Kodamaite, Na3(Ca5Na)6Si16O36(OH)4F2·14−xH2O, x = ∼5, A New Layered Alkali-Alkaline Earth Fluorosilicate-Hydrate and Member of the Gyrolite Supergroup, from Mont Saint-Hilaire, Quebec, Canada: Description, Crystal-Chemical Considerations, and Genetic Implications
Authors: McDonald Andrew M. , Chao George Y. 
Published: December 2023

Abstract
Kodamaite, ideally Na3(Ca5Na)6Si16O36(OH)4F2·14−xH2O, where x = ∼5, is a new mineral discovered at the Poudrette quarry, Mont Saint-Hilaire, Quebec, Canada. It develops in spherulitic aggregates up to ∼2 mm across, comprising thin, platy to bladed crystals, with individual crystals averaging &lt;0.05 × 0.5 mm. Crystals are dominated by the form pinacoid {100} with a lesser {hk0} form and typically range from tan to creamy-yellow to white in color, less often being colorless, light or dark green, or white. Associated minerals include eudialyte-group minerals, aegirine, a clinoamphibole, serandite-group minerals, natrolite, molybdenite, pyrite, fluorite, sodalite, catapleiite, rinkite-group minerals, albite, villiaumite, pyrochlore, fluorapatite, microcline, and erdite. Kodamaite is translucent to transparent and has a silky to slightly vitreous luster, a white streak, a Mohs hardness of ∼2, a perfect {001} cleavage, a sectile tenacity, and a smooth fracture. The mineral shows a pronounced bluish-white fluorescence under both long- and medium-wave ultraviolet radiation, much less so under short-wave radiation. The calculated density is 2.16 g/cm3. Optically, kodamaite is presumed to be optically biaxial, but its optical properties could not be readily discerned. An average nD = 1.507 (for λ = 590 nm) was calculated using data from the refined crystal structure. It is non-pleochroic, has low birefringence (first-order gray) and crystals are length slow and show no dispersion. The Raman spectrum shows two strong peaks at 607 (O–Si–O stretch) and 1070 cm−1 (Si–O stretch), along with several weaker ones over the range of ∼100–500 cm−1 arising from a combination of alkali–O bonds and lattice vibrations. An average of 11 energy-dispersive spectroscopic analyses gave Na2O 8.55, CaO 16.80, Fe2O3 0.39, SiO2 60.76, F 2.57, Cl 0.11, H2O (calc.) 12.49, O=F −1.08, O=Cl −0.02, total 100.98 wt.%. The empirical formula [based on Σ16(Si+S6+) cations apfu] is Na3.14(Ca4.72Na1.30Fe0.08)Σ6.00(Si15.92S6+0.08)Σ16O36.09(OH3.82F0.13Cl0.08)Σ4.00F2·9H2O. The crystal-structure formula is NaNa2Ca2Ca2(NaCa)2Si16O36(OH)4F2·14−xH2O with x = ∼5, which may be simplified to Na3(Ca5Na)6Si16O36(OH)4F2·9H2O. The mineral is triclinic, crystallizing in space group P, with a = 9.609(2), b = 9.630(2), c = 15.739(3) Å, α = 75.21(3), β = 85.22(3), γ = 60.12(3) °, V = 1219.3(1) Å3, Z = 1. The strongest six lines of the X-ray powder-diffraction pattern [dobs in Å (Iobs) (hkl)] are: 15.197 (100) (001), 3.833 (7) (004), 3.068 (7) (005), 2.996 (18) , 2.780 (14) , 1.831 (9) . The crystal structure was refined to R = 8.43%, wR2 = 23.50% using single-crystal X-ray diffraction data (1578 reflections with Fo &gt; 4σFo). Kodamaite has a strongly layered crystal structure, with a motif based on single sheets of SiO4 tetrahedra (T) that are composed of regular, planar, six-membered rings (with a u6 arrangement; apices of the tetrahedra all point in a common direction, up = u). The regular rings of tetrahedra are linked in the (001) plane by SiO3(OH) tetrahedra, whose apical directions are opposite to those in the ring (down; d). Linkages between the two types of tetrahedra give rise to an irregular, six-membered ring of tetrahedra with a u2du2d arrangement. The combination of regular and irregular six-membered rings of tetrahedra produces a three-connected (63)4 net. Two symmetry-related T layers, designated T2 and −T2, sandwich a closest-packed layer of edge-sharing Naφ6 and Caφ6 octahedra and Naφ8 polyhedra (φ = undefined ligand; O, F), forming a quasi-layer of octahedra (O). Refinement of the crystal structure shows the Ca sites are dominated by Ca with small amounts of Na (9:1 ratio); one of the sites has an equal concentration of Ca and Na, suggesting ordering of the two in a single site. The arrangement of T and O layers produces a TOT module with the interlayer space being occupied by H2O groups. Charge-balance arguments and observed anion–anion distances support ordering of F in a single, distinct site. The complete crystal structure of kodamaite conforms to a scheme, which is similar to that of the closely related minerals martinite and ellingsenite. All are classed in the gyrolite supergroup, which includes minerals in five groups, all having similar but distinct crystal-structure features. The crystal-chemical relationships among the groups and implications in terms of the stacking of various modules are presented and discussed. Kodamaite is a late-stage mineral that developed under hydrothermal conditions at T &lt; 200 °C, most likely under high to very high alkalinity. The interaction of these fluids with pre-existing minerals, including pectolite, eudialyte-group minerals, and sodalite, is likely essential to the formation of the kodamaite.