American Mineralogist, Volume 94, pages 1361–1370, 2009 Kumtyubeite Ca5(SiO4)2F2—A new calcium mineral of the humite group from Northern Caucasus, Kabardino-Balkaria, Russia IrIna О. GaluskIna,1,* BIljana lazIc,2 Thomas armBrusTer,2 evGeny v. GaluskIn,1 vIkTor m. Gazeev,3 aleksander e. zadov,4 nIkolaI n. PerTsev,3 lIdIa jeżak,5 roman WrzalIk,6 and anaToly G. GurBanov3 Faculty of Earth Sciences, Department of Geochemistry, Mineralogy and Petrography, University of Silesia, Będzińska 60, 41-200 Sosnowiec, Poland 2 Mineralogical Crystallography, Institute of Geological Sciences, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland 3 Institute of Geology of Ore Deposits, Geochemistry, Mineralogy and Petrography (IGEM) RAS, Staromonetny 35, 119017 Moscow, Russia 4 OOO NPP TEPLOCHIM, Dmitrovskoye av. 71, 127238 Moscow, Russia 5 Institute of Geochemistry, Mineralogy and Petrology, University of Warsaw, al. Żwirki i Wigury 93, 02-089 Warszawa, Poland 6 Institute of Physics, University of Silesia, Uniwersytecka 4, 40-007 Katowice, Poland 1 aBsTracT Kumtyubeite, Ca5(SiO4)2F2—the fluorine analog of reinhardbraunsite with a chondrodite-type structure—is a rock-forming mineral found in skarn carbonate-xenoliths in ignimbrites of the Upper Chegem volcanic structure, Kabardino-Balkaria, Northern Caucasus, Russia. The new mineral occurs in spurrite-rondorfite-ellestadite zones of skarn. The empirical formula of kumtyubeite from the holotype sample is Ca5(Si1.99Ti0.01)∑2O8(F1.39OH0.61)∑2. Single-crystal X-ray data were collected for a grain of Ca5(SiO4)2(F1.3OH0.7) composition, and the structure refinement, including a partially occupied H position, converged to R = 1.56%: monoclinic, space group P21/a, Z = 2, a = 11.44637(18), b = 5.05135(8), c = 8.85234(13) Å, β = 108.8625(7)°, V = 484.352(13) Å3. For direct comparison, the structure of reinhardbraunsite Ca5(SiO4)2(OH1.3F0.7) from the same locality has also been refined to R = 1.9%, and both symmetry independent, partially occupied H sites were determined: space group P21/a, Z = 2, a = 11.4542(2), b = 5.06180(10), c = 8.89170(10) Å, β = 108.7698(9)°, V = 488.114(14) Å3. The following main absorption bands were observed in kumtyubeite FTIR spectra (cm–1): 427, 507, 530, 561, 638, 779, 865, 934, 1113, and 3551. Raman spectra are characterized by the following strong bands (cm–1) at: 281, 323, 397 (ν2), 547 (ν4), 822 (ν1), 849 (ν1), 901 (ν3), 925 (ν3), 3553 (νOH). Kumtyubeite with compositions between Ca5(SiO4)2F2 and Ca5(SiO4)2(OH1.0F1.0) has only the hydrogen bond O5-H1···F5′, whereas reinhardbraunsite with compositions between Ca5(SiO4)2(OH1.0F1.0) and Ca5(SiO4)2(OH)2 has the following hydrogen bonds: O5-H1···F5′, O5-H1···O5′, and O5-H2···O2. Keywords: Kumtyubeite, new mineral, reinhardbraunsite, crystal structure, chondrodite, composition, Raman, FTIR, Northern Caucasus, Russia InTroducTIon Kumtyubeite, Ca5(SiO4)2F2, the calcium analog of chondrodite, was discovered in contact-metasomatic rocks formed by interaction of carbonate xenoliths with subvolcanic magma and volcanites of the Upper Chegem caldera structure, Kabardino-Balkaria, Northern Caucasus, Russia (Gazeev et al. 2006). Kumtyubeite is the fluorine analog of reinhardbraunsite, Ca5(SiO4)2(ОН)2. Reinhardbraunsite (Karimova et al. 2008) and Ca7(SiO4)3(ОН)2 (IMA2008-38, Galuskin et al. in preparation), the Ca and hydroxyl analog of humite, were also discovered in the same xenoliths. Minerals belonging to the humite group are characterized by the crystal-chemical formula nA2SiO4 × A(F,OH)2, where А = Mg, Fe2+, Mn2+, Zn, Ca, and others with n = 1, 2, 3, or 4. The humite group of minerals, sensu stricto, comprises nesosilicates with cations in octahedral sites and additional (OH)–, F– groups * E-mail: irina.galuskina@us.edu.pl 0003-004X/09/0010–1361$05.00/DOI: 10.2138/am.2009.3256 (Jones et al. 1969; Ribbe and Gibbs 1971). From a structural point of view (Thompson 1978), they are polysomes assembled of norbergite and olivine modules. Strunz and Nickel (2001) classify the corresponding minerals as norbergite-chondrodite group distinct from the ribbeite-leucophoenicite-jerrygibbsite group, which may also be derived by the same crystal-chemical formula given above but the M(O,OH)6 chains are linked by dimers of edge-sharing SiO4 tetrahedra with only one Si site of the dimer being statistically occupied. Among minerals, hydroxyl analogs of the humite group seem to predominate. Only in case of Mg endmembers does a complete polysomatic series of F-dominant phases exist: norbergite Mg 3 (SiO 4 )(F,OH), chondrodite (Mg,Fe2+)5(SiO4)2(F,OH)2, humite (Mg,Fe2+)7(SiO4)3(F,OH)2, and clinohumite (Mg,Fe2+)9(SiO4)4(F,OH), whereas only one hydroxyl analog of the Mg end-members is known: hydroxylclinohumite (Gekimyants et al. 1999). Within the chondrodite series three mineral species have been defined up to now: alleghanyite Mn5(SiO4)2(OH)2, chondrodite (Mg,Fe2+)5(SiO4)2(F,OH)2, 1361 1362 GALUSKINA ET AL.: KUMTyUBEITE—A NEW CALCIUM MINERAL OF THE HUMITE GROUP and reinhardbraunsite Ca5(SiO4)2(OH,F)2. Reinhardbraunsite was discovered in altered xenoliths within volcanic rocks of Eifel, Germany (Hamm and Hentschel 1983; Kirfel et al. 1983); later it was described in pyrometamorphic rocks of Hatrurim, Negev, Israel (Sokol et al. 2007). Analytical data of alleged high-F reinhardbraunsite were presented from carbonatites of Fort Portal, Uganda (Barker and Nixon 1989), but the low total and the significant Na content cast some doubts on its identity. Analogs of kumtyubeite were found in burned coal dumps at Kopeisk, Southern Ural, Russia (Chesnokov et al. 1993). In addition, the phase