Description: Two new mineral species of the mayenite group, fluormayenite Ca 12 Al 14 O 32 [ 4 F 2 ] ( I $$\overline{4}$$ 3 d , a = 11.9894(2) Å, V = 1723.42(5) Å 3 , Z = 2) and fluorkyuygenite Ca 12 Al 14 O 32 [(H 2 O) 4 F 2 ] ( I $$\overline{4}$$ 3 d , a = 11.966(2) Å, V = 1713.4(1) Å 3 , Z = 2), are major constituents of larnite pyrometamorphic rocks of the Hatrurim Complex (Mottled Zone) distributed along the Dead Sea rift on the territory of Israel, Palestinian Autonomy and Jordan. Holotype specimens of fluormayenite and fluorkyuygenite were collected at the Jabel Harmun, Judean Mts., Palestinian Autonomy and in the Hatrurim Basin, Negev Desert, Israel, respectively. Mineral associations of holotype fluormayenite and fluorkyuygenite are similar and include larnite, shulamitite, Cr-containing spinel–magnesioferrite series, ye’elimite, fluorapatite–fluorellestadite, periclase, brownmillerite, oldhamite as well as the retrograde phases portlandite, hematite, hillebrandite, afwillite, foshagite, ettringite, katoite and hydrocalumite. Fluormayenite and fluorkyuygenite crystals, usually 〈 20 μm in size, are colourless, in places with greenish or yellowish tint, the streak is white. Both minerals are transparent with a vitreous lustre; they do not show fluorescence. Fluormayenite and fluorkyuygenite are isotropic and have similar refractive indices: n = 1.612(3) and n = 1.610(3) (589 nm), respectively. The hardness of fluormayenite and fluorkyuygenite is H (Mohs) 51/2–6; VHN load 50 g, 771(38) kg mm –2 ; and 5–51/2; VHN load 50 g, 712(83) kg m –2 , respectively. Both minerals have the microporous tetrahedral framework structure characteristic of the mayenite supergroup. In fluormayenite 1/3 of the structural cages are occupied by fluorine. In fluorkyuygenite, in addition to fluorine and negligible amounts of OH, H 2 O molecules occupy about 2/3 of the cages. The holotype fluormayenite from Jabel Harmun has the crystal chemical formula (Ca 11.951 Na 0.037 ) 11.987 (Al 13.675 Fe 3+ 0.270 Mg 0.040 Si 0.009 P 0.005 S 6+ 0.013 ) 14.013 O 31.503 (OH) 1.492 [ 4.581 F 1.375 Cl 0.044 ] 6 , fluorkyuygenite from the Hatrurim Basin has the composition Ca 12.034 (Al 13.344 Fe 3+ 0.398 Si 0.224 ) 13.966 O 32 [(H 2 O) 3.810 F 1.894 (OH) 0.296 ] 6 . Raman spectra of fluormayenite and fluorkyuygenite in the spectral region 200–1000 cm –1 are similar and are characterized by the four strong main bands at about 320 ( 2 AlO 4 ), 520 ( 4 AlO 4 ), 700, 770 ( 1 AlO 4 ) cm –1 . In the O-H vibration region fluorkyuygenite shows a broad band between 2600–3500 cm –1 (H 2 O). The molecular water is completely released from the fluorkyuygenite structure at about 400°C. Fluorkyuygenite crystallized initially as fluormayenite, which later was altered under influence of water vapour-enriched gases during a combustion process. Fluormayenite has been synthesized and fluorkyuygenite is an analogue of the recently discovered chlorkyuygenite, Ca 12 Al 14 O 32 [(H 2 O) 4 Cl 2 ], from the Northern Caucasus, Russia.

Description: Opal has long fascinated scientists. It is one of the few minerals with an amorphous structure, and yet, compared to silica glass, it is highly organized on the mesoscale. By means of inelastic neutron scattering (INS), we could document that in four samples of opal at low temperature an ice-like structure of water is present, with details depending on microstructural characteristics. While FTIR spectra for all samples are nearly identical and thus not very informative, INS shows clear differences, highlighting the significance of microstructures. Neutron diffraction at 100 K on one of the opal samples provides evidence for crystalline cubic ice.

