Melansonite, (Na,□)□2KZrSi8O19·5H2O, a New Member of the Rhodesite Group, from Mont Saint-Hilaire, Québec, Canada: Characterization, Crystal-Structure Determination, and Origin
Authors: Gore Thomas E. , McDonald Andrew M. 
Published: July 2023

Melansonite, ideally (Na,□)□2KZrSi8O19·5H2O, is a new mineral discovered at the Poudrette Quarry, Mont Saint-Hilaire, Québec, Canada. It develops as loose clusters of square to rectangular, euhedral plates with average dimensions of 35 × 50 μm. Individual crystals are ∼2 μm in thickness, dominated by the pinacoid {100}, and colorless to pale yellow in color. Associated minerals include albite, calcite, aegirine, quartz, gaidonnayite, a sodic clinoamphibole, an apophyllite-group mineral, and pyrrhotite (?). Melansonite has a vitreous luster, is transparent, and has a white streak. It exhibits a perfect {100} cleavage, is brittle, and has an irregular fracture. The mineral shows a green fluorescence under short-wave radiation. The calculated density is 2.352 g/cm3. Optically, it is non-pleochroic and presumed to be optically biaxial, with α = 1.536(1), β = 1.537(1), γ = 1.538(1) (for λ = 590 nm), 2Vcalc = 89.9(1)°. No dispersion was noted. The optical orientation is inferred to be Z = b, Y = c, Z = a, based on the observed symmetry and crystal morphology. An average of 15 electron microprobe analyses gave Na2O 2.82, Al2O3 6.32, SiO2 53.50, SO3 0.91, K2O 2.64, CaO 1.75, MnO 1.09, Fe2O3 0.46, Y2O3 0.07, ZrO2 15.41, BaO 1.35, H2O (calc.) 11.32, total 97.64 wt.%. The empirical formula (based on 24 anions) is: (Na0.63Ca0.25Mn0.12)Σ1.00(□1.91Na0.09)Σ2.00(□0.48K0.45Ba0.07)Σ1.00(Zr0.99Fe0.05Y0.01)Σ1.05(Si7.08Al0.99S0.09)Σ8.16O19·5H2O. The mineral is orthorhombic, crystallizing in the space group Pmma (No. 51), with a = 24.063(5), b = 6.9820(14), c = 6.5260(13) Å, V = 1096.4(4) Å3, Z = 2. The strongest six lines of the X-ray powder-diffraction pattern [d in Å (I) (hkl)] are 11.988 (100) (200), 3.060 (29) (121), 6.971 (27) (010), 2.881 (25) (321), 6.529 (19) (001), 3.429 (15) (601). A Raman spectrum is presented; it is very similar to that of monteregianite-(Y), with shifts between the two minerals being consistent with Zr ↔ Y substitution. The crystal structure was refined to R = 5.39%, wR2 = 15.45%. It is strongly layered, with a motif based on double sheets of tetrahedra (T), single sheets of octahedra (O), and extra-framework cations. Double layers of tetrahedra (T) are composed of both eight- and four-membered rings of corner-sharing (Si,Al)O4 tetrahedra that form infinite sheets on (100). Individual layers of the double sheet are joined via branching Si(2) tetrahedra along [100]. Channels created by eight-membered rings of (Si,Al)O4 tetrahedra are occupied by Na, K, and H2O groups. The O layers are composed of edge-sharing Mϕ6 (M = Zr, Na, ϕ = O2−, H2O) octahedra that form infinite sheets on (001). Alternating T and O layers give rise to a TOT topology reminiscent of that observed in micas and characteristic of minerals, including rhodesite, delhayelite, hydrodelhayelite, and monteregianite-(Y), with which melansonite shares a crystal-chemical relationship. Melansonite is paragenetically a late-stage mineral that is considered to have formed as a result of the interaction of late-stage alkaline fluids, enriched in SiO2 and ZrO2, that interacted with a marble xenolith under conditions of low P and T (&lt;200 °C).