Ca5(SiO4)2F2, usually reported as 2C2S⋅CaF2, is a component of cement clinker (Gutt and Osborne 1966; Taylor 1997; Watanabe et al. 2002). Gutt and Osborne (1966) also published approximate cell dimensions and optical data for the synthetic product indicating identity with kumtyubeite. Kumtyubeite Ca5(SiO4)2F2 was approved as a new mineral species in December 2008 by the IMA-CNMNC. In this paper, composition, crystal structure, spectroscopy, and mineral association of this new mineral are reported. The name kumtyubeite derives from Kum-Tyube—the name of a mountain plateau situated at the locality where this mineral was found. Type material is deposited in the collection of the Museum of Natural History, Bern, specimen number NMBE 39572 and in the collection of the Fersman Mineralogical Museum, Moscow, specimen number 3732/1. meThods of InvesTIGaTIon Morphology and mineral association were investigated with a Philips XL 30 ESEM scanning electron microscope. Chemical analyses were carried out by means of a СAMECA SX-100 electron microprobe (WDS mode, 15 kV, 15 nA, 1–3 µm beam diameter). The following standards and lines were used: TiKα = rutile; CaKα, SiKα = wollastonite, diopside; MgKα = diopside; PKα = apatite; NaKα = albite; FeKα = Fe2O3, andradite; MnKα = rhodonite; ClKα = tugtupite; FKα = synthetic fluorphlogopite; CrKα = synthetic Cr2O3; SKα = barite. Recommendations on fluorine measurements (Ottolini et al. 2000) in Mg-rich humites were taken into consideration. Corrections were calculated using the PAP procedures suggested by CAMECA. An X-ray powder pattern of kumtyubeite was recorded using a Philips X’Pert PW 3710 powder diffractometer (filtered CoKα-radiation, 45 kV, 30 mA, 0.01° step, 100 s/step). The selection of the two crystals of reinhardbraunsite and kumtyubeite for structure refinement was solely based on crystal quality (low concentration of inclusions, sharp X-ray reflections). For this reason the crystals do not correspond to the most OH- or F-rich compositions determined from this locality. Single crystals of kumtyubeite Ca5(SiO4)2(F1.3OH0.7) and reihardbraunsite Ca5(SiO4)2(OH1.3F0.7) from the same locality were mounted on a Bruker Apex II three-circle CCD diffractometer using graphite monochromated MoKα X-radiation for diffraction intensity-data collection. Preliminary lattice parameters and an orientation matrix were obtained from three sets of frames and refined during the integration process of the intensity data. Diffraction data were collected with a combination of ω and ϕ scans (program SMART, Bruker 1999). Integrated intensities were obtained using the SAINT computer program (Bruker 1999). An empirical absorption correction using SADABS (Sheldrick 1996) was applied. Reflection statistics and systematic absences were consistent with space group P21/a (no. 14) already known for reinhardbraunsite. The standard setting of this space group is P21/c but in accordance with the refinement of reinhardbraunsite (Kirfel et al. 1983) the P21/a setting was preferred. In addition, we used the same atom labeling scheme as Kirfel et al. (1983). Structural refinement was performed using SHELXL-97 (Sheldrick 1997). Scattering factors for neutral atoms were employed. Positions of the hydrogen atoms of the hydroxyl groups were derived from difference-Fourier syntheses. Subsequently, partially occupied hydrogen positions were refined at a fixed value of Uiso = 0.05 Å2, and O-H distances were restrained to 0.95(2) Å. Data collection and refinement details are summarized in Table 1. CIF files are available online.1 IR spectra of kumtyubeite were obtained on a Bio-Rad FTS-6000 spectrophotometer with a micro-ATR accessory (MIRacle), equipped with KRS-5 lenses and a single-reflection diamond ATR. A small amount of sample was simply placed onto the ATR crystal. During analysis a pressure of up to 75 psi was applied to the sample. FTIR spectra (32 scans) were averaged to obtain background and sample spectra. Spectra were collected from 360 to 6000 cm–1 with a resolution of 4 cm–1. Raman spectra of single crystals of kumtyubeite (20–30 µm) were recorded using a LabRam System spectrophotometer (Jobin-yvone-Horiba), equipped with a grating monochromator, a charge-coupled device (CCD), a Peltier-cooled detector (1024 × 256) and an Olympus BX40 confocal microscope. The incident laser excitation was provided by a water-cooled argon laser source operating at 514.5 nm. The power at the exit of a 50× or 100× objective lens varied from 20 to 40 mW. To avoid undesirable Rayleigh scattering, two notch-filters were used, cutting the laser line at 200 cm–1. Raman spectra were recorded at 0° geometry in the range 200–4000 cm–1 with a resolution of 3.5 cm–1. The monochromator was calibrated using the Raman scattering line of a silicon plate (520.7 cm–1). 