Abstract

Hogarthite, (Na,K)2CaTi2Si10O26·8H2O, a new mineral of the lemoynite group, was discovered in vugs within metasomatically altered marble xenoliths in the Poudrette quarry, Mont Saint-Hilaire, La-Vallée-du-Richelieu RCM, Montérégie, Quebec, Canada. Crystals are bladed to blocky, average 0.05 × 0.15 × 2 mm in size, and range in color from tan to white to colorless. The mineral develops in dense, radiating crystal aggregates up to 0.5 × 3 mm in size, and is associated with calcite (several generations), quartz, haineaultite, labuntsovite-Mn, lemoynite, chabazite, and gmelinite-Na. Crystals are elongate along [100], flattened on {010} and are bounded by the forms pinacoid {010} (dominant), pinacoid {100} (minor), and pinacoid {001} (minor). The mineral has a white streak, is non-fluorescent under short-, medium-, and long-wave ultraviolet radiation, is translucent, and has a satiny to silky to subvitreous luster. It has a brittle fracture, a perfect {010} cleavage, a hackly to splintery fracture, an estimated hardness of 4, and Dcalc = 2.40(1) g/cm3. Hogarthite is biaxial (+) with α 1.567(1), β 1.591(1), and γ 1.618(1) with 2Vmeas = 87(1)º and 2Vcalc = 88(1)º. The optical orientation is X = b, Y˄c = 15º (measured in the obtuse angle β), Z = a. A total of 21 analyses, obtained from seven crystals, gave an average (range) of (wt.%): Na2O 2.37 (1.69–2.92), K2O 2.88 (2.61–3.17), CaO 6.00 (5.40–6.31), TiO2 14.44 (13.70–15.83), ZrO2 1.11 (0.48–1.73), Nb2O5 0.78 (0.42–1.24), SiO2 59.27 (57.32–60.64), H2O 14.10 (calc.), total 100.95. The empirical formula (based on 34 anions) is (Na0.78K0.620.51Ca0.09)Σ2.00Ca(Ti1.85Zr0.09Nb0.06)Σ2.00Si10.09O26·8H2O, and the simplified formula is (Na,K)2CaTi2Si10O26·8H2O. The presence of (OH) and H2O were confirmed via refinement of the crystal-structure and data from combined FTIR and Raman spectroscopies. The mineral crystallizes in space group C2/m with a 10.1839(5), b 15.8244(6), c 9.1327(7) Å, β 104.463(2)º, V 1425.1(1) Å3, and Z = 2. The strongest six lines on the X-ray powder diffraction pattern [d in Å (I) (hkl)] are: 8.835 (85) (001), 7.913 (100) (020), 6.849 (70) (

1¯
11), 4.336 (45) (
1¯
31,
1¯
12), 3.514 (80) (221), 3.426 (55) (
2¯
22,
1¯
32). The crystal structure, refined to R = 0.049 for 1335 reflections (Fo > 4σFo), consists of a silicate component, composed of interconnected, non-planar 10-membered rings, arranged into thick slabs and stacked along [001]. These are interleaved with layers of independent TiO6 octahedra, producing a framework of composition [TiSi5O13]2–. Numerous channels within the framework, notably the continuous ones developed by 10-membered silicate rings, are occupied by Na, Ca, and H2O groups. Hogarthite is considered to be a product of late-stage alkaline fluid enriched in SiO2 and TiO2, and forming under conditions of low P at T < 200 ºC, possibly through crystallization of a gel.

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