ALBERT

Description: Microanalysis of Fe 3+ /Fe in geological samples using synchrotron-based X-ray absorption spectroscopy has become routine since the introduction of standards and model compounds. Existing calibrations commonly use least-squares linear combinations of pre-edge data from standard reference spectra with known coordination number and valence state acquired on powdered samples to avoid preferred orientation. However, application of these methods to single mineral grains is appropriate only for isometric minerals and limits their application to analysis of in situ grains in thin sections. In this work, a calibration suite developed by acquiring X-ray absorption near-edge spectroscopy (XANES) data from amphibole single crystals with the beam polarized along the major optical directions (X, Y, and Z) is employed. Seven different methods for predicting %Fe 3+ were employed based on (1) area-normalized pre-edge peak centroid, (2) the energy of the main absorption edge at the location where the normalized edge intensity has the highest R 2 correlation with Fe 3+ /Fe, (3) the ratio of spectral intensities at two energies determined by highest R 2 correlation with Fe 3+ /Fe, (4) use of the slope (first derivative) at every channel to select the best predictor channel, (5 and 6) partial least-squares models with variable and constant numbers of components, and (7) least absolute shrinkage and selection operator models. The latter three sophisticated multivariate analysis techniques for predicting Fe 3+ /Fe show significant improvements in accuracy over the former four types of univariate models. Fe 3+ /Fe can be measured in randomly oriented amphibole single crystals with an accuracy of ±5.5–6.2% absolute. Multivariate approaches demonstrate that for amphiboles main edge and EXAFS regions contain important features for predicting valence state. This suggests that in this mineral group, local structural changes accommodating site occupancy by Fe 3+ vs. Fe 2+ have a pronounced (and diagnostic) effect on the XAS spectra that can be reliably used to precisely constrain Fe 3+ /Fe.

Description: 〈span〉Natural garnets are robust silicate minerals stable over large ranges of temperature and pressure that can provide useful geochemical constraints on petrogenetic conditions. The purpose of this study is to determine how the size of cation sites controls rare earth elements (REE) incorporation in mantle eclogitic garnets. Major and lanthanide element data in a suite of mantle-derived eclogite garnets were combined with new single-crystal structure refinements (SREFs) to examine the effects of major-element chemistry and site dimension on the incorporation of REE into the garnet structure. Several distinct trends are apparent. Pyrope-rich samples similar to mantle lherzolitic garnets are enriched in the smaller heavy rare earth elements (HREE). Almandine-rich garnets are also HREE-enriched, but low rare earth elements (LREE) values are lower than in the pyrope-rich garnets. Intermediate garnet compositions are more depleted in HREE and enriched in LREE (Ce, Nd, and Sm). Finally, Ca-rich garnets (50% grossular component) are depleted in LREE and HREE, but are enriched in MREE. Hence, the X site dimension does exert a crucial role in REE incorporation into the garnet structure. Crystal structure refinements provide further evidence of this influence.〈/span〉

Description: Magnesio-arfvedsonite, the C Fe 3+ -dominant analogue of eckermannite, has been found in a sample of "szechenyite" in the mineral collection of the American Museum of Natural History (AMNH H35024). It comes from the northern part of the Jade Mine Tract near Hpakan, Kachin State, Myanmar. Associated minerals are kosmochlor–jadeite solid-solution pyroxene and clinochlore. The ideal formula of magnesio-arfvedsonite is A Na B Na 2 C (Mg 4 Fe 3+ ) T Si 8 O 22 W (OH) 2 , and the empirical formula derived from electron microprobe analysis and single-crystal structure refinement for the sample of this work is A (Na 0.96 K 0.04 ) =1.00 B (Na 1.57 Ca 0.40 $${\mathrm{Fe}}_{0.02}^{2+}$$ Mn 0.01 ) =2.00 C (Mg 4.26 $${\mathrm{Fe}}_{0.19}^{2+}$$ $${\mathrm{Fe}}_{0.41}^{3+}$$ Al 0.11 $${\mathrm{Ti}}_{0.03}^{4+}$$ ) =5.00 T (Si 7.99 Al 0.01 ) =8.00 O 22 W [F 0.02 (OH) 1.98 ] =2.00 . The unit-cell dimensions are a = 9.867(1), b = 17.928(2), c = 5.284(1) Å, β = 103.80(2)°, V = 907.7 (2) Å 3 , Z = 2. Magnesio-arfvedsonite is biaxial (–), with α = 1.624, β = 1.636, = 1.637, all ± 0.002 and 2V obs = 36(1)°, 2V calc = 32°. The ten strongest reflections in the X-ray powder pattern [ d values (in Å), I , ( hkl )] are: 2.708, 100, (151); 3.399, 68, (131); 3.144, 63, (310); 2.526, 60, (2I02); 8.451, 46, (110); 3.273, 39, (240); 2.167, 37, (261); 2.582, 34, (061); 2.970, 34, (221); 2.326, 33, [(2I51) (4I21)].