Description: The Al-B substitution in the system albite–reedmergnerite has been investigated experimentally by performing cold-seal hydrothermal and piston-cylinder synthesis experiments at temperatures of 450 °C and 750 °C in the pressure range 0.2–3.0 GPa. As starting material, glasses close to NaBSi 3 O 8 (R100), NaB 0.75 Al 0.25 Si 3 O 8 (R75), and NaB 0.5 Al 0.5 Si 3 O 8 (R50) composition were used. Run products were characterised by SEM, powder-XRD applying the Rietveld method, EMP- and TEM-analyses. In all but one experiment reedmergnerite or (Al, B)-bearing feldspar formed as the main phase beside quartz, coesite or jadeite in one run at 3.0 GPa, 750 °C. Amorphous phases were observed in the nominally dry experiments and were assumed to be due to incongruent melting under participation of absorbed water. Synthetic (Al, B)-feldspar from experimental runs show considerable Al-B solid solution. TEM-investigations show that the (Al, B)-feldspars are strongly twinned. No exsolution lamellae of varying compositions are visible, indicating that the (Al, B)-feldspars are chemically homogeneous. The reedmergnerite (Rd)-component in albite at constant temperature of 750 °C increases strongly with increasing pressure, with the highest Rd-content of about 50 mol% just below the albite jadeite + quartz equilibrium. The amount of Al substituting into reedmergnerite is small and the variation with pressure is not well constrained. At 3.0 GPa, 750 °C an albite content of 4 mol% is observed in reedmergnerite. Consequently, the wide miscibility gap of albite-reedmergnerite solid solution in albitic feldspars observed at low pressures strongly diminishes to higher pressures. The degree of ordering in the synthetic B-bearing feldspars was estimated from Rietveld-determined mean (T-O) distances of the four tetrahedral positions T 1o , T 1m , T 2o , T 2m . For reedmergnerite a pressure-induced high/low transition appears to occur within the pressure range 1.0–1.5 GPa at 750 °C. This experimental study indicates that albitic feldspars can reach maximum Rd-contents of up to about 20 mol% under late-stage Band Na-enriched pegmatitic conditions. At fluid-saturated conditions such feldspars can be transported during subduction to depths of about 40 km until their breakdown and formation of a B-bearing melt. Natural albitic feldspars, typically containing only traces of B, will decompose in subduction zones below about 50 km depths and produce a melt with low B-contents.

Description: Albitization of K-feldspar has been studied experimentally at 500 and 750 °C and 200 MPa using concentrated NaCl solutions. The partially reacted feldspar grains were characterized using X-ray diffraction, electron probe microanalysis, transmission electron microscopy as well as Raman and laser-induced photoluminescence spectroscopy. The replacement of K-feldspar by albite occurs via coupled dissolution-reprecipitation due to the mobilization of K in exchange for Na and results in the mobilization of minor elements like Ba, Fe, Ti, and Mg. The release of tetrahedrally incorporated Fe 3+ and Ti 4+ in K-feldspar is temperature-dependent resulting in increasing mobilization with increasing temperature. Barium shows a positive correlation with K in the reacted areas. Replacement also leads to the formation of cogenetic rutile and Fe-rich pseudobrookite inclusions in the albitized regions. This supports the proposal that the Fe responsible for the formation of hematite inclusions in altered, natural alkali feldspars can originate from the feldspar.