1 Deposit item AM-09-045, CIF files. Deposit items are available two ways: For a paper copy contact the Business Office of the Mineralogical Society of America (see inside front cover of recent issue) for price information. For an electronic copy visit the MSA web site at http://www.minsocam.org, go to the American Mineralogist Contents, find the table of contents for the specific volume/issue wanted, and then click on the deposit link there. TABLE 1. Parameters for X-ray data collection and crystal-structure refinement of kumtyubeite and reinhardbraunsite Crystal data Unit-cell dimensions (Å)* Volume (Å3) Space group Z Chemical formula µ (mm–1) Intensity measurement Crystal shape Crystal size (mm) Diffractometer X-ray radiation X-ray power Monochromator Temperature Detector to sample distance Measurement method Rotation width Total number of frames Frame size Time per frame Max. θ-range for data collection Index ranges Kumtyubeite a = 11.44637(18) b = 5.05135(8) c = 8.85234(13) β = 108.8625(7)° 484.352(13) P21/a (no. 14) 2 Ca5(SiO4)2(OH0.7F1.3) 3.06 Reinhardbraunsite a = 11.4542(2) b = 5.06180(10) c = 8.89170(10) β = 108.7698(9)° 488.114(14) P21/a (no. 14) 2 Ca5(SiO4)2(OH1.3F0.7) 3.03 prism prism 0.100 × 0.060 × 0.040 0.080 × 0.060 × 0.015 APEX II SMART APEX II SMART MoKα, λ = 0.71073 Å MoKα, λ = 0.71073 Å 50 kV, 30 mA 50 kV, 30 mA graphite graphite 293 K 293 K 5.95 cm 3.95 cm phi, omega scans phi, omega scans 0.5° 0.5° 1366 1746 512 × 512 pixels 512 × 512 pixels 20 s 45 s 28.25 29.98 –14 ≤ h ≤ 15 –16 ≤ h ≤ 15 –6 ≤ k ≤ 5 –7 ≤ k ≤ 7 –11 ≤ l ≤ 11 –12 ≤ l ≤ 12 6905 9218 1202 1419 1153 1249 No. of measured reflections No. of unique reflections No. of observed reflections [I > 2σ(I)] Refinement of the structure No. of parameters used in refinement 81 85 Rint 0.0231 0.0350 Rσ 0.0139 0.0224 R1, I > 2σ(I) 0.0156 0.0190 R1, all data 0.0163 0.0236 2 wR2 (on F ) 0.0444 0.046 GooF 1.103 1.115 –3 ∆ρmin (–e Å ) –0.27 close to Si1 –0.33 close to H1 ∆ρmax (e Å–3) 0.50 close to F 0.33 close to O1 * Based on 5305 (kumtyubeite) and 3987 reflections (reinhardbraunsite); the low e.s.d. values are probably not realistic but are a consequence of the high number of reflections applied. GALUSKINA ET AL.: KUMTyUBEITE—A NEW CALCIUM MINERAL OF THE HUMITE GROUP occurrence, mIneral assocIaTIon, and resulTs of kumTyuBeITe InvesTIGaTIons Kumtyubeite was discovered in xenolith 1 located between Lakargi and Vorlan mountain peaks within the Upper Chegem volcanic structure (xenolith numbering after Gazeev et al. 2006). The exposure of xenolith 1 reaches 20–25 m2 (Gazeev et al. 2006). Its central part is composed of bluish-gray marble with banding relics. Massive dark gray spurrite-calcite rocks encase the marble core. Some small xenolith outcrops (1–1.5 m across) occur along the west and northwest contacts of xenolith 1. They are light colored from white to yellow or red and contain abundant calcium minerals of the humite group: Ca7(SiO4)3(OH)2, and reinhardbraunsite or kumtyubeite. More rarely, small fragments of light spurrite rocks, 50 cm in size, with striking pink spots of reinhardbraunsite and yellow oval rondorfite patches are noted. At the contact of xenolith with ignimbrites, cuspidine rocks with larnite, rondorfite, rustumite, and wadalite are developed. Reinhardbraunsite, the new mineral Ca7(SiO4)3(ОН)2, ron- “Ca-hum” A 1363 dorfite, P- and As-bearing ellestadite-(OH), wadalite, Sn- and U-bearing lakargiite, srebrodolskite, and magnesioferrite are commonly distributed in different types of skarn rocks. Perovskite, ferrigarnet of kimzeyite type, periclase, and sphalerite are considerably less abundant. Secondary calcium hydrosilicates (bultfonteinite, hillebrandite, afwillite), minerals of the ettringite group, hydrocalumite, hydrogarnets, calcite, and brucite are fairly common. Kumtyubeite is noted in spurrite-rondorfite-ellestadite zones of skarn, developed along the west contact of the xenolith (Figs. 1a–1c). In hand specimens of the grayish-reddish matrix, kumtyubeite forms oval spots up to 1 cm across, composed of lightpink grains 250 µm in maximum dimension. Kumtyubeite of the holotype specimen has an invariable composition corresponding to Ca5(Si1.99Ti0.01)∑2O8(F1.39OH0.61)∑2 (Table 2; analysis 1). The kumtyubeite composition in rocks enriched in bultfonteinite (Table 2; analysis 12) is very similar: Ca5.01(Si1.98Ti0.01)∑1.99O8 (F1.36OH0.62)∑1.98 (Table 2, analysis 2). Kumtyubeite contains inclusions of the potential new mineral “Ca-humite” Ca7(SiO4)3F2, rondorfite, rarely lakargiite and kimzeyite (Figs. 1a–1b). If B Rndr Kmt 7 Kmt Kmt 3 Kmt “Ca-hum” 13 4 Rndr 100 µm Kmt C C 100 µ m D Blt Wad Cal Blt Rhb 9 10 12 Srb Rndr Spu Lak Rhb 8 Kmt 100 µ m Cal 100 µm fIGure 1. А = kumtyubeite grain with “Ca-humite” inclusions; B = kumtyubeite with rounded rondorite inclusions, black spots = products of rondorite alteration, Ca-hydrosilicates; C = kumtyubeite associated with srebrodolskite, bultfonteinite, and Ca-hydrosilicates, and ettringite (dark gray); D = reinhardbraunsite substituting for spurrite along the rims of spurrite grains; numbers correspond to the number of analyses in Table 1. Blt = bultfonteinite, “Ca-hum” = “Ca-humite,” Cal = calcite, Kmt = kumtyubeite, Lak = lakargiite, Rndr = rondorite, Spu = spurrite, Srb = srebrodolskite, Wad = wadalite. GALUSKINA ET AL.: KUMTyUBEITE—A NEW CALCIUM MINERAL OF THE HUMITE GROUP 1364 TABLE 2. Chemical composition (wt%) of kumtyubeite and associated minerals mean 28 SO3 n.d. P2O5 n.d. TiO2 0.09 SiO2 28.20 Al2O3 n.d. Fe2O3 0.04 Cr2O3 n.d. CaO 66.19 MgO 0.05 MnO n.d. Na2O n.d. F 6.22 Cl n.d. H2O† 1.25 –O ≡ F + Cl 2.64 Total 99.40 1 SD range mean 29 n.d. n.d. 0.07 0–0.29 0.10 0.15 27.85–28.54 28.02 n.d. 0.05 0–0.29 0.02 n.d. 0.68 64.51–67.26 66.38 0.02 0–0.09 0.04 0.03 n.d. 0.32 5.54–6.73 6.09 0.03 1.33 2.56 99.48 2 SD range 0.06 0.02–0.25 0.17 27.71–28.38 0.03 0–0.11 0.29 65.46–66.88 0.02 0.01–0.08 0.04 0–0.13 0.36 5.42–6.77 0.02 0–0.05 0.17 1.02–1.67 3 n.d. n.d. 0.27 27.92 n.d. n.d. n.d. 65.78 0.05 n.d. n.d. 6.25 n.d. 1.30 2.65 98.92 4 n.d. n.d. 0.10 29.94 n.d. n.d. 0.05 65.47 0.03 0.07 4.22 n.d. 1.02 1.79 99.11 5 n.d. n.d. 0.20 27.86 n.d. 0.02 n.d. 66.42 0.03 0.02 n.d. 5.26 0.04 0.88 2.23 98.50 6 n.d. n.d. 0.14 30.02 n.d. n.d. n.d. 65.99 0.03 n.d. n.d. 3.68 n.d. 1.22 1.55 99.53 7 n.d. n.d. 0.23 27.94 n.d. n.d. n.d. 66.08 0.05 n.d. n.d. 6.46 n.d. 1.19 2.74 99.21 8 0.07 n.d. 0.41 27.91 n.d. n.d. n.d. 66.23 0.03 0.02 5.78 0.04 1.48 2.44 99.53 9 10 