ALBERT

Description: An experimental study of an Al-rich schorl sample from Cruzeiro mine (Minas Gerais, Brazil) was carried out using electron microprobe analysis, structural refinement and Mössbauer, infrared and optical absorption spectroscopy in order to explore the disordering of Fe 2+ over the Y and Z sites of the tourmaline structure. A structural formula was obtained by merging chemical and structural data. The cation distribution at the two non-equivalent octahedrally coordinated sites ( Y and Z ) was obtained by two different optimization procedures which, minimizing the residuals between observed and calculated data, converged to the formula: X (Na 0.65 0.32 Ca 0.02 K 0.01 ) 1.00 Y ( $${\mathrm{Fe}}_{1.65}^{2+}$$ Al 1.15 $${\mathrm{Fe}}_{0.06}^{3+}$$ $${\mathrm{Mn}}_{0.05}^{2+}$$ Zn 0.05 $${\mathrm{Ti}}_{0.04}^{4+}$$ ) 3.00 Z (Al 5.52 $${\mathrm{Fe}}_{0.30}^{2+}$$ Mg 0.18 ) 6.00 [ T (Si 5.87 Al 0.13 ) 6.00 O 18 ]( B BO 3 ) 3 V (OH) 3 W [(OH) 0.34 F 0.28 O 0.38 ] 1.00 . This result shows a partial disordering of Fe 2+ over the Y and Z sites which explains adequately the mean atomic number observed for the Z site (13.5±0.1). Such a disordering is also in line with the shoulder recorded in the Mössbauer spectrum (fitted by a doublet with isomer shift of 1.00 mm/s and quadrupole splitting of 1.38 mm/s) as well as with the asymmetric bands recorded in the optical absorption spectrum at ~9000 and 14,500 cm –1 (modelled by four Gaussian bands, centred at 7677 and 9418 cm –1 , and 13,154 and 14,994 cm –1 , respectively). The high degree of consistency in the results obtained using the different methods suggests that the controversy over Fe 2+ order can be ascribed to the failure to detect small amounts of Fe 2+ at Z (typically 〈〈10% atoms/site) rather than a steric effect of Fe 2+ on the tourmaline structure.

Description: Fluor-tsilaisite, NaMn 3 Al 6 (Si 6 O 18 )(BO 3 ) 3 (OH) 3 F, is a new mineral of the tourmaline supergroup. It occurs in an aplitic dyke of a LCT-type pegmatite body from Grotta d'Oggi, San Piero in Campo, Elba Island, Italy, in association with quartz, K-feldspar, plagioclase, elbaite, schorl, fluor-elbaite and tsilaisite. Crystals are greenish yellow with a vitreous lustre, sub-conchoidal fracture and white streak. Fluor-tsilaisite has a Mohs hardness of ~7 and a calculated density of 3.134 g/cm 3 . In plane-polarized light, fluor-tsilaisite is pleochroic (O = pale greenish yellow and E = very pale greenish yellow), uniaxial negative. Fluor-tsilaisite is rhombohedral, space group R 3 m , a = 15.9398(6), c = 7.1363(3) Å, V = 1570.25(11) Å 3 , Z = 3. The crystal structure of fluor-tsilaisite was refined to R 1 = 3.36% using 3496 unique reflections collected with Mo K α X-ray intensity data. Crystal-chemical analysis resulted in the empirical formula: X (Na 0.69 0.29 Ca 0.02 ) 1.00 Y ( $${\mathrm{Mn}}_{1.29}^{2+}$$ Al 1.21 Li 0.56 Ti 0.03 ) 6.00 Z Al 6 T (Si 5.98 Al 0.03 ) 6.00 B 2.92 O 27 V (OH) 3 W [F 0.39 (OH) 0.25 O 0.36 ] 1.00 . Comparisons were performed between fluor-tsilaisite and a tsilaisitic tourmaline from the same locality as the holotype specimen. This latter tourmaline sample was selected for this study due to its remarkable composition (MnO = 11.63 wt.%), the largest Mn content found in tourmaline so far. Fluor-tsilaisite is related to tsilaisite through the substitution W F W (OH) and with fluor-elbaite through the substitution Y (Al + Li) 2 Y Mn 2+ , and appears to be a stepwise intermediate during tourmaline evolution from tsilaisite to fluor-elbaite.