Description: Many studies dealing with the synthesis of tourmaline report a sharp intragranular chemical gradient and extensive porosity in the core zones of the crystals, both of which still lack a reliable explanation. Using the example of olenitic tourmaline, we show that these features are likely explained by the occurrence of a precursor phase during tourmaline formation. Time-dependent piston–cylinder synthesis experiments were performed in the system SiO 2 –Al 2 O 3 –B 2 O 3 –NaCl–H 2 O at 700 °C/40 kbar with run durations of 0.5 h, 2.5 h and 216 h, starting from quartz–Al 2 O 3 –H 3 BO 3 solid mixtures and NaCl solutions. Sharply zoned olenitic tourmaline ( [4] B-rich cores, [4] B-poor rims) formed in all experiments, with its abundance increasing with increasing run duration. The amount of the additional solid product phases coesite and jeremejevite decreased with time. Extensive porosity is recognized in jeremejevite and in the cores of early grown acicular tourmaline. Textural relationships indicate that olenitic tourmaline grows at the expense of jeremejevite which acts as a crystalline precursor in this system. A possible reaction is: \[ 3{\mathrm{Al}}_{6}{\left({\mathrm{BO}}_{3}\right)}_{5}{\left(\mathrm{OH}\right)}_{3}+(12-2x){\mathrm{SiO}}_{2(\mathrm{aq})}+2y{\mathrm{NaCl}}_{(\mathrm{aq})}\to 2{\mathrm{Na}}_{y}{\mathrm{Al}}_{3}{\mathrm{Al}}_{6}({\mathrm{Si}}_{6-x}{\mathrm{B}}_{x}){\mathrm{O}}_{18}{\left({\mathrm{BO}}_{3}\right)}_{3}({\mathrm{O}}_{3-x},{\mathrm{OH}}_{1+x})+(4.5-x){\mathrm{B}}_{2}{\mathrm{O}}_{3(\mathrm{aq})}+(2.5-x){\mathrm{H}}_{2}\mathrm{O}+2y{\mathrm{HCl}}_{(\mathrm{aq})}. \] The transformation likely proceeds via a dissolution/re-precipitation mechanism, which triggers the sharp chemical zonation in olenite. Based on Rayleigh fractionation modelling, we estimate a minimum B isotope fractionation between jeremejevite and fluid with 11 B jer-fluid –2.8 at 700 °C/40 kbar. Due to the progressive dissolution of jeremejevite, the fluids 11 B values continuously decrease with increasing run duration. Hence, olenite growth concomitant with the dissolution of jeremejevite will produce scattered or inverse boron isotope patterns (heavy cores, light rims) in tourmaline, which cannot result from simple Rayleigh fractionation. Similar reactions involving jeremejevite or other precursor phases might explain chemical zonation and porous textures in tourmaline core zones reported in many experimental studies. The occurrence of natural tourmaline overgrowing jeremejevite in pegmatites of the Erongo Mountains, Namibia, gives rise to the assumption that jeremejevite might also act as a precursor for tourmaline formation in natural systems.

Description: Shulamitite, ideally Ca 3 TiFe 3+ AlO 8 , is a mineral intermediate between perovskite CaTiO 3 and brownmillerite Ca 2 (Fe,Al) 2 O 5 . It was discovered as a major mineral in a high-temperature larnite-mayenite rock from the Hatrurim Basin, Israel. Shulamitite is associated with larnite, F-rich mayenite, Cr-containing spinel, ye'elimite, fluorapatite, and magnesioferrite, and retrograde phases (portlandite, hematite, hillebrandite, afwillite, foshagite and katoite). The mineral forms reddish brown subhedral grains or prismatic platelets up to 200 μm and intergrowths up to 500 μm. The empirical formula of the holotype shulamitite (mean of 73 analyses) is (Ca 2.992 Sr 0.007 LREE 0.007 )(Ti 0.981 Zr 0.014 Nb 0.001 )(Fe 3+ 0.947 Mg 0.022 Cr 0.012 Fe 2+ 0.012 Mn 0.001 )(Al 0.658 Fe 3+ 0.288 Si 0.054 )O 8 . The X-ray diffraction powder-pattern (Mo Kα -radiation) shows the strongest lines {d [Å]( I obs )} at: 2.677(100), 2.755(40), 1.940(40), 11.12(19), 1.585(17), 1.842(16), 1.559(16), 3.89 (13), 1.527(13). The unit-cell parameters and space group are: a = 5.4200(6), b = 11.064(1), c = 5.5383(7) Å, V= 332.12(1) Å 3 , Pmma, Z = 2. The calculated density is 3.84 g/cm 3 . The crystal structure of shulamitite has been refined from X-ray single-crystal data to R 1 = 0.029 %. No partitioning among octahedral sites was found for Ti and Fe 3+ in the structure of shulamitite, these cations are randomly distributed among all octahedra indicating an example of "valency-imposed double site occupancy". The strong bands in the Raman spectrum of shulamitite are at: 238,250, 388,561, and 742 cm –1 . Shulamitite from the Hatrurim Basin crystallized under combustion metamorphism conditions characterized by very high temperatures (1150–1170 °C) and low pressures (high- T -region of the spurrite-merwinite facies). Chemical data for shulamitite and its Fe-analog from other metacarbonate occurrences (natural and anthropogenic) are given here.