Description: The crystal-chemical characterization of an amphibole with an unusual composition, A (Na 0.76 K 0.24 ) B (Ca 1.42 Na 0.56 Mn 2+ 0.02 ) C (Mg 2.64 Fe 2+ 1.95 Mn 2+ 0.07 Mg 2.64 Zn 0.01 Fe 3+ 0.01 Ti 4+ 0.32 ) T (Si 7.18 Al 0.82 )O 22 W [(OH) 0.58 O 0.27 F 1.15 ], found in pegmatitic veins at Kariåsen, Larvik Plutonic Complex, Norway, provides an excellent example of the detection and estimation of the oxo component in amphibole. The use of Ti as a proxy for the oxo component is discussed and a procedure to derive accurate Ti partitioning from the results of structure refinement is described. Because the presence and amount of oxo component in amphiboles are important in order to determine values of f O 2 and f H 2 O , especially in igneous and magmatic systems, this procedure should be applied any time the compositional data or the petrological context indicate the presence of significant Ti, or suggest that the oxo component may be a relevant issue.

Description: Katophorite has the ideal formula A Na B (NaCa) C (Mg 4 Al) T (Si 7 Al)O 22 W (OH) 2 ( Hawthorne et al. , 2012 ). No published analyses of amphiboles fall in the katophorite compositional field, except that of Harlow and Olds ( 1987 ) for an amphibole from near Hpakan in the Jade Mine Tract, Myanmar. This amphibole was approved by the International Mineralogical Association Commission on New Minerals, Nomenclature and Classification (vote 2013-140) as katophorite, and is reported here. Holotype katophorite is monoclinic, space group C 2/ m, a = 9.8573(8), b = 17.9617(15), c = 5.2833(4) Å, β = 104.707(2)°, V = 904.78(13) Å 3 , Z = 2. The calculated density is 3.091 g cm –3 . In plane-polarized light, katophorite is pleochroic, X = pale blue (medium), Y = light blue-green (strongest), Z = colourless; X ^ a = 30.6° (β obtuse), Y || b, Z ^ c = 15.8 (β acute). It is biaxial negative, α = 1.638, β = 1.642, = 1.644, all ± 0.002; 2V obs = 73(1)°, 2V calc = 70°. The eight strongest lines in the powder X-ray diffraction pattern are [ d in Å ( I )( hkl )]: 2.700 (100)(151), 3.129 (69)(310), 2.536 (65)(2I02), 3.378 (61)(131), 8.421 (55)(110), 2.583 (46)(061), 2.942 (43)(221) and 2.334 (41)(3I51). Electron-microprobe analysis of the refined crystal gave SiO 2 51.74, Al 2 O 3 7.38, TiO 2 0.14, FeO 1.55, Fe 2 O 3 2.82, MgO 18.09, CaO 8.17, Na 2 O 6.02, K 2 O 0.24, F 0.06, H 2 O calc. 1.80, Li 2 O calc. 0.09, sum 100.55 wt.% (Li 2 O and H 2 O based on the results of single-crystal structure refinement). The formula unit, calculated on the basis of 24 (O,OH,F) with (OH + F + O) = 2 is: A (Na 0.85 K 0.04 ) =0.89 B (Ca 1.22 Na 0.78 ) =2.00 C (Mg 3.76 Al 0.43 $${\mathrm{Fe}}_{0.30}^{3+}$$ $${\mathrm{Cr}}_{0.27}^{3+}$$ $${\mathrm{Fe}}_{0.18}^{2+}$$ Li 0.05 $${\mathrm{Ti}}_{0.01}^{4+}$$ ) =5.00 T (Si 7.21 Al 0.79 ) =8.00 O 22 W [(OH) 1.67 O 0.30 F 0.03 )] =2.00 .