11 n.d. 0.05 n.d. n.d. n.d. n.d. 0.05 n.d. 0.18 28.54 26.98 28.21 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 66.74 62.97 66.27 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 4.24 n.d. 1.72 n.d. n.d. n.d. 2.14 n.d. 3.41 1.80 99.91 99.90* 99.06 12 0.69 n.d. 0.05 27.20 0.02 0.10 n.d. 53.36 n.d. n.d. n.d. 8.63 n.d. 12.98 3.66 99.37 13 n.d. 0.19 n.d. 29.99 0.13 0.08 0.02 56.50 4.63 n.d. 0.04 n.d. 8.72 1.94 98.36 Na 0.01 Ca 5.00 5.01 5 6.99 5.00 7.01 5.01 5 5 5 5 2.02 8.03 Mn2+ 0.01 Mg 0.92 X 5 5.01 5 7 5 7.01 5.01 5 5 5 5 2.02 Si 1.99 1.98 1.98 2.99 1.98 2.98 1.98 1.98 2 2 1.99 0.96 3.98 Ti4+ 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.02 0.01 3+ Fe 0.01 Al 0.02 P 0.02 S6+ 0.02 Z 2.00 1.99 1.99 3 1.99 2.99 1.99 2 2 2 2 0.98 F 1.39 1.36 1.40 1.33 1.17 1.16 1.45 1.29 0.94 0.39 0.97 OH 0.60 0.62 0.60 0.67 0.83 0.82 0.55 0.71 1.06 1.61 3.03 Cl 1.96 C 1 Notes: 1 = analyses of holotype kumtyubeite; 2 = kumtyubeite from bultfonteinite-bearing rock; 3–6 = kumtyubeite grain (3,5) with inclusions of “Ca-humite” (4,6); 7 = kumtyubeite with rondorfite inclusions (13); 8 = kumtyubeite with srebrodolskite and bultfonteinite (12); 9 = reinhardbraunsite rim on spurrite (10); 10 = reinhardbraunsite associated with “Ca-OH-humite.” The sums of cations on the octahedral X and the tetrahedral Z sites are printed in bold. * CO2 = 9.90 wt% in total. † Calculated on the basis of charge balance. Kumtyubeite and reinhardbraunsite analyses were normalized on 7 cations, “Ca-humites” were normalized on 10 cations, bultfonteinite was normalized on 3 cations, rondorfite on 13 cations. replaced by kumtyubeite, relic “Ca-humite” Ca7(SiO4)3F2 is preserved as lentil-like, elongated inclusions, the boundary of which is sometimes transformed to secondary hydrosilicates (Fig. 1a). It is interesting that in the case of partial replacement, the F/OH ratio in newly formed kumtyubeite is more or less the same as in primary “Ca-humite” (Table 2, analyses 3–6). Rondorfite is also characteristically found in this association, occurring as rounded, isometric inclusions in kumtyubeite, partly replaced by calcium hydrosilicates (Fig. 1b; Table 2, analyses 7 and 13). P- and As-bearing ellestadite-(OH) associated with kumtyubeite is often replaced by minerals of the ettringite and hydrogarnet groups. Srebrodolskite and magnesioferrite, forming aggregates reaching 500 µm in size, and also ferrigarnet of kimzeyite type, wadalite, and sphalerite are accessory minerals. Representative analyses of srebrodolskite, magnesioferrite, and lakargiite are given in Table 3. Kumtyubeite associated with srebrodolskite is characterized by increased Ti content (Fig. 1c; Table 2, analysis 8). Reinhardbraunsite is confined to spurrite zones of the skarn, which change to kumtyubeite-spurrite-rondorfite-ellestadite zones toward the xenolith center. Kumtyubeite appears predominantly replacing Ca7(SiO4)3F2, whereas reinhardbraunsite replaces spurrite and Ca7(SiO4)3(OH)2 (Fig. 1d). It is interesting that larnite relics are absent within kumtyubeite and reinhardbraunsite, whereas these relics are characteristic of Ca7(SiO4)3(OH)2 (Galuskin et al. in preparation). Ca7(SiO4)3(OH)2 coexists with reinhardbraunsite having low fluorine content (Table 2, analysis 11). Kumtyubeite shows vitreous luster and is transparent. The mean Moh’s hardness is 5–6, the mean measured micro-hardness VHN50 = 300 (280–320) kg/mm2 (11 measurements), the mineral is brittle, the cleavage is distinct on (001), in particular for crystals with a ratio OH:F ≈ 1. Kumtyubeite is transparent and colorless in thin-section. It is biaxial negative, with nα = 1.594(2), nβ = 1.605(2), nγ = 1.608(2), δ = 0.014, 2VX (meas.) = 40–55°, 2VX (calc.) = 54.8°, X ∧ c = 15(2)°, Z = b. Simple, rarely polysynthetic, twins on (001) are characteristic for kumtyubeite. Dcalc is 2.866 g/cm3. Numerous inclusions in kumtyubeite did not permit selection of a pure fraction for a powder X-ray diffraction investigation using a Philips X’Pert PW 3710 powder diffractometer (CoKα radiation). The collected pattern contained strong peaks for “Ca-humite,” ettringite and weak peaks for hydrogrossular, spurrite, wadalite-mayenite, hydrocalumite, and other phases. Nevertheless, we were able to extract the strongest peaks of the kumtyubeite pattern (Table 4) and to refine cell parameters. The PowderCell for Windows version 2.4 program (Kraus and Nolze 1996), based on the Rietveld method, was used to refine lattice parameters (R =13). Starting parameters were taken from the single-crystal data of kumtyubeite. Refined cell dimensions from GALUSKINA ET AL.: KUMTyUBEITE—A NEW CALCIUM MINERAL OF THE HUMITE GROUP 222 512 114 510 014 513 222 421 023 511 114 603 313 423 514 403 005 130 604 131 131 132 712 331 314 132 332 605 Measured d (Å) I 2.1369 3 2.0511 2 2.0126 4 1.9926 3 1.9360 2 1.9128 2 1.9040 100 1.8952 37 1.8748 2 1.8209 4 1.8145 2 1.8063 14 1.8027 23 1.7267 3 1.7216 6 1.6915 4 1.6766 9 1.6655 1 1.6587 28 1.6492 3 1.6183 2 1.5748 2 1.5551 6 1.5403 8 1.5274 2 1.5221 1 1.5026 2 1.4925 4 Calculated d (Å) I 2.1369 7 2.0514 5 2.0125 11 1.9929 7 1.9352 2 1.9123 2 1.9043 100 1.8955 45 1.8751 5 1.8208 13 1.8151 1 1.8064 22 1.8029 28 1.7266 11 1.7218 14 1.6915 17 1.6765 34 1.6652 4 1.6588 39 1.6492 7 1.6181 8 1.5747 4 1.5551 25 1.5402 6 1.5273 8 1.5220 5 1.5027 5 1.4924 7 3558 3600 A 3541 3551 B 1200 3550 1000 800 3500 3450 600 461 427 530 507 561 656 638 760 the powder data are given below: monoclinic, space group: P21/a, a = 11.454(2), b = 5.056(1), c = 8.857(1) Å, β = 108.84(1)°, V = 485.51(2) Å3, Z = 2. FTIR and Raman spectra of kumtyubeite resemble spectra of humite-group