Publication Type:

Journal Article

Authors:

Haring, Monika M. M.; McDonald, Andrew M.

Source:

American MineralogistAmerican Mineralogist, Mineralogical Society of America, Washington, DC, United States, Volume 102, Number 11, p.2333-2340 (2017)

ISBN:

0003-004X

Keywords:

Albite, Canada, Carbonates, chemical composition, chemical fractionation, coexisting minerals, crystal chemistry, crystal form, crystal structure, eastern canada, feldspar group, formula, framework silicates, hydroxides, igneous rocks, lattice parameters, metals, mica group, Mont-Saint-Hilaire Quebec, muscovite, nepheline syenite, new minerals, niobium, optical properties, oxides, physical properties, plagioclase, plutonic rocks, pyrite, pyrrhotite, quartz, quebec, Rouville County Quebec, sheet silicates, siderite, silica minerals, silicates, space groups, sulfides, syenites, tantalum, unit cell, x-ray diffraction data

Abstract:

Charleshatchettite, CaNb (sub 4) O (sub 10) (OH) (sub 2) .8H (sub 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 ( approximately 0.15-0.20 mm in diameter) composed of radiating crystals with individual crystals having average dimensions of approximately 0.002 X 0.010 X 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 alpha ' = approximately 1.72(2) and gamma ' = approximately 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 (sub 2) O (sub 3) 0.13 (b.d.-0.49), SiO (sub 2) 1.04 (0.49-1.88), TiO (sub 2) 3.64 (2.45-5.05), Nb (sub 2) O (sub 5) 68.07 (64.83-71.01), and H (sub 2) O (calc) 22.96, total 104.04 wt% gives the average empirical formula: (Ca (sub 1.00) Mg (sub 0.04) ) (sub Sigma 1.04) (Nb (sub 3.62) Ti (sub 0.32) Si (sub 0.12) Al (sub 0.02) ) (sub Sigma 4.08) O (sub 10) (OH) (sub 2) .8H (sub 2) O (based on 20 anions). This is similar to that of hochelagaite (CaNb (sub 4) O (sub 11) .nH (sub 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 beta = 103.958(3) degrees , space group C2/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) (sub 6) octahedra, linked through shared corners to adjacent clusters, forming layers of Nb(O,OH) (sub 6) octahedra. These alternate along [100] with layers composed of Ca(H (sub 2) O) (sub 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 (<150 degrees C) aqueous fluids with an alkali-, Nb-rich precursor under slightly reducing conditions and a highly alkaline pH. The precursor mineral(s) is unknown but is considered to have been Nb-dominant, relatively unstable under slightly reducing as well as alkaline conditions, and likely itself would have been a product of near-complete Nb/Ta fractionation due to the paucity of Ta in charleshatchettite. Charleshatchettite is crystallochemically related to Sandia Octahedral Molecular Sieves [SOMS; Na (sub 2) Nb (sub 2-x) M (sub x) O (sub 6-x) (OH) (sub x) .H (sub 2) O with M = Ti, Zr, Hf], a group of synthetic compounds with strong ion exchange capabilities.

Notes:

GeoRef, Copyright 2018, American Geological Institute.<br/>2018-021725<br/>ancylite<br/>charleshatchettite<br/>franconite<br/>hochelagaite<br/>Sandia Octahedral Molecular Sieves