Magnesio-lucchesiite, CaMg3Al6(Si6O18)(BO3)3(OH)3O, a new species of the tourmaline supergroup

Abstract

Always consult and cite the final, published document. See http:/www.minsocam.org or GeoscienceWorld [ T (Si 5.87 Al 0.13) 6.00 O 18 ] (BO 3) V (OH) 3 W [O 0.61 (OH) 0.39 ] 1.00 (sample #2). Magnesio-lucchesiite is an oxy-species belonging to the calcic group of the tourmaline supergroup. It is related to lucchesiite by the homovalent substitution Y Fe ↔ Y Mg, and to feruvite by the homovalent and heterovalent substitutions Y Fe ↔ Y Mg and Z Al 3+ + W O 2-↔ Z Mg 2+ + W (OH) 1-. The new mineral was approved by the International Mineralogical Association Commission on New Minerals, Nomenclature and Classification (IMA 2019-025). Occurrences of magnesio-lucchesiite show that its presence is not restricted to replacement of mafic minerals only; it may form also in metacarbonate rocks by fluctuations of F and Al during crystallization of common uvitic tourmaline. High miscibility with other tourmaline endmembers indicates large petrogenetic potential of magnesio-lucchesiite in Mg,Al-rich calcsilicate rocks, as well as contactmetamorphic, and metasomatic rocks.

Crystal chemistry and evolution of tourmaline in tourmalinites from Zlatá Idka, Slovakia

“…Another possible mechanism for substituting Ca into X site in low Li-tourmalines is the MgCaOAl -1 □ -1 (OH) -1 substitution (Henry and Dutrow 1990). The simpler variation of this substitution, the CaONa -1 (OH) -1 substitution, results in the magnesio-lucchesiite end member (Scribner et al 2021) and can have an influence on Ca-rich and F-poor compositions in the studied samples. Another common Ca-substitution in tourmalines, the X Na + Y Al = X Ca + Y Li substitution, produces liddicoatite end member (Henry and Guidotti 1985) and is unlikely in these low-Li tourmalines.…”

“…Along with the common schorl-dravite composition, their fluor-and oxy-dominant counterparts -fluor-schorl, fluor-dravite, oxy-schorl and oxy-dravite -are also present. Moreover, portions of the tourmaline are enriched in Ca resulting in fluor-uvite and magnesio-lucchesiite, which is tourmaline derived from uvite with O 2dominant at the W site and the sum of trivalent cations at Z above 5.5 apfu (Scribner et al 2021). Moreover, the areas with Ti-rich compositions having Ti > 0.25 apfu likely correspond to a potential new mineral species, "magnesio-dutrowite", which would be the Mgdominant analog of dutrowite (Biagioni et al 2020).…”

Uvite, CaMg3(Al5Mg)(Si6O18)(BO3)3(OH)3(OH), a new, but long-anticipated mineral species of the tourmaline supergroup from San Piero in Campo, Elba Island, Italy

“…Such veins contain tourmaline extremely enriched in Al (∼7.7 apfu) with the spinel phase corresponding to Fe-bearing spinel, (Mg 0.7 Fe 2+ 0.3 ) Σ1.0 (Al 1.9 Fe 3+ 0.1 ) Σ2.0 O 4 , as determined by energy dispersive spectroscopy analyses. Scribner et al (2021) identified in San Rocco a second occurrence of a solid solution between magnesio-lucchesiite and uvite relatively depleted in Al (∼5.6 apfu), in veins along fractures within a basic hornfels derived by contact metamorphism of serpentinite.…”

“…Prism faces are striated parallel to the c axis. It has a grey streak and shows no fluorescence, has a Mohs hardness of ~7½ (estimated by analogy with magnesio-lucchesiite; Scribner et al , 2021) and is brittle with a conchoidal fracture. The calculated density, based on the empirical formula and unit-cell volume refined from single-crystal X-ray diffraction (XRD) data, is 3.115 g/cm 3 .…”

“…Dini and Pezzotta, 2021). Among them, elbaite (Vernadsky, 1914), tsilaisite (Bosi et al ., 2012), fluor-tsilaisite (Bosi et al ., 2015b), magnesio-lucchesiite (Scribner et al ., 2021), celleriite (Bosi et al ., 2022) and uvite (this study) have Elba Island as type locality. Uvite is the second calcic tourmaline, after magnesio-lucchesiite, discovered in the San Piero in Campo area.…”