Description: In this work a single crystal of synthetic hercynite, FeAl 2 O 4 , was investigated by X-ray diffraction up to 7.5 GPa and at room temperature, in order to determine its pressure–volume equation of state. The unit-cell volume decreases non-linearly with a reduction of 3.4% (i.e. 18.43 Å 3 ). The pressure–volume data were fitted to a third-order Birch-Murnaghan equation of state providing the following coefficients: V 0 = 542.58(3) Å 3 , K T0 = 193.9(1.7) GPa, K ' = 6.0(5). These results are consistent with previous investigations of Cr and Al spinels measured with the same experimental approach but the K T0 differs significantly from the experimental determination carried out more than 40 years ago by Wang and Simmons ( 1972 ) by the pulse echo overlap method. Our new results were used to redetermine the FeAl 2 O 4(hercynite) = FeO (wüstite) + Al 2 O 3(corundum) equilibrium in P–T space and obtain geobarometric information for Cr-Al spinels found as inclusions in diamond.

Description: The elastic constants C ij of a set of synthetic single crystals belonging to the join MgAl 2 O 4 –MnAl 2 O 4 (spinel sensu stricto–galaxite) were determined by Brillouin spectroscopy at ambient conditions. The C 11 component tends to remain constant for Mg-rich compositions ( X Mn 〈 0.5) and decreases in Mn-rich compositions, whereas C 12 increases and C 44 decreases almost linearly from MgAl 2 O 4 to MnAl 2 O 4 . The bulk modulus K S is weakly dependent upon Mg-Mn substitution within experimental uncertainties, whereas the shear modulus G decreases with increasing Mn 2+ content. For MnAl 2 O 4 , C 11 = 271.3(1.3) GPa, C 12 = 164.8(1.3) GPa, C 44 = 124.9(5) GPa, K S = 200(1) GPa, and G = 88.7(5) GPa. Based on the "polyhedral approach," we developed a model that describes the crystal bulk moduli of the MgAl 2 O 4 –MnAl 2 O 4 spinels in terms of their cation distribution and the polyhedral bulk moduli of the different cations. We refined a set of values for the effective polyhedral bulk modulus of Mg, Mn 2+ , and Al in tetrahedral (T) and octahedral (M) sites, which span from 153 to 270 GPa ranking as follows: K M Mn 〈 K M Mg 〈 K T Mg K T Mn 〈 K M Al 〈〈 K T Al . Crystal bulk modulus was perfectly reproduced within 0.1% for all Mn 2+ -bearing samples. We also found a high linear correlation between the effective polyhedral bulk modulus and the ionic potential, IP, of the coordinating cations: K i j (GPa) = 20(2) IP + 108(10) (where i indicates the cation and j the polyhedral site). We tested this simple correlation by calculating the specific effective polyhedral bulk modulus of several cations in T and M coordination and then predicting the crystal bulk modulus for several spinel compositions. The success of our simple correlation in modeling the bulk modulus of spinels outside the MgAl 2 O 4 –MnAl 2 O 4 system is encouraging, and suggests that the relationships between chemical composition, cation distribution and elastic properties in spinel-structured minerals and materials can indeed be expressed by relatively simple models.

Description: Fluor-elbaite, Na(Li 1.5 Al 1.5 )Al 6 (Si 6 O 18 )(BO 3 ) 3 (OH) 3 F, is a new mineral of the tourmaline supergroup. It is found in miarolitic cavities in association with quartz, pink muscovite, lepidolite, spodumene, spessartine, and pink beryl in the Cruzeiro and Urubu mines (Minas Gerais, Brazil), and apparently formed from late-stage hydrothermal solutions related to the granitic pegmatite. Crystals are blue-green with a vitreous luster, sub-conchoidal fracture and white streak. Fluor-elbaite has a Mohs hardness of approximately 7.5, and has a calculated density of about 3.1 g/cm 3 . In plane-polarized light, fluor-elbaite is pleochroic (O = green/bluish green, E = pale green), uniaxial negative. Fluor-elbaite is rhombohedral, space group R 3 m, a = 15.8933(2), c = 7.1222(1) Å, V = 1558.02(4) Å 3 , Z = 3 (for the Cruzeiro material). The strongest eight X-ray-diffraction lines in the powder pattern [ d in Å( I )( hkl )] are: 2.568(100)(051), 2.939(92)(122), 3.447(67)(012), 3.974(58)(220), 2.031(57)(152), 4.200(49)(211), 1.444(32)(642), and 1.650(31)(063). Analysis by a combination of electron microprobe, secondary ion mass spectrometry, and Mössbauer spectroscopy gives SiO 2 = 37.48, Al 2 O 3 = 37.81, FeO = 3.39, MnO = 2.09, ZnO = 0.27, CaO = 0.34, Na 2 O = 2.51, K 2 O = 0.06, F = 1.49, B 2 O 3 = 10.83, Li 2 O = 1.58, H 2 O = 3.03, sum 100.25 wt%. The unit formula is: X (Na 0.78 0.15 Ca 0.06 K 0.01 ) Y (Al 1.15 Li 1.02 Fe 2+ 0.46 Mn 2+ 0.28 Zn 0.03 ) Z Al 6 T (Si 6.02 O 18 ) B (BO 3 ) 3 V (OH) 3 W (F 0.76 OH 024 ). The crystal structure of fluor-elbaite was refined to statistical indices R 1 for all reflections less then 2% using Mo K α X-ray intensity data. Fluor-elbaite shows relations with elbaite and tsilaisite through the substitutions W F W OH and Y (Al + Li) + W F 2 Y Mn 2+ + W OH, respectively.