Description: 〈span〉The ideal formulae of two root-names of the hellandite group, 〈span〉i.e.〈/span〉 mottanaite and ciprianiite, were originally given as 〈sup〉〈span〉X〈/span〉〈/sup〉Ca〈sub〉4〈/sub〉〈sup〉〈span〉Y〈/span〉〈/sup〉(CeCa)〈sub〉2〈/sub〉〈sup〉〈span〉Z〈/span〉〈/sup〉Al〈sup〉〈span〉T〈/span〉〈/sup〉Be〈sub〉2〈/sub〉[B〈sub〉4〈/sub〉Si〈sub〉4〈/sub〉O〈sub〉22〈/sub〉]〈sup〉〈span〉W〈/span〉〈/sup〉O〈sub〉2〈/sub〉 and 〈sup〉〈span〉X〈/span〉〈/sup〉Ca〈sub〉4〈/sub〉〈sup〉〈span〉Y〈/span〉〈/sup〉[(Th,U)REE]〈sub〉Σ2〈/sub〉〈sup〉〈span〉Z〈/span〉〈/sup〉Al〈sup〉〈span〉T〈/span〉〈/sup〉□〈sub〉2〈/sub〉[B〈sub〉4〈/sub〉Si〈sub〉4〈/sub〉O〈sub〉22〈/sub〉]〈sup〉〈span〉W〈/span〉〈/sup〉(OH)〈sub〉2〈/sub〉. In order to conform to the later introduced dominant-valency nomenclature rule, they have been redefined as 〈sup〉〈span〉X〈/span〉〈/sup〉Ca〈sub〉4〈/sub〉〈sup〉〈span〉Y〈/span〉〈/sup〉REE〈sub〉2〈/sub〉〈sup〉〈span〉Z〈/span〉〈/sup〉Al〈sup〉〈span〉T〈/span〉〈/sup〉(Be〈sub〉1.5〈/sub〉□〈sub〉0.5〈/sub〉)[B〈sub〉4〈/sub〉Si〈sub〉4〈/sub〉O〈sub〉22〈/sub〉]〈sup〉〈span〉W〈/span〉〈/sup〉O〈sub〉2〈/sub〉 and 〈sup〉〈span〉X〈/span〉〈/sup〉Ca〈sub〉4〈/sub〉〈sup〉〈span〉Y〈/span〉〈/sup〉[(Th,U)Ca]〈sub〉Σ2〈/sub〉〈sup〉〈span〉Z〈/span〉〈/sup〉Al〈sup〉〈span〉T〈/span〉〈/sup〉(Be〈sub〉1.5〈/sub〉□〈sub〉0.5〈/sub〉)〈sub〉Σ2〈/sub〉[B〈sub〉4〈/sub〉Si〈sub〉4〈/sub〉O〈sub〉22〈/sub〉]〈sup〉〈span〉W〈/span〉〈/sup〉O〈sub〉2〈/sub〉, respectively (IMA-CNMNC vote 18D). Also, the mineral description is provided for the first Fe〈sup〉3+〈/sup〉-dominant species of the hellandite group, ferri-mottanaite-(Ce), ideally 〈sup〉〈span〉X