Silicate Tubes in the Crystal Structure of Manaksite
Unique tubular ribbons silicate radicals with composition [Si8O20]∞ have been found in the crystal structure of manaksite. Group of minerals with tubular silicate radicals is not large. Miserite (Scott 1976 ), canasite (Rozdestvenskaya et al. 1988 ), fran
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Unique tubular ribbons silicate radicals with composition [Si8 O20 ]∞ have been found in the crystal structure of manaksite. Group of minerals with tubular silicate radicals is not large. Miserite (Scott 1976), canasite (Rozdestvenskaya et al. 1988), frankamenite (Rozdestvenskaya et al. 1996), narsarsukite (Peacor and Buerger 1962), and manaksite are minerals that contain different types of silicate tubes. Among them, manaksite is the only structure containing tubes with a hexagonal cross-section. A rare mineral manaksite, ideally NaKMnSi4 O10 , was discovered in 1992 in Lovozero alkali massif, Kola Peninsula, Russia (Khomyakov et al. 1992). Manaksite is a Mn analog of fenaksite, NaKFeSi4 O10 , which was found only in Khibiny alkaline massif, Russia (Dorfman et al. 1959; Sokolova et al. 1993). Recently, intermediate compositions between these minerals were described and an isomorphous series fenaksite-manaksite was established (Ageeva 2000). Crystal structure of fenaksite was studied twice (Golovachev et al. 1970, Prencipe et al. 1998). In current study, a CCD-based detector mounted on an automated single-crystal diffractometer has been used to collect X-ray diffraction data, resulting in successful elucidation of the structure. Crystals with different Fe content were chosen from the mineralogical collection of Dr. Maya Sokolova and Dr. Olga Ageeva (IGEM RAS, Moscow, Russia). Crystals were mounted on a Bruker three-circle diffractometer equipped with a SMART APEX CCD detector, using monochromated MoKα X-radiation. The unit cell dimensions were refined using least-squares techniques (Table 1). The three-dimensional data were reduced and corrected for Lorentz, polarization, and background effects using the Bruker program SAINT. An empirical absorption correction was done for crystal by modeling it as an ellipsoid. Oxana Karimova Department of Mineralogy, Institute of Geology of Ore Deposits Russian Academy of Science, 35 Staromonetny, Moscow 119017, Russia, e-mail: [email protected] Peter C. Burns Department of Civil Engineering and Geological Sciences, University of Notre Dame, 156 Fitzpatrick Hall, Notre Dame, IN 46556-0767, USA
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Table 1 Crystallographic data and refinement parameters for manaksite Sample
1
2
˚ a (A) ˚ b (A) ˚ c (A) α (◦ ) β (◦ ) γ (◦ ) ˚ 3) V (A Space group Z Total Ref. Unique Ref. Unique |F0 | ≥ 4σF R1 wR2 S
6.9774(5) 8.1778(6) 9.9664(8) 105.782(2) 99.743(2) 114.440(1) 471.74(6) P-1 2 9520 3798 2622 0.029 0.055 0.835
6.9752(6) 8.1505(7) 9.9464(8) 105.803(2) 99.990(2) 114.332(2) 468.71(4) P-1 2 9514 3793 3673 0.034 0.084 1.023
The Bruker SHELXTL Version 5 system of programs was used for the determination and refinement of crystal structure. Structure was solved by direct methods and refined to R = 0.029 (1) and 0.034 (2) (Table 1). After collecting the X-ray-diffraction data, the crystal 1 was polished and analyzed with an electron microprobe. As expected K, Na, Si, Fe and Mn were major elements. Quantitative analysis gave the following: SiO2 63.6, Na2 O 7.76, M
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