Charleshatchettite, CaNb4O10(OH)2·8H2O, a new mineral from Mont Saint-Hilaire, Québec, Canada: Description, crystal-structure determination, and origin

Abstract

Charleshatchettite, CaNb 4 O 10 (OH) 2 ·8H 2 O, is a new mineral related to franconite and hochelagaite, discovered on a fracture surface of a nepheline syenite at Mont Saint-Hilaire, Quebec, Canada. The mineral occurs in white globules (~0.15–0.20 mm in diameter) composed of radiating crystals with individual crystals having average dimensions of ~0.002 × 0.010 × 0.040 mm. Crystals are euhedral, bladed (flattened on [100]), and are transparent to translucent. The mineral is associated with albite, quartz, muscovite, pyrrhotite, pyrite, ancylite-(Ce), and siderite. Charleshatchettite is inferred to be biaxial (−) with α′ = ~1.72(2) and γ′ = ~1.82(2). Data from chemical analyses (SEM-EDS, n = 8): CaO 7.96 (7.04–8.63), MgO 0.24 (0.08–0.78), Al 2 O 3 0.13 (b.d.–0.49), SiO 2 1.04 (0.49–1.88), TiO 2 3.64 (2.45–5.05), Nb 2 O 5 68.07 (64.83–71.01), and H 2 O (calc) 22.96, total 104.04 wt% gives the average empirical formula: (Ca 1.00 Mg 0.04 ) ∑1.04 (Nb 3.62 Ti 0.32 Si 0.12 Al 0.02 ) ∑4.08 O 10 (OH) 2 ·8H 2 O (based on 20 anions). This is similar to that of hochelagaite (CaNb 4 O 11 · n H 2 O), although the two are readily distinguished by their powder X-ray diffraction patterns. Results from single-crystal X-ray diffraction analysis give a = 21.151(4), b = 6.496(2), c = 12.714(3) A, and β = 103.958(3)°, space group C 2/ c (no. 15). The crystal structure, refined to R = 5.64%, contains 1 Ca site, 2 distorted octahedral Nb sites, and 10 O sites. It consists of clusters of four edge-sharing Nb(O,OH) 6 octahedra, linked through shared corners to adjacent clusters, forming layers of Nb(O,OH) 6 octahedra. These alternate along [100] with layers composed of Ca(H 2 O) 8 polyhedra, the two being linked together by H-bonding. Charleshatchettite is a late-stage mineral, interpreted to have developed through the interaction of low- T ( 2 Nb 2−x M x O 6−x (OH) x ·H 2 O with M = Ti, Zr, Hf], a group of synthetic compounds with strong ion exchange capabilities.