Description: The crystal chemistry of Cr-spinels included in spinel peridotite mantle xenoliths from Baker Rocks and Greene Point (northern Victoria Land, Antarctica) has been studied by single-crystal structure refinement and electron microprobe analysis. All crystals are characterized by a dominance of Al Cr substitution with minor evidences of Mg Fe 2+ substitution and pertain to the Mg-rich portion of the spinel sensu stricto-chromite join. The two groups of samples, Baker Rocks (BR) and Greene Point (GP), show distinct degree of cation order with the inversion parameter ranging from 0.17 to 0.20 for BR spinels and from 0.06 to 0.13 for GP crystals. Closure temperatures, computed by a geothermometer based on the M Al+ T Mg T Al+ M Mg intracrystalline exchange, range from 883 to 911 °C for BR spinels and from 592 to 675 °C for GP spinels. We show that this difference is due to the higher concentration in Fe 3+ in GP spinels that enabled a faster kinetics of the intracrystalline cation ordering reaction, allowing the GP spinels to reach a higher degree of cation ordering and then lower closure temperatures.

Description: Natural blue Co-bearing spinel crystals are rare and actively sought as gemstones, while synthetic blue Co-bearing spinel powders are largely used as ceramic pigments. High-quality spinel single crystals with compositions closely corresponding to the solid-solution series spinel sensu stricto (MgAl 2 O 4 )-cobalt spinel (CoAl 2 O 4 ) were produced by flux growth method, with Na 2 B 4 O 7 as flux. Low-cooling rates (2 °C/h) and linear temperature profiles were applied in the thermal interval 1200–800 °C, followed by rapid cooling. Thermal runs were performed in reducing atmosphere ( f O2 10 –8 –10 –15 bars) created by a continuous flow of a CO 2 :H 2 mix with a ratio of 100:4 (cm 3 /min). Ten experiments were successfully carried out and hundreds of inclusion-free gem-quality single crystals (up to 1 mm large) were produced in each of them, sometimes together with crusty aggregates and microcrystalline powder. Selected crystals were investigated by SEM/EDS X-ray mapping to check for compositional homogeneity and by electron-microprobe analysis to obtain the chemical formula. Crystals were found to be chemically homogeneous and entirely representing the MgAl 2 O 4 -CoAl 2 O 4 solid-solution series, with the latter component ranging from 7 to 100%. With increasing Co 2+ contents, the crystals vary in color from light blue to intensely dark blue in daylight. The unit-cell parameter a increases from 8.084 to 8.105 Å along the solid-solution series, and the observed increase is determined more by the inversion degree than by the variation in Co contents. The composition of crystal products does not correspond to the composition of the starting oxide mixture, being cobalt enriched in the crystals. A tentative explanation of this behavior is suggested by considering possible ionic potential as well crystal field stabilization effects.