minerals (Prasad and Sarma 2004; Frost et al. 2007a, 2007b). The FTIR spectrum of kumtyubeite shows the following bands (main bands are italic): 427, 507, 530, 561, 638, 656, 722, 760, 779, 822, 837, 865, 904, 934, 957, 993, 1113, 1165, 3541, 3551, and 3558 cm–1 (Fig. 2). The following bands appear in the Raman spectrum of kumtyubeite (strong bands are italic): 281, 299, 323, 397, 420, 525, 547, 822, 849, 901, 925, 3544, 3553, and 3561 cm–1 (Fig. 3). In the low-frequency (<1000 cm–1) region, the Raman spectra of kumtyubeite and reinhardbraunsite of xenolith 1 (Fig. 3) strongly resemble each other. FTIR and Raman bands measured for the reinhardbraunsite–kumtyubeite series may be divided into four groups following Frost et al. (2007a, hkl 722 Ca 0.031 2.001 0.991 Mg 0.519 0.001 Mn2+ 0.354 Sr 0.003 Fe2+ 0.063 Zn 0.032 Ce3+ 0.003 3+ La 0.002 Th 0.002 Х 0.999 2.001 1.002 3+ Fe 1.924 1.647 0.067 Al 0.063 0.213 Mn4+ 0.042 Mn2+ 0.073 Mg 0.001 Si 0.004 0.001 Ti4+ 0.009 0.019 0.226 Zr 0.002 0.588 Sn 0.064 Sc 0.006 6+ U 0.026 Cr 0.002 0.002 Nb5+ 0.012 Hf 0.008 Y 2.000 2.000 0.999 Note: 1 = magnesioferrite, normalized on 3 cations; 2 = srebrodolskite, normalized on 4 cations; 3 = lakargiite, normalized on 2 cations; n.m. = not measured; n.d. = not detected. Calculated d (Å) I 8.4083 8 5.4215 33 4.5859 2 4.3319 10 4.1948 7 4.0003 14 3.8102 34 3.6987 10 3.3274 55 3.2281 4 3.1384 5 3.0340 78 2.9410 45 2.9016 67 2.8942 59 2.8639 5 2.7959 8 2.7744 60 2.7103 18 2.5718 32 2.5463 40 2.5276 8 2.5115 33 2.4922 26 2.4637 14 2.3642 7 2.3171 6 2.2539 6 Intensity [a.u.] n.d. n.m. 0.22 101.41 001 200 110 011 002 201 111 210 112 012 211 311 310 112 203 401 003 312 400 311 113 020 213 411 120 403 121 113 Measured d (Å) I 8.3830 1 5.4202 30 4.5824 1 4.3297 3 4.1915 3 4.0006 4 3.8081 13 3.6973 5 3.3273 21 3.2269 4 3.1374 3 3.0344 37 2.9399 17 2.9018 16 2.8932 13 2.8635 3 2.7943 2 2.7737 25 2.7101 12 2.5713 20 2.5452 12 2.5282 2 2.5112 17 2.4917 10 2.4621 2 2.3640 2 2.3168 2 2.2530 2 822 779 1.94 n.m. n.m. 99.02 hkl 957 934 904 865 837 3 4.25 0.95 10.53 42.24 0.01 0.93 0.31 5.62 n.d. 3.13 n.d. n.d. 0.23 0.07 0.15 0.33 32.42 0.02 993 2 n.m. n.d. 0.55 0.08 0.01 n.d. n.m. n.d. 1.36 48.99 4.04 n.d. n.m. 0.06 n.m. n.m. 41.79 0.20 X-ray powder diffraction data (d in angstroms) of kumtyubeite 1113 1 n.m. n.d. 0.35 n.m. 0.11 n.m. n.m. n.d. n.d. 71.80 1.49 0.05 n.m. n.d. n.m. n.m. 0.80 9.79 2.09 11.74 1.20 n.m. 99.42 TABLE 4. 1165 UO3 wt% Nb2O5 TiO2 ZrO2 SiO2 HfO2 ThO2 SnO2 MnO2 Fe2O3 Al2O3 V2O3 Sc2O3 Cr2O3 La2O3 Ce2O3 CaO MgO FeO MnO ZnO SrO Total Chemical composition of oxides associated with kumtyubeite Absorbance [a.u.] TABLE 3. 1365 400 fIGure 2. FTIR spectra of a kumtyubeite grain measured with microATR accessory in the region: A = 1250–400 cm–1, B = 3600–3450 cm–1. 2007b): group 1 = modes lower than 400 cm–1 corresponding to the vibrations of CaO6 octahedra; group 2 = modes 400–700 cm–1 are related to bending vibrations of SiO4 tetrahedra; group 3 = modes 700–1000 cm–1 are related to the stretching vibrations of SiO4; and group 4 = modes between 3480 and 3562 cm–1 are characteristic of (OH)-group vibrations. The main difference between the reinhardbraunsite and kumtyubeite spectra consists in the absence of a strong band near 3480 cm–1 in the kumtyubeite spectrum. In addition, the intensity 820.3 GALUSKINA ET AL.: KUMTyUBEITE—A NEW CALCIUM MINERAL OF THE HUMITE GROUP 1366 A 8000 3560.8 3553.1 3543.8 3528.8 6000 8000 2 224.7 318.8 290.4 419.3 396.9 420 397.3 4000 2000 322.5 298.5 281.3 548.7 524.1 2000 848.6 924.8 901.1 2 546.5 525.4 4000 849.1 6000 924.3 899.8 Intensity [a.u.] 10000 3483.3 12000 3561.3 3549.8 821.8 B 1 1 1200 0 3700 0 1000 800 600 -1 400 200 3600 3500 3400 3300 Raman Shift (cm ) fIGure 3. Raman spectra of kumtyubeite (1) and reinhardbraunsite (2) in the region: A = 1200–200 cm–1, B = 3700–3300 cm–1. fIGure 4. [010] projection of the polyhedral structure model of kumtyubeite. Three symmetry independent CaO6 octahedra are labeled M1 to M3; the SiO4 tetrahedron is black with light gray outlines; (OH,F) sites are indicated by black circles with white rim. Disordered H1 is shown as small gray sphere. of the band near 3550 cm–1 (Figs. 2 and 3) is also weaker in kumtyubeite than in reinhardbraunsite. The two main bands are related to the hydrogen sites H1 and H2. In kumtyubeite, which is characterized by high F content, the H2 site is not occupied (see single-crystal structure investigations below). At this point one may easily be misled by assuming that the 3480 cm–1 absorption is caused by the hydrogen bond related to H2. However, as shown below, the situation is much more complex. sInGle-crysTal sTrucTural InvesTIGaTIon of kumTyuBeITe and reInhardBraunsITe The crystal structures of kumtyubeite and reinhardbraunsite (Figs. 4 and 5; Tables 1 and 5–7) correspond to the chondrodite structure-type (Kirfel et al. 1983). A projection of a polyhedral structure model of kumtyubeite along [010] (Fig. 4) is indis- fIGure 5. Disordered hydrogen-bond system in reinhardbraunsite solid-solution members with compositions between Ca5(SiO4)2(OH)2 and Ca5(SiO4)2(OH)F: SiO4 tetrahedra are represented by dark polyhedra, bonds are drawn for Ca-O and O-H, dashed lines indicate O···H acceptor interactions. The two opposite structural fragments are related to each other by an inversion center between H1 and H1'. The sub-site F5 centers a triangle formed by 2 × Ca3 and 1 × Ca2, whereas the sub-site O5 is shifted toward the symmetry center. H1 is at maximum 50% occupied avoiding short H1-H1' distances. In kumtyubeite solid-solution members with compositions between Ca5(SiO4)2(F)2 and Ca5(SiO4)2F(OH), H2 