Bobshannonite, Na₂KBa(Mn,Na)₈(Nb,Ti)₄(Si₂O₇)₄O₄(OH)₄(O,F)₂, a new TS-block mineral from Mont Saint-Hilaire, Québec, Canada: Description and crystal structure

Abstract Bobshannonite, Na 2 KBa(Mn,Na) 8 (Nb,Ti) 4 (Si 2 O 7 ) 4 O 4 (OH) 4 (O,F) 2 , is a new TS-block mineral from Mont Saint-Hilaire, Québec, Canada. It occurs as blocky crystals 0.5–1 mm across,perched on sérandite and albite. Other associated minerals are epididymite, catapleiite, aegirine, kupletskite, rhodochrosite and rhabdophane-(Ce). Bobshannonite occurs as vitreous to frosty, transparent to translucent very pale brown to orange brown crystals, has a very pale brown streak, hackly fracture and does not fluoresce under cathode or ultraviolet light. Cleavage is {001} very good, no parting was observed, Mohs hardness is ∼4, it is brittle and D calc. = 3.787 g/cm 3 . Crystals are twinned extensively and do not extinguish in cross-polarized light. Bobshannonite is triclinic, C 1, a = 10.839(6), b = 13.912(8), c = 20.98(1) Å, α = 89.99(1), β = 95.05(2), γ = 89.998(9)°, V = 3152(5) Å 3 . The six strongest reflections in the powder X-ray diffraction data [ d (Å),I, ( hkl )] are: 2.873, 100, (241, 241, 044, 044, 241, 241); 3.477, 60, (006); 3.193, 59, (224, 224); 2.648, 40, (402, 243, 243); 2.608, 35, (008, 226, 226); 1.776, 30, (249). Chemical analysis by electron microprobe gave Ta 2 O 5 0.52, Nb 2 O 5 19.69,TiO 2 5.50, SiO 2 26.31, Al 2 O 3 0.06, BaO 7.92, ZnO 1.02, FeO 0.89, MnO 26.34, MgO 0.06, Rb 2 O 0.42, K 2 O 2.38, Na 2 O 4.05, F 0.70, H 2 O calc. 1.96, O = –0.29, total 97.53 wt.%, where the H 2 O content was calculated from the crystal-structure analysis. The empirical formula on the basis of 38 anions is Na 1.89 (K 0.93 Rb 0.08 ) Σ1.01 Ba 0.95 (Mn 6.85 Na 0.52 Zn 0.23 Fe 0.23 2+ Mg 0.03 Al 0.02 ) Σ7.88 (Nb 2.73 Ti 1.27 Ta 0.04 ) Σ4.04 (Si 8.07 O 28 )O 9.32 H 4.01 F 0.68 , Z = 4. The crystal structure was refined to R 1 = 2.55% on the basis of 7277 unique reflections [ F > 4σ( F )] and can be described as a combination of a TS (Titanium Silicate) block and an I (Intermediate) block. The TS block consists ofHOH sheets (H – heteropolyhedral, O – octahedral). The topology of the TS block is as in Group II of the Ti disilicates: Ti + Nb = 2 a.p.f.u. per (Si 2 O 7 ) 2 [as defined by Sokolova (2006)]. In the O sheet, ten [6] M O sitesare occupied mainly by Mn, less Na and minor Zn, Fe 2+ , Mg and Al, with <M O –ϕ> = 2.223 Å. In the H sheet, four [6] M H sites are occupied by Nb and Ti (Nb > Ti), with <M H –ϕ> = 1.975 Å,and eight [4] Si sites are occupied by Si, with <Si–O> = 1.625 Å. The M H octahedra and Si 2 O 7 groups constitute the H sheet. The TS blocks link via common vertices of M H octahedra. In the I block, Ba and Kare ordered at the A P (1) and A P (2) sites with Ba:K = 1:1 and the two B P sites are occupied by Na. The ideal composition of the I block is Na 2 KBa a.p.f.u. Bobshannonite, perraultite, surkhobite and jinshajiangite are topologically identical Group-II TS-block minerals. Bobshannonite is the Nb-analogue of perraultite. The mineral is named bobshannonite after Dr. Robert (Bob) D. Shannon (b. 1935), in recognition of his major contributions to the field of crystal chemistry in particular and mineralogy in general through his development of accurate and comprehensive ionic radii and his work on dielectric properties of minerals.

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