Description: The pressure-volume equation of state for the two spinel end-member compositions chromite FeCr 2 O 4 and magnesiochromite MgCr 2 O 4 was determined for flux-grown synthetic single crystals at room temperature up to 8.2 and 9.2 GPa, respectively, by single-crystal X-ray diffraction using a diamond-anvil cell. The pressure-volume data show that the linear volume compressibility (here used only for purpose of comparison), calculated as β V = |[( V / V 0 )/ P ]|, is 0.00468 and 0.00470 GPa –1 , for chromite and magnesiochromite, respectively, with a negligible difference below 0.5%. The experimental data were fitted to a third-order Birch-Murnaghan equation of state (BM3) allowing a simultaneous refining of the following coefficients: V 0 = 588.47(4) Å 3 , K T0 = 184.8(1.7) GPa, and K' = 6.1(5) for chromite and V 0 = 579.30(4) Å 3 , K T0 = 182.5(1.4) GPa, and K' = 5.8(4) for magnesiochromite. The difference in K T0 is reduced to 〈1.5% going from Fe to Mg end-member composition, whereas the first pressure derivative seems not to be affected by the chemical variability. The limited difference in the equation of state coefficients recorded for FeCr 2 O 4 and MgCr 2 O 4 allowed us to fit the pressure-volume data of both to a single BM3 equation, resulting in a K T0 = 184.4(2.2) GPa and K' = 5.7(6).

Description: Four gahnite single crystals with variable colors from pale blue to green have been studied by a multi-analytical approach with the aim to evaluate existing assignments of optical absorption bands. Combined information from electron microprobe analyses, Mössbauer spectroscopy, IR-spectroscopy, single-crystal X-ray structure refinements, and optical absorption spectroscopy confirms the conclusions of earlier studies that the absorption bands recorded in the visible spectral region up to ~540 nm (above ~18 500 cm –1 ) are related to electronic d-d transitions in tetrahedrally coordinated Fe 2+ . It also demonstrates that a set of absorption bands between ~550–625 nm (~16 000–18 200 cm –1 ) are caused by spin-allowed and spin-forbidden d-d transitions in tetrahedrally coordinated Co 2+ . Two absorption bands at higher wavelengths (~680 and ~800 nm, i.e., ~14 700 and ~12 500 cm –1 ) are assigned to electronic transitions in exchange coupled VI Fe 3+ - IV Fe 2+ pairs and a band at ~950 nm (~10 500 cm –1 ) is assigned to a spin-allowed electronic transition in VI Fe 2+ . Low-Fe gahnite crystals owe their blue color to traces of cobalt at concentration levels in the order of 200 ppm and less, while the green color of gahnite crystals with higher Fe-contents is due to a combination of electronic ligand-metal transitions causing strong UV-absorption and electronic transitions in exchange coupled Fe 2+ -Fe 3+ cation pairs that absorb in the red region of the visible spectrum. A detailed characterization of samples that includes cation site occupancy and iron valency data is demonstrated to be crucial for interpreting optical absorption spectra. Also electronic transitions in trace element chromophores below the detection limit of electron microprobe may participate to light absorption. All this information contribute to the comprehension of the causes of crystal color of minerals, gemstones, and ceramic pigments.

Description: Optical absorption spectroscopy and X-ray structural refinements were used to characterize short-range and long-range structures of 10 gem-quality, blue spinel single crystals synthesized on the (Mg 1– x Co x )Al 2 O 4 solid solution ( x = 0.07–1.00). The site distributions of Mg, Co 2+ , and Al show that the tetrahedrally coordinated site (T) is mainly populated by Mg and Co 2+ , with a marked preference of Co 2+ for tetrahedral coordination with respect to Mg, while the octahedrally coordinated site (M) is dominated by Al. Crystals also show a certain degree of inversion, i.e., occurrence of Al at T counterbalanced by the occurrence of divalent cations at M, which decreases from 0.24 to 0.13 with increasing Co 2+ content. Short-range information based on the crystal field splitting parameter Dq derived from single-crystal optical spectra suggests that the local Co 2+ -O bond length at the T-site may increase marginally at increasing Co 2+ content. An almost constant value for the Racah B -parameter, also derived from optical spectra, for tetrahedrally coordinated Co 2+ suggests that any influence of substitutional second nearest neighbor cations on the ionicity of Co 2+ -O bonds at the T-site is very small. Long-range information shows that variations in the unit-cell parameter from 8.084 to 8.105 Å along the solid-solution series are mainly related to the ordering of Al at the M site as a result of the replacement of Mg by Co 2+ . Therefore, the spinel structure responds to the chemical variation by ordering of Al in such a manner that M-O remains almost constant and T-O increases. In this way, the lengths of shared octahedral edges are reduced and the destabilization effect due to the increased octahedral cation-cation repulsion is minimized. In line with other studies, the importance of steric factors for controlling the cation distributions in the spinel structure has also been shown to be valid in the MgAl 2 O 4 -CoAl 2 O 4 solid-solution series.