An outstanding gem sarcolite gemstone, orange in color and weighing 2.55 carats, has been cut from a large fragment of a crystal hosted in xenolitic rock from the type locality of Monte Somma (Vesuvius, Italy) [1]. This sample was found in a metamorphosed volcanic ejecta in association with augite, gehlenite and nepheline. It represents the largest cut gemstone of sarcolite known so far. Sarcolite has an ideal chemical formula of NaCa6Al4Si6O24F and is tetragonal with space group I4/m. Gemmological properties give refraction indices of = 1.604(1) and ! = 1.613(2) and a density of 2.91 g/cm3 [2]. The crystal structure of sarcolite was solved using single crystal X-ray diffraction (SCXRD) by [3] who obtained an R value of 0.054. Chemical investigations of sarcolite were performed by [4] and [5]. We have performed a crystal-chemistry study using SCXRD and WDS electron microprobe analysis. The CHNS elemental analysis technique was also used for the determination of volatile elements. Crystallographic results give lattice parameters of a = 12.3678(5), b = 12.3678(5), c = 15.4777(10) Å, V = 2367.51 Å3, and an R value of 0.0414. CO3 and PO4 groups and Na(K) polyhedrons are situated along the 4-fold axis. C is 3-fold coordinated in planar CO3 groups showing four alternative positions, thus having the CO3 plane parallel to {110}. There are P tetrahedra with two alternative configurations, i.e., with two opposed edges parallel to (001), thus showing a local violation of the space group. Na is 5-fold coordinated and is locally split. Al is 5-fold coordinated; two polyhedra are connected through a common apex being F (or minor OH). Silicon is present as one disilicate group and four isolated tetrahedra per unit cell. There are three alkali sites: two Ca sites and a Na site, which can have half occupancy at maximum. Electron microprobe analysis revealed an average composition of SiO2 35.45 wt%, CaO 32.93 wt%, Al2O3 18.79 wt%, Na2O 3.98 wt%, P2O5 3.23 wt%, F 2.86 wt%, Fe2O3 0.50 wt%, MgO 0.28 wt%, MnO 0.03 wt%, K2O 0.85 wt%, TiO2 0.04 wt%, Cl– 0.02 wt%, and SrO 0.83 wt%. The CHNS yielded CO2 1.99 wt%, SO3 0.07 wt%, and OH– 0.22 wt%. The chemical formula (a.p.f.u.) calculated for the sarcolite specimen by combining single crystal XRD, EMPA, and CHNS is [(CO3)0.46(PO4)0.47 (Na0.82K0.18)] Na0.47 (Ca6.01Sr0.08)"=6.09 (Al3.79Fe2+ 0.07Mg0.07Ti4+ 0.01Mn2+ 0.01)"=3.94 (Si2O7) (Si1.02O4)4 [F1.52(OH)0.21Cl0.01]" =1.74.
Crystal chemistry of a sarcolite gemstone.
CAMARA ARTIGAS, Fernando
2010-01-01
Abstract
An outstanding gem sarcolite gemstone, orange in color and weighing 2.55 carats, has been cut from a large fragment of a crystal hosted in xenolitic rock from the type locality of Monte Somma (Vesuvius, Italy) [1]. This sample was found in a metamorphosed volcanic ejecta in association with augite, gehlenite and nepheline. It represents the largest cut gemstone of sarcolite known so far. Sarcolite has an ideal chemical formula of NaCa6Al4Si6O24F and is tetragonal with space group I4/m. Gemmological properties give refraction indices of = 1.604(1) and ! = 1.613(2) and a density of 2.91 g/cm3 [2]. The crystal structure of sarcolite was solved using single crystal X-ray diffraction (SCXRD) by [3] who obtained an R value of 0.054. Chemical investigations of sarcolite were performed by [4] and [5]. We have performed a crystal-chemistry study using SCXRD and WDS electron microprobe analysis. The CHNS elemental analysis technique was also used for the determination of volatile elements. Crystallographic results give lattice parameters of a = 12.3678(5), b = 12.3678(5), c = 15.4777(10) Å, V = 2367.51 Å3, and an R value of 0.0414. CO3 and PO4 groups and Na(K) polyhedrons are situated along the 4-fold axis. C is 3-fold coordinated in planar CO3 groups showing four alternative positions, thus having the CO3 plane parallel to {110}. There are P tetrahedra with two alternative configurations, i.e., with two opposed edges parallel to (001), thus showing a local violation of the space group. Na is 5-fold coordinated and is locally split. Al is 5-fold coordinated; two polyhedra are connected through a common apex being F (or minor OH). Silicon is present as one disilicate group and four isolated tetrahedra per unit cell. There are three alkali sites: two Ca sites and a Na site, which can have half occupancy at maximum. Electron microprobe analysis revealed an average composition of SiO2 35.45 wt%, CaO 32.93 wt%, Al2O3 18.79 wt%, Na2O 3.98 wt%, P2O5 3.23 wt%, F 2.86 wt%, Fe2O3 0.50 wt%, MgO 0.28 wt%, MnO 0.03 wt%, K2O 0.85 wt%, TiO2 0.04 wt%, Cl– 0.02 wt%, and SrO 0.83 wt%. The CHNS yielded CO2 1.99 wt%, SO3 0.07 wt%, and OH– 0.22 wt%. The chemical formula (a.p.f.u.) calculated for the sarcolite specimen by combining single crystal XRD, EMPA, and CHNS is [(CO3)0.46(PO4)0.47 (Na0.82K0.18)] Na0.47 (Ca6.01Sr0.08)"=6.09 (Al3.79Fe2+ 0.07Mg0.07Ti4+ 0.01Mn2+ 0.01)"=3.94 (Si2O7) (Si1.02O4)4 [F1.52(OH)0.21Cl0.01]" =1.74.
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