is empty. tinguishable from a corresponding drawing for chondrodite (Gibbs et al. 1970), with the only obvious difference being that the cell volume of kumtyubeite is ca. 26% greater than that of chondrodite due to the difference in octahedral ionic radii, 0.72 Å for Mg vs. 1.00 Å for Ca (Shannon 1976). The reinhardbraunsite-kumtyubeite series is characterized by an increase in the unit-cell volume from the F end-member toward the OH end-member (Hamm and Hentchel 1983). This relation is also evident from this single-crystal X-ray study (Table 1), with the advantage that using the same equipment and data collection strategy for both crystals reduces systematic errors. There are already several structure refinements of reinhardbraunsite (Kirfel et al. 1983; Karimova et al. 2008) and its synthetic analog (Ganiev et al. 1969; Kuznetsova et al. 1980), but the quality of these previous studies was not sufficient to extract H sites GALUSKINA ET AL.: KUMTyUBEITE—A NEW CALCIUM MINERAL OF THE HUMITE GROUP and to discuss the structural differences with kumtyubeite. A special structural feature of solid-solution members between kumtyubeite and reinhardbraunsite is the strongly anisotropic smearing of O5 related to positional and chemical disorder. O5 is occupied by OH and by F. This average O5/F5 site lies on a triangular plane with 2 × Ca3 and 1 × Ca2 at the corners, and the disorder is most pronounced perpendicular to this plane. In the Ca5(SiO4)2(OH)2, end-member reinhardbraunsite, O5 acts simultaneously as donor and acceptor of a hydrogen bond. The reinhardbraunsite/kumtyubeite structure has a center of inversion halfway between two adjacent O5/F5 sites (Fig. 5). Thus, on a statistical basis, H1 between the two O5 sites can only be 50% occupied (Fig. 5). If both H1 sites were to be occupied (H1 occupancy = 100%), the two centrosymmetric H1 counterparts would be too close. For stoichiometric reasons, the reinhardbraunsite OH end-member must have 2 OH pfu. However, with H1 50% TABLE 5a. Atomic coordinates and isotropic displacement parameters (Ueq) for kumtyubeite Atom Ca1 Ca2 Ca3 Si1 O1 O2 O3 O4 O5 F5 H1 Occup. 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.3 0.7 0.3 x 1/2 0.16958(2) 0.38125(2) 0.35382(3) 0.48464(8) 0.24752(8) 0.33001(8) 0.35279(8) 0.0466(4) 0.05554(14) 0.015(9) y 0 0.00430(5) 0.00820(5) 0.57472(7) 0.70458(17) 0.70644(17) 0.70447(17) 0.25353(18) 0.2161(9) 0.2581(3) 0.052(11) z 1/2 0.30946(3) 0.07805(3) 0.29754(4) 0.29303(10) 0.14423(10) 0.45559(10) 0.29649(10) 0.0743(5) 0.09880(17) 0.024(12) Ueq (Å2) 0.00872(10) 0.00788(9) 0.00829(9) 0.00617(10) 0.00864(18) 0.00875(18) 0.00855(18) 0.00938(18) 0.0130(2) 0.0130(2) 0.050 TABLE 5b. Atomic coordinates and isotropic displacement parameters (Ueq) for reinhardbraunsite Atom Occup. x Ca1 1.0 1/2 Ca2 1.0 0.16976(3) Ca3 1.0 0.38090(3) Si1 1.0 0.35425(4) O1 1.0 0.48447(10) O2 1.0 0.24736(10) O3 1.0 0.33012(10) O4 1.0 0.35324(10) O5 0.697(11) 0.0505(2) F5 0.303(11) 0.0636(4) H1 0.5 0.024(5) H2 0.2 0.099(11) y 0 0.00251(6) 0.00879(6) 0.57435(8) 0.7052(2) 0.7042(2) 0.7043(2) 0.2540(2) 0.2325(7) 0.2829(13) 0.080(8) 0.36(2) z 1/2 0.31087(4) 0.07952(4) 0.29833(5) 0.29332(12) 0.14547(13) 0.45581(12) 0.29808(13) 0.0841(4) 0.1163(8) 0.025(6) 0.158(13) Ueq (Å2) 0.00921(10) 0.00875(8) 0.00909(8) 0.00652(10) 0.0087(2) 0.0090(2) 0.0090(2) 0.0095(2) 0.0124(3) 0.0124(3) 0.050 0.050 1367 occupied, it only contributes to 1 OH pfu. If O5 (Fig. 5) acts as acceptor of a hydrogen bond from O5′ (O5′-H1′···O5), O5 will also be hydroxylated but the alternate O5-H2 vector points almost opposite to the weak O5···H1 interaction (Fig. 5). This additional hydrogen bond system can be described as O5-H2···O2 (Fig. 5). Disordered hydrogen bonds are characteristic of the OH endmembers of all humite-group minerals, as has been confirmed by neutron powder diffraction for synthetic hydroxylclinohumite (Berry and James 2001) and “hydroxyl-chondrodite” (Lager et al. 2001; Berry and James 2002). The good quality of our diffraction data for the two solidsolution members invited splitting of the smeared O5 position into two sub-sites (O5 and F5). Subsequently, a refinement was done with individual O5 and F5 sites having a common isotropic displacement parameter and a constraint that the occupancies of both sub-sites sum to 1. Such a sub-site refinement involves one parameter less than the anisotropic O5 model. In the O5/F5 sub-site model for reinhardbraunsite, O5 and F5 are separated by 0.38 Å, whereas the corresponding distance in kumtyubeite is only 0.29 Å. In the case of reinhardbraunsite, we also refined F5/O5 populations and the occupancies agreed with values from the chemical analyses. A corresponding refinement for kumtyubeite was not so successful, probably because of the shorter O5-F5 distance, leading to strong overlap of the corresponding electron clouds. Thus, O5 and F5 occupancies were fixed to the value obtained from electron-microprobe analyses. In reinhardbraunsite and kumtyubeite, F5 is on the plane formed by the triangle with 2 × Ca3 and 1 × Ca2 at the apices, whereas O5 is displaced from the triangular plane and shifted toward the symmetry center (Fig. 5). Thus, in accordance with the smaller ionic radius of F– (1.30 Å) compared to O2– (1.36 Å) for threefold coordination (Shannon 1976), corresponding distances between O5 and Ca are significantly longer than those between F5 and Ca (Table 7). A distance restraint [0.95(2) Å] between O5 and the two proton sites (H1 and H2) was applied to obtain physically meaningful O-H distances (Lager et al. 1987). The sub-site refinement for reinhardbraunsite yielded the following