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Abstract:

Steedeite, ideally NaMn (sub 2) Si (sub 3) BO (sub 9) (OH) (sub 2) , is a new mineral discovered in altered sodalite syenite at the Poudrette quarry, La Vallee-du-Richelieu, Monteregie (formerly Rouville County), Quebec, Canada. Crystals of steedeite are colorless to pale pink, and acicular with average dimensions of 0.006 X 0.011 X 0.51 mm. They occur as radiating to loose, randomly oriented groupings within vugs associated with aegirine, nepheline, sodalite, eudialyte-group minerals, analcime, natron, pyrrhotite, catapleiite, and two other unidentified minerals temporarily designated UK78 and UK80. Steedeite is transparent to translucent with a vitreous luster and has a weak pale green to pale yellow fluorescence under medium-wave radiation. No partings or cleavages were observed, although crystals exhibit an uneven fracture. The calculated density is 3.106 g/cm (super 3) . Steedeite is nonpleochroic, biaxial with n (sub min) = 1.636(2) and n (sub max) = 1.656(2) and a positive elongation. Chemical analyses (n = 14) from seven crystals gave an average (range, standard deviation) of: Na (sub 2) O 7.51 (6.78-8.32, 0.44), CaO 0.17 (0.08-0.22, 0.03), MnO 31.02 (29.91-32.83, 0.93), FeO 0.86 (0.76-1.01, 0.07), SiO (sub 2) 46.34 (40.39-49.29, 2.56), S 0.39 (*b.d.- 2.36, 0.71) (*b.d. = below detection), B (sub 2) O (sub 3) (calc.) 8.73, and H (sub 2) O (calc.) 4.52, total 99.53 wt.%. The empirical formula is: Na (sub 0.97) (Mn (sub 1.75) Fe (sub 0.05) Ca (sub 0.01) ) (sub Sigma 1.83) (Si (sub 3.07) S (sub 0.02) ) (sub Sigma 3.09) BO (sub 9) (OH) (sub 2) , or ideally aMn (sub 2) Si (sub 3) BO (sub 9) (OH) (sub 2) . The presence of both B and OH in steedeite were inferred from the refinement of the crystal structure. The Raman spectrum for steedeite show bands at 3250-3500 cm (super -1) attributed to O-H stretching, strong sharp bands at 575-750 cm (super -1) and 825-1075 cm (super -1) attributed to Si-O bonds and possibly B-O bonding, as well as weak to strong bands at 50-500 cm (super -1) attributed to Na-O/Mn-O bonds. Steedeite crystallizes in space group P 1( with a 6.837(1), b 7.575(2), c 8.841(2) Aa, alpha 99.91(3), beta 102.19, gamma 102.78(3) degrees , V 424.81 Aa (super 3) , and Z = 2. The strongest six lines on the X-ray powder-diffraction pattern [d in Aa (I) (hkl)] are: 8.454 (100) (00 1), 7.234 (39) (00 1), 3.331 (83) (1 2 1, 0 1 2, 20 1, 1 1 2), 3.081 (38) (0 1 2), 2.859 (52) (0 1 3), and 2.823 (80) (21 1). The crystal structure of steedeite was refined to R = 1.68% and wR (sub 2) = 4.96% for 2409 reflections (F (sub o) > 4sigma F (sub o) ). It is based on silicate chains with a periodicity of three (i.e., dreier chain) consisting of four-membered borosilicate rings of composition [BSi (sub 3) O (sub 9) (OH) (sub 2) ] (super 5-) . The borosilicate chains are classed as single loop-branched dreier chains that are linked together through shared corners to bands of edge-sharing MnO (sub 5) (OH) octahedra. Steedeite is a chain silicate mineral closely related to the serandite-pectolite series of the pyroxenoid group and is the first mineral found to contain single loop-branched dreier silicate chains.

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GeoRef, Copyright 2018, American Geological Institute.<br/>2014-046921<br/>catapleiite<br/>La Vallee-du-Richelieu<br/>natron<br/>Poudrette Quarry<br/>pyroxenoids<br/>serandite<br/>steedeite