FLUORCHEGEMITE, Ca7(SiO4)3F2, A NEW MINERAL FROM THE EDGREWITE-BEARING ENDOSKARN ZONE OF AN ALTERED XENOLITH IN IGNIMBRITES FROM UPPER CHEGEM CALDERA, NORTHERN CAUCASUS, KABARDINO-BALKARIA, RUSSIA: OCCURRENCE, CRYSTAL STRUCTURE, AND NEW DATA ON THE MINERAL ASSEMBLAGES

Abstract

Fluorchegemite, Ca 7 (SiO 4 ) 3 F 2 [ Pbnm , a 5.0620(1), b 11.3917(2), c 23.5180(3) A, V 1356.16(4) A 3 , Z = 4], the F-analog of chegemite, Ca 7 (SiO 4 ) 3 (ОН) 2 [ Pbnm , a 5.0696(1), b 11.3955(1), c 23.5571(3) A, V 1360.91(4) A 3 , Z = 4], was found in an edgrewite-bearing zone of endoskarn at the contact of a large altered calciferous xenolith within ignimbrites of the Upper Chegem Caldera, Northern Caucasus, Kabardino-Balkaria, Russia. Fluorchegemite is associated with the high-temperature minerals larnite, edgrewite, wadalite, eltyubyuite, rondorfite, lakargiite, Th-rich kerimasite, and with their alteration products such as bultfonteinite, killalaite, hillebrandite, afwillite, trabzonite, and jennite. Fluorchegemite is colorless, and the streak is white. It forms irregular grains up to 0.2 mm. Fluorchegemite gave the following optical data: biaxial (–), α 1.610(2), β 1.6150(2), γ 1.619(2) (589 nm), 2 V z (meas.) = 80(8)°, 2 V z (calc.) = 84°, Δ = 0.009, dispersion r > v, weak; non-pleochroic. Density (calc.) = 2.91 g cm −3 . The micro-hardness VHN (loaded 50 g) ranges between 480 and 511 kg mm −2 , with an average value (mean of four measurements) of 499(10) kg mm −2 , corresponding to a Mohs hardness of 5.5–6. The main bands in the Raman spectrum of fluorchegemite are at 258, 297, 410, 422, 560, 817, 843, 922, 3539, 3548, and 3552 cm −1 . Strong lines of the calculated X-ray diffraction pattern are [ d ( hkl ) I ]: 2.531(200)100, 1.905(227)90, 2.718(117)63, 3.013(131)57, 2.991(116)56, 3.636(114)52, 2.832(133)51, 2.699(134)46. The empirical formula of fluorchegemite from the holotype specimen is (Ca 7.01 Mg 0.01 ) Σ7.02 (Si 2.98 Ti 4+ 0.01 ) Σ 2.99 O 12 [F 1.40 (OH) 0.60 ] Σ2.00 . The Ca-humite minerals often form lens-shaped aggregates within the endoskarn zone. The elongated axis of the lens is oriented subperpendicular to the front of metasomatic replacement (skarn zonality). Lens-shaped aggregates of grains are interpreted as contours of replaced quartz phenocrysts in the ignimbrite. The mechanism of this lens formation is discussed. Moreover, lens-shaped aggregates of hydroxyledgrewite occur as rock-forming minerals in an endoskarn zone of a newly discovered xenolith within the Upper Chegem Caldera. In addition, the second finding of the Ca-humite minerals fluorchegemite and kumtyubeite, in an altered xenolith of the Shadil-Khokh volcano, Southern Ossetia, is reported.