O···O distances for the various hydrogen-bond systems: O5-H1···O5′ 2.827 Å; O5-H1···F5′ 3.190 Å; O5H2···O2 3.209 Å. According to the correlations between O-H stretching frequencies and O-H···O hydrogen bond lengths TABLE 6a. Anisotropic displacement parameters Uij (Å2) for kumtyubeite Atom Ca1 Ca2 Ca3 Si1 O1 O2 O3 O4 U11 0.00913(18) 0.00735(14) 0.00878(14) 0.00674(16) 0.0075(4) 0.0086(4) 0.0099(4) 0.0107(4) U22 0.00746(18) 0.00832(14) 0.00876(15) 0.00521(17) 0.0088(4) 0.0087(4) 0.0085(4) 0.0061(4) U33 0.00819(17) 0.00829(13) 0.00740(13) 0.00647(16) 0.0098(4) 0.0078(4) 0.0077(4) 0.0113(4) U23 –0.00114(11) –0.00020(8) 0.00086(8) –0.00006(11) –0.0002(3) 0.0007(3) –0.0003(3) –0.0001(3) U13 0.00090(14) 0.00295(10) 0.00269(10) 0.00201(12) 0.0031(3) 0.0010(3) 0.0035(3) 0.0035(3) U23 –0.00147(15) –0.00043(10) 0.00118(11) –0.00004(14) –0.0002(4) 0.0001(4) 0.0000(4) –0.0002(4) U13 0.00059(16) 0.00321(12) 0.00273(12) 0.00194(15) 0.0022(4) 0.0015(4) 0.0033(4) 0.0029(4) U12 –0.00112(11) 0.00047(8) –0.00004(8) 0.00006(11) –0.0001(3) 0.0002(3) 0.0004(3) 0.0000(3) TABLE 6b. Anisotropic displacement parameters Uij (Å2) for reinhardbraunsite Atom Ca1 Ca2 Ca3 Si1 O1 O2 O3 O4 U11 0.0092(2) 0.00784(16) 0.00943(16) 0.0069(2) 0.0065(5) 0.0089(5) 0.0103(5) 0.0098(5) U22 0.0078(2) 0.00975(15) 0.00949(15) 0.00536(19) 0.0090(5) 0.0090(5) 0.0090(5) 0.0072(5) U33 0.0089(2) 0.00901(15) 0.00827(15) 0.00703(19) 0.0102(5) 0.0083(5) 0.0079(5) 0.0113(5) U12 –0.00105(15) 0.00078(11) 0.00005(11) 0.00015(15) –0.0005(4) –0.0004(4) 0.0003(4) –0.0001(4) 1368 TABLE 7. GALUSKINA ET AL.: KUMTyUBEITE—A NEW CALCIUM MINERAL OF THE HUMITE GROUP Selected interatomic distances (Å) for reinhardbraunsite and kumtyubeite Kumtyubeite Reinhardbraunsite Synth. OH* Reinhardbr.† Ca1 O1 ×2 2.3251(8) 2.3293(11) 2.335 2.328(2) Ca1 O3 ×2 2.3821(8) 2.3853(11) 2.391 2.382(2) Ca1 O4 ×2 2.4005(8) 2.3999(11) 2.404 2.401(2) <Ca1-O> 2.3692 2.3715 2.377 2.370 Ca2 F5 2.2917(16) 2.266(5) Ca2 O5 2.359(4) 2.349(3) 2.325 2.295(5) Ca2 O3 2.3113(9) 2.3119(11) 2.312 2.312(2) Ca2 O1 2.3275(9) 2.3282(11) 2.323 2.324(2) Ca2 O3 2.4090(9) 2.4062(12) 2.404 2.411(2) Ca2 O2 2.4582(9) 2.4645(12) 2.470 2.457(2) Ca2 O4 2.4802(9) 2.4909(12) 2.501 2.477(2) <Ca2-O> 2.3909 2.3918 2.389 2.379 Ca3 O2 2.2745(8) 2.2908(11) 2.315 2.268(2) Ca3 F5 2.2737(16) 2.271(5) Ca3 O5 2.358(4) 2.332(3) 2.316 2.279(5) Ca3 F5 2.3026(16) 2.336(4) Ca3 O5 2.324(5) 2.330(2) 2.343 2.300(4) Ca3 O2 2.3643(9) 2.3744(12) 2.379 2.369(2) Ca3 O4 2.4060(9) 2.4111(11) 2.416 2.406(2) Ca3 O1 2.4327(9) 2.4356(11) 2.443 2.434(2) <Ca3-O> 2.3599 2.3623 2.369 2.343 Si1 O4 1.6225(9) 1.6218(11) 1.620 1.625(2) Si1 O2 1.6424(9) 1.6455(12) 1.653 1.643(2) Si1 O3 1.6456(9) 1.6455(12) 1.646 1.646(2) Si1 O1 1.6471(9) 1.6489(12) 1.642 1.649(2) <Si1-O> 1.6394 1.6404 1.6403 1.641 O5 H1 0.96(2) 0.94(2) H1 F5 1.92(2) 2.26(2) H1 O5 1.64(2) 1.90(2) O5 O5 2.594(9) 2.827(9) O5 F5 0.297(4) 0.377(13) O5 F5 2.880(4) 3.190(13) F5 F5 3.169(3) 3.56(2) O5 O2 3.300(2) 3.209(4) O5 H2 0.96(2) H2 O2 2.47(2) * Ca5(SiO4)2(OH)2 Kuznetsova et al. (1980). † Ca5(SiO4)2(OH)F Kirfel et al. (1983). (Libowitzky 1999), the low-frequency band at ca. 3480 cm–1 in the reinhardbraunsite Raman spectrum (Fig. 3) is related to O5-H1···O5′, whereas the weaker hydrogen bonds O5-H1···F5′ and O5-H2···O2 are related to the bands near 3550–3560 cm–1. In kumtyubeite, the only hydrogen bond system is O5-H1···F5′ with an O5···F5′ distance of 2.880 Å. The corresponding Raman and FTIR spectra (Figs. 2 and 3) display a set of closely spaced absorptions around 3553 cm–1, which must be assigned to the hydrogen bond O5-H1···F5′. If the correlation between O-H stretching frequencies and O-H···O hydrogen bond length (Libowitzky 1999) is assumed to be qualitatively correct even for a rather narrow range of O···O distances between ca. 2.8 and 3.3 Å, the characteristic IR and Raman absorption at ca. 3480 cm–1, found only in the OH-dominant reinhardbraunsite but not in F-dominant kumtyubeite, is caused by the short O5-H1···O5′ hydrogen bond system (2.827 Å) but not by O5-H2···O2 (3.209 Å), as one might expect at first glance. The O5-H1···O5′ bond system is also unique to reinhardbraunsite, whereas kumtyubeite has a corresponding O5-H1···F5′ system, which is considerably weaker, as evidenced by the increased absorption frequency near 3550–3560 cm–1. However, this assignment is only suggested for the reinhardbraunsite–kumtyubeite series, but not, e.g., for the isotypic Mg-rich members chondrodite and synthetic “OH-chondrodite.” Humite-group minerals are based on a hexagonal closest-packing of anions. Due to the smaller radius of Mg compared to Ca, 1 2 3 4 Reinhardbraunsite 0.00 F apfu 0.50 5 Kumtyubeite 1.00 1.50 2.00 F apfu fIGure 6. Classiication diagram of minerals of the reinhardbraunsitekumtyubeite series: 1 = kumtyubeite from xenolith 1, holotype specimen, associated with rondorite and spurrite; 2 = kumtyubeite from xenolith 1, associated with bultfonteinite; 3 = minerals of the reinhardbraunsitekumtyubeite series, rims on spurrite grains; 4 = reinhardbraunsite from xenolith 1; 5 = analysis of holotype reinhardbraunsite, Eifel, Germany (Hamm and Hentschel 1983; Kirfel et al. 1983). O···O distances in reinhardbraunsite-kumtyubeite are in general significantly longer than those in chondrodite. In chondrodite, H2 participates in a bifurcated hydrogen-bond system with O2 and O1 as acceptors. The O5-H2···O2 distance in “OH-chondrodite” is 2.88 Å and O5-H2···O1 is 3.01 Å (yamamoto 1977; Lager et al. 2001), whereas the corresponding distances in reinhardbraunsite are 3.208 and 3.611 Å (this study). A special case is the O5-H1···O5′ distance, which is 3.07 