minerals (chrysotile, lizardite and antigorite) are common rock-forming hydrous magnesium phyllosilicates, with ideal chemical formula Mg3Si2O5(OH)4. They form under a wide range of temperatures, including Earth surface conditions and hot hydrothermal temperatures (Evans et al., 2013). When characterized by a compact microstructure and fine and uniform colors (generally green), they are used in gemology for ornamental carvings and gems, often referred as "noble" or "precious" serpentine (O'Donoghue, 2006). In Italy, gem-quality serpentine occurs in various deposits in the Alps and the Apennines. This study aims to provide a gemological and mineralogical characterization of serpentine from Pizzo Tremogge, Val Malenco, Central Alps, which is one of the most known Italian source for "noble" serpentine (Bedogné et al., 1993). At Pizzo Tremogge serpentine minerals are included in forsterite-bearing marbles Paleozoic in age. The suite of rough and cut gem-quality samples from this locality, kindly provided by Mr. P. Nana (Sondrio, Italy), were investigated by means of gemological analyses, X-Ray Powder Diffraction, EMP chemical analyses, and Raman spectroscopy. The gemological properties of serpentine from Val Malenco are in the range of typical gem-quality serpentine. In particular, the specimens have all a massive and opaque aspect, with a color ranging from green to yellowish green, sometimes with white veins and black spots. The refractive index, measured by the distant vision method, is about 1.55, whereas the specific gravity ranges from 2.49-2.67, with variations related to the occurrence of other phases. The samples are inert to UV (254 and 366 nm) radiations and have a Mohs hardness of ~4. Four different samples have been analyzed by X-ray powder diffraction and Raman spectroscopy, which are considered the more efficient methods for identifying different species of serpentine. The identified phases are: lizardite, antigorite, calcite, dolomite, brucite and chlorite. In addition EMP analyses allowed us to evidence chemical differences between lizardite and antigorite that mainly arise from the extent of cationic substitution: lizardite is always Al and Fe -enriched (0.10-0.18 and 0.11-0.16 a.p.f.u., calculated on the basis of seven anhydrous oxygens respectively) in respect to antigorite (0.02-0.08 and 0.04- 0.08 a.p.f.u, respectively). Minor amounts of Ti (0.001-0.008 a.p.f.u.) and trace amounts of Ca, Cr, Ni, and Zn have been also detected. The compact and fine-grained microstructure and the typical green color confer to the serpentine from Pizzo Tremogge an agreeable aspect and make it noteworthy as gemological material.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
