Origin of gem corundum in calcite marble : the Revelstoke occurrence in the Canadian Cordillera of British Columbia

The calcite marble-hosted gem corundum (ruby, sapphire) occurrence near Revelstoke, British Columbia, Canada, occurs in the Monashee Complex of the Omineca Belt of the Canadian Cordillera. Corundum occurs in thin, folded and stretched layers with green muscovite + Ba-bearing K-feldspar + anorthite (An(0.85-1)) +/- phlogopite +/- Na-poor scapolite. Other silicate layers within the marble are composed of: (1) diopside + tremolite quartz and (2) garnet (Alm(0.7-05)Grs(0.2-0.4)) + Na-rich scapolite + diopside + tremolite + Na,K-amphiboles. Non-silicate layers in the marble are either magnetite- or graphite-bearing. Predominantly pink (locally red or purple) opaque to transparent corundum crystals have elevated Cr2O3 (<= 0.21 wt.%) and variable amounts of TiO2; rare blue rims on the corundum crystals contain higher amounts of TiO2 (<= 0.53 wt.%) and Fe2O3 (<= 0.07 wt.%). The associated micas have elevated Cr, V, Ti, and Ba contents. Petrography of the silicate layers show that corundum formed from muscovite at the peak of metamorphism (similar to 650-700 degrees C at 8.5-9 kbar). Because the marble is almost pure calcite (dolomite is very rare), the corundum was preserved because it did not react with dolomite to spinel + calcite during decompression. The scapolite-bearing assemblages formed during or after decompression of the rock at similar to 650 degrees C and 4-6 kbar. Gem-quality corundum crystals formed especially on borders of the mica-feldspar layers in an assemblage with calcite. Whole rock geochemistry data show that the corundum-bearing silicate (mica-feldspar) layers formed by mechanical mixing of carbonate with the host gneiss protolith; the bulk composition of the silicate layers was modified by Si and Fe depletion during prograde metamorphism. High element mobility is supported by the homogenization of delta O-18 and delta C-13 values in carbonates and silicates for the marble and silicate layers. The silicate layers and the gneiss contain elevated contents of Cr and V due to the volcanoclastic component of their protolith

Princivalleite, Na(Mn2Al)Al6(Si6O18)(BO3)3(OH)3O, a new mineral species of the tourmaline supergroup from Veddasca Valley, Varese, Italy

Princivalleite, Na(Mn2Al)Al6(Si6O18)(BO3)3(OH)3O, is a new mineral of the tourmaline supergroup. It occurs in the Veddasca Valley, Luino area (Varese, Lombardy, Italy) (46°03'30.74''N - 8°48'24.47''E) at the center of a narrow (2-3 cm wide) vertical pegmatitic vein, a few meters long, crosscutting a lens of flaser gneiss. Crystals are subhedral (up to 10 mm in size), azure with a vitreous luster, conchoidal fracture and white streak. Princivalleite has a Mohs hardness of approximately 7, a calculated density of 3.168 g/cm3and is uniaxial negative. Princivalleite has trigonal symmetry, space group R3m, a = 15.9155(2) Å, c = 7.11660(10) Å, V = 1561.15(4) Å3, Z = 3. The crystal structure was refined to R1 = 1.36 % using 1758 unique reflections collected with MoKa X-ray intensity data. Crystal-chemical analysis resulted in the empirical crystal-chemical formula {equation presented} which recast in its ordered form for classification purposes is:{equation presented} Princivalleite is an oxy-species belonging to the alkali group of the tourmaline supergroup. The closest end-member compositions of valid tourmaline species are those of oxy-schorl and darrellhenryite, to which princivalleite is related by the substitutions Mn2+↔ Fe2+and Mn2+↔ 0.5Al3++ 0.5Li+(respectively). The new mineral was approved by the International Mineralogical Association's Commission on New Minerals, Nomenclature and Classification (IMA 2020-056). Princivalleite from Veddasca Valley is a geochemical anomaly, originated in a B-rich and peraluminous anatectic pegmatitic melt formed in situ, poor in Fe and characterized by reducing conditions in the late-stage metamorphic fluids derived by the flaser gneiss. The Mn-enrichment in this new tourmaline is due to absence of other minerals competing for Mn such as garnet

Origin of scapolite-hosted sapphire (corundum) near Kimmirut, Baffin island, Nunavut, Canada

Gem-quality corundum (sapphire) occurs in scapolite-rich calc-silicate rock hosted in marble of the Lake Harbour Group near Kimmirut, southern Baffin Island. A deposit of blue and colorless gem corundum (Beluga occurrence) is compared to a similar calc-silicate pod generally lacking corundum but containing nepheline (Bowhead occurrence) and located 170 m to the SSW. Corundum formation was made possible by three equally important sequential metamorphic reactions: (1) formation of nepheline, diopside, and K-feldspar (inferred) at granulite facies peak metamorphic conditions; (2) partial retrograde replacement of the peak assemblage by phlogopite, oligoclase, calcite, and scapolite (Me-50-Me-67) as a result of CO2-, H2O-, Cl-, F-bearing fluid influx at 1782.5 +/- 3.7 Ma (P-T < 720 degrees C, 6.2 kbar); and (3) retrograde breakdown of scapolite + nepheline (with CO2-and H2O-bearing fluid) to form albite, muscovite, corundum, and calcite. Late, low-temperature zeolite mineralization is common in corundum-bearing zones. Based on thermodynamic models, the corundum-forming reaction only occurs in a <100 degrees C window with an upper limit determined by scapolite-nepheline stability, and a lower limit determined by the formation of Al-silicate rather than corundum. The protolith is inferred to be dolomitic argillaceous marl with no evidence to suggest the initial presence of evaporites. The enrichment of trace metals V and Cr, and the depletion of Co, Ni, and Mn, suggest reducing diagenetic conditions in the initial sediment. Beluga calc-silicate rock is strongly depleted in REE (Total REE similar to 18 ppm). Oxy-dravite delta B-11 (+3.9 +/- 0.7%) is consistent with a marine boron source. The oxygen isotope composition of corundum (delta O-18(VSMOW) = 16.4 +/- 0.1%) is comparable to that of corundum in marble or desilicated pegmatite associated with marble. Phlogopite and muscovite Ar-40/(39) Ar ages and calculated closure temperatures (considered estimates) are ca. 1640 Ma (T-c = 455 to 515 degrees C) and 1510 Ma (Tc = 410 to 425 degrees C), respectively. In the Lake Harbour Group, the most prospective areas for gem corundum exploration are expected to be contiguous to the thrust fault separating the Lake Harbour Group and Narsajuaq terranes, where the retrograde, amphibolite facies overprint of the granulite peak assemblages was most pervasive

