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Tinaksite and Tokkoite: X‐ray Powder Diffraction, Optical and Vibrational Properties

https://doi.org/10.3390/cryst12030377

“In this study, natural tinaksite (K2Ca2NaTi[Si7O18OH]O) and tokkoite (K2Ca4[Si7O18OH](OH,F)) collected in charoite rocks of the Murun alkaline massif (Siberia, Russia) were examined by X-ray diffraction and optical and vibrational spectroscopic methods. A comparative analysis of the experimental diffraction patterns with respect to the calculated X-ray powder diffraction patterns was carried out for tinaksite and tokkoite powders. The shift in the diffraction peaks of tinaksite is explained by the smaller values of the unit cell parameters a and b as compared with those of tokkoite. A similar shift of the peaks is also observed in the Raman and infrared absorption spectra; however, this feature is explained by the difference in the chemical composition of the minerals. The shoulder in the absorption spectra at about 800 nm in tinaksite and 700 nm in tokkoite corresponds to the presence of Mn2+ and Fe3+ absorption bands, the presence of which determines the color of tinaksite and tokkoite. The luminescence band with a maximum at about 540–550 nm in the photoluminescence spectra is related to Mn2+ centers, while an additional band at about 610 nm can be associated with Ti3+ centers in tinaksite. The intensity of the Fe3+ ESR signal increases in both samples after heating, while the intensities of the bands associated with OH groups decrease in tinaksite and tokkoite. This characteristic is the result of iron oxidation and dehydrogenation reaction.”

“The X-ray powder diffraction data of the studied samples were collected at room temperature with a Bruker D8 ADVANCE (Bruker AXS, Berlin, Germany) powder diffractometer (Cu Kα radiation, 40 kV, 40 Ma) and linear VANTEC detector. The samples for measurement were prepared by packing and leveling the powder in a special cuvette. Profiles were obtained between 3° and 120° 2θ. The step size of 2θ was 0.02°, and the counting time was 4 s per step. The measured patterns were used without any corrections or other processing. Diffraction patterns were analyzed using the EVA V4.2.1 software suite [22] (Bruker AXS, Madison, WI, USA). To establish the features of powder diffraction patterns and compare them with those previously obtained, the Powder Diffraction File (PDF-2, Release 2007) database, maintained and updated by the International Center for Diffraction Data [23,24], was used. In this study, the VESTA (version 4.3.0) software [20] was used to simulate the X-ray diffraction patterns of tinaksite and tokkoite using the crystal structure models of [18,21]. The unit cell parameters of the studied samples were determined by the Rietveld method using TOPAS 4.2 (Bruker AXS, Berlin, Germany) [25]. Refinements were stable and gave relatively low R-factors (4.7 and 5.1% for tinaksite and tokkoite, respectively). Pseudo-Voigt line shapes were used for the peaks. A three-parameter 2nd order polynomial function was used for the background. The measured patterns were used without any corrections or other processing; Lorentz polarization, absorption, and sample displacement corrections were applied to the calculated patterns.”

3.1. X-ray Powder Diffraction

Powder X-ray diffraction data for the studied samples are represented in Figure 3 and Tables S1 and S2 of Supplementary Materials. Peak positions are expressed as d-spacing in Å. Intensities are given in relative percentages. It is noted that the reflections and their intensities, obtained experimentally, were in agreement with the results of the powder pattern simulation on the basis of the structural model determined by [18] using single-crystal X-ray diffraction. The unit cell parameters derived from the X-ray powder diffraction data for tinaksite and tokkoite powders were consistent with the data obtained for single crystals by [18] (see Table 1).
Figure 3. (a) X-ray powder diffraction patterns (XRPDP) of tinaksite and tokkoite; (b) experimental and calculated XRPDP of tinaksite; (c) experimental and calculated XRPDP of tokkoite. The range of 5–70° 2θ is shown.
Table 1. Unit cell parameters, space group, and crystal chemical formulas of tinaksite and tokkoite as compared with the data in the literature and PDF-2 files. PDF 00–018-1382—[1], tinaksite from Murun massif, Cr Kα radiation; PDF 00–054-0646—Karimova O. IGEM RAS, Moscow, Russia, ICDD Grant-in-Aid, 2002, tinaksite from the Khibiny massif, Cu Kα radiation; PDF 01–072-1823—calculated from ICSD using POWD-12++ (2004), [28], tinaksite from Murun massif, Cu Kα radiation; PDF 01-071-1758—calculated from ICSD using POWD-12++ (2004), [29], tinaksite from Murun massif, Cu Kα radiation; PDF 00-040-0517—[2], tokkoite from Murun massif, unknown radiation; PDF 01-079-1981—calculated from ICSD using POWD-12++ (2004), [30], tokkoite from Murun massif, Cu Kα radiation.
a (Å) b (Å) c (Å) α (°) β (°) γ (°) Sp. Gr. Source
Tinaksite
10.371 (6) 12.167 (6) 7.060 (5) 90.91 (2) 99.37 (3) 92.79 (3) P1¯�1¯ This work
10.375 (2) 12.190 (2) 7.057 (1) 90.75 (3) 99.25 (3) 92.81 (3) P1¯�1¯ [18]
10.370 (1) 12.162 (1) 7.057 (1) 90.89 (1) 99.20 (1) 92.80 (1) P1¯�1¯ [18]
10.361 (2) 12.153 (2) 7.044 (1) 90.79 (2) 99.22 (2) 92.83 (2) P1¯�1¯ [31]
10.350 12.170 7.050 91.00 99.33 92.50 P1¯�1¯ PDF 00-018-1382
10.369 12.177 7.052 90.00 99.00 92.00 P1¯�1¯ PDF 00-054-0646
10.350 12.170 7.050 91.00 99.33 92.50 P1 PDF 01-072-1823
10.377 12.166 7.059 90.91 99.30 92.76 P1¯�1¯ PDF 01-071-1758
Tokkoite
10.436 (5) 12.474 (7) 7.085 (4) 89.98 (3) 99.50 (3) 92.89 (3) P1¯�1¯ This work
10.423 (1) 12.492 (1) 7.116 (1) 89.89 (1) 99.69 (1) 92.95 (1) P1¯�1¯ [18]
10.424 (1) 12.477 (1) 7.113 (1) 89.88 (1) 99.68 (1) 92.99 (1) P1¯�1¯ [18]
10.423 (1) 12.462 (1) 7.106 (1) 89.98 (1) 99.68 (1) 92.95 (1) P1¯�1¯ [18]
10.370 25.390 7.270 91.67 100.66 92.09 P1¯�1¯ PDF 00-040-0517
10.438 12.511 7.112 89.92 99.75 92.89 P1¯�1¯ PDF 01-079-1981

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