Å in “OH-chondrodite” (yamamoto 1977; Lager et al. 2001), but 2.827 Å in reinhardbraunsite (this study). “OH-chondrodite” displays IR absorptions characteristic of O-H stretching at 3527.2, 3562.0, and 3606.7 (weak) cm–1 (Liu et al. 2003). For fluorine-dominant natural chondrodite, Frost et al. (2007b) report OH-specific Raman absorptions at 3561, 3570, and 3576 (shoulder) cm–1. Thus, the closer range of hydrogen bonded O···O distances and stretching frequencies does not allow a definite band assignment. One could also imagine a chondrodite or reinhardbraunsite structure with OH/F = 1 and H2 50% occupied but H1 vacant. However, this hypothetical model is in contradiction to all experimental studies based on direct localization of H or D by neutron or X-ray diffraction (e.g., Berry and James 2002; Friedrich et al. 2001; this study). Only if OH > 1 pfu does the H2 site become occupied (yamamoto 1977; Lager et al. 2001; this study). The obvious preference of H1 over H2 has been discussed by several authors (e.g., Abbot et al. 1989; Berry and James 2002), however, without any convincing argument for the striking H1 selectivity. Average Ca-O distances for Ca2 and Ca3 (Table 7) calculated from our reinhardbraunsite structure refinement are significantly longer than those cited by Kirfel et al. (1983). Kirfel et al. (1983) reported average O/F distances, which are shorter than pure Ca-O of our O/F sub-site model. In reinhardbraunsite, kumtyubeite (this paper), and “OHchondrodite” (yamamoto 1977), the sequence of angular distortions for octahedra is M1O6 > M2O6 > M3O6. Differences in bond-length distortion are much less pronounced, and corresponding octahedra in reinhardbraunsite, kumtyubeite, and chondrodite have similar bond-length distortions. However, if the bond-angle variance (Robinson et al. 1971) is applied as a measure of polyhedral distortion (Table 8), variances for corresponding octahedra in reinhardbraunsite and kumtyubeite are almost twice as high as those in chondrodite. On the other hand, SiO4 tetrahedra in reinhardbraunsite and kumtyubeite are GALUSKINA ET AL.: KUMTyUBEITE—A NEW CALCIUM MINERAL OF THE HUMITE GROUP TABLE 8. Bond-angle variance for MO6 octahedra and SiO4 tetrahedra in reinhardbraunsite, kumtyubeite (this study), and “OHchondrodite” (Yamamoto 1977) Polyhedron M1 octahedron M2 octahedron M3 octahedron Si1 tetrahedron Reinhardbraunsite (216°)2 (159°)2 (119°)2 (22°)2 Kumtyubeite (216°)2 (172°)2 (120°)2 (22°)2 “OH-chondrodite” (106°)2 (76°)2 (71°)2 (45°)2 less distorted in their O-T-O angles than those in chondrodite (Table 8). The different distortion behavior in Ca and Mg minerals is related to a cation-size effect (Ca vs. Mg). Within a larger CaO6 octahedron oxygen atoms have a higher flexibility in their arrangement without coming too close to each other, whereas the chondrodite structure (yamamoto 1977) is much closer to a hexagonal closest-packed anion (O,F) arrangement giving rise to more ideal octahedral geometry. As the CaO6 moieties are the flexible links in the reinhardbraunsite and kumtyubeite structure, SiO4 tetrahedra are less distorted than in chondrodite. dIscussIon About 150 compositions from microprobe analyses of reinhardbraunsite-kumtyubeite solid-solution members from skarn xenoliths of the Upper Chegem volcanic structure are plotted on a classification diagram (Fig. 6). They form a continuous solid-solution series with common crystal-chemical formula Ca5(SiO4)2(OH,F)2 from F/(F + OH) ≈ 0.15 (maximally hydrated reinhardbraunsite) to F/(F + OH) ≈ 0.75 (maximally fluorinated kumtyubeite). The absence of kumtyubeite with higher F content up to F/(F + OH) = 1 may be explained by the stabilizing role of hydrogen bonds in humite-group structures (Berry and James 2002). Available data allow us to ascertain a minimal OH amount near 0.25 pfu necessary for kumtyubeite formation. For this minimum OH concentration, the proton participates only in the hydrogen bond O5-H1⋅⋅⋅F5′. Kumtyubeite crystallization is connected with the early stage of alteration of primary minerals (larnite and spurrite) in high-temperature skarns, formed according to Korzhinsky (1940) at the larnite-mervinite depth facies, corresponding to the P-T conditions of the sanidinite facies of metamorphism (Zharikov and Shmulovich 1969; Pertsev 1977; Korzhinski 1940; Gazeev et al. 2006; Callegari and Pertsev 2007; Galuskin et al. 2008). In Caucasian xenoliths, kumtyubeite occurs in mineral associations characteristic of higher temperature than those where reinhardbraunsite formed. Kumtyubeite and reinhardbraunsite replace “Ca-humites,” which, in turn, formed at the expense of larnite and spurrite. The most fluorine-rich kumtyubeite formed in the internal part of skarns together with cuspidine Ca4Si2O7[F1.9(OH)0.1], which replaces larnite at the contact with the ignimbrite. acknoWledGmenTs The research was supported by the Russian Foundation for Fundamental Research (no. 08-05-00181) and the Swiss National Science Foundation (project T.A.: crystal chemistry of minerals, no. 20-122122). The thoughtful comments by the Associate Editor G. Diego Gatta (Milan) and the reviewers Stefano Merlino (Pisa) and Giancarlo Della Ventura (Rome) are highly appreciated. references cITed Abbot, R.N., Burnham, C.W., and Post, J.E. (1989) Hydrogen in humite-group minerals: Structure-energy calculations. American Mineralogist, 74, 1300–1306. Barker, D.S. and Nixon, P.H. (1989) High-Ca, low-alkali carbonatite volcanism 1369 at Fort Portal, Uganda. Contributions to Mineralogy and Petrology, 103, 166–177. Berry, A.J. and James, M. 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