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

Magnesio-lucchesiite, ideally CaMg3Al6(Si6O18)(BO3)3(OH)3O, is a new mineral species of the tourmaline supergroup. The holotype material was discovered within a lamprophyre dike that cross-cuts tourmaline-rich metapelites within the exocontact of the O'Grady Batholith, Northwest Territories (Canada). Two additional samples were found at San Piero in Campo, Elba Island, Tuscany (Italy) in hydrothermal veins embedded in meta- serpentinites within the contact aureole of the Monte Capanne intrusion. The studied crystals of magnesio-lucchesiite are black in a hand sample with vitreous luster, conchoidal fracture, an estimated hardness of 7-8, and a calculated density of 3.168 (Canada) and 3.175 g/cm3 (Italy). In plane-polarized light, magnesio-lucchesiite is pleochroic (O = dark brown, E = colorless) and uniaxial (-); its refractive index values are nω = 1.668(3) and nϵ = 1.644(3) (Canada), and nω = 1.665(5) and nϵ = 1.645(5) (Italy). Magnesio-lucchesiite is trigonal, space group R3m, Z = 3, with a = 15.9910(3) Å, c = 7.2224(2) Å, V = 1599.42(7) Å3 (Canada) and with a = 15.9270(10) Å, c = 7.1270(5) Å, V = 1565.7(2) Å3 (Italy, sample 1). The crystal structure of magnesio-lucchesiite was refined to R1 = 3.06% using 2953 reflections with Fo &gt; 4σ(Fo) (Canadian sample; 1.96% / 1225 for the Italian sample) The Canadian (holotype) sample has the ordered empirical formula X(Ca0.60Na0.39K0.01)ς1.00YMg2.02Fe0.622+Fe0.093+Ti0.25 V0.01Cr0.01ς3.00zAl5.31Fe0.693+ς6.00TSi5.98Al0.02ς6.00O18BO33v(OH)2.59O0.41ς3.00 ${ }^{mathrm{Y}}left(mathrm{Mg}_{2.02} mathrm{Fe}_{0.62}^{2+} mathrm{Fe}_{0.09}^{3+} mathrm{Ti}_{0.25} mathrm{~V}_{0.01} mathrm{Cr}_{0.01} ight)_{Sigma 3.00} mathrm{z}left(mathrm{Al}_{5.31} mathrm{Fe}_{0.69}^{3+} ight)_{Sigma 6.00}left[{ }^{mathrm{T}}left(mathrm{S} mathrm{i}_{5.98} mathrm{Al}_{0.02} ight)_{Sigma 6.00} mathrm{O}_{18} ight]left(mathrm{BO}_{3} ight)_{3}{ }^{mathrm{v}}left[(mathrm{OH})_{2.59} mathrm{O}_{0.41} ight]_{Sigma 3.00}$W(O0.78F0.22)ς1.00. The Italian (co-type) material shows a wider chemical variability, with two different samples from the same locality having ordered chemical formulas: xCa0.88Na0.12ς1.00YMg1.45Fe0.402+Al0.79Fe0.363+ς3.00 ${ }^{mathrm{x}}left(mathrm{Ca}_{0.88} mathrm{Na}_{0.12} ight)_{Sigma 1.00} mathrm{Y}left(mathrm{Mg}_{1.45} mathrm{Fe}_{0.40}^{2+} mathrm{Al}_{0.79} mathrm{Fe}_{0.36}^{3+} ight)_{Sigma 3.00}$ZAl6[T(Si5.05Al0.95)ς6.00O18](BO3)3V[(OH)2.90O0.10]ς3.00W(O0.98F0.02)ς1.00 (sample 1) and X(Ca0.71Na0.210.08)ς1.00YMg2.49Fe0.412+Ti0.10ς3.00ZAl5.44Fe0.463+Mg0.09 V0.01ς6.00TSi5.87Al0.13ς6.00O18BO33V(OH)3WO0.61(OH)0.39ς1.00 (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 YFe YMg, and to feruvite by the homovalent and heterovalent substitutions YFe YMg and ZAl3+ + WO2-ZMg2+ + 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 also form in metacarbonate rocks by fluctuations of F and Al during crystallization of common uvitic tourmaline. High miscibility with other tourmaline end-members indicates the large petrogenetic potential of magnesio-lucchesiite in Mg,Al-rich calc-silicate rocks, as well as contact-metamorphic and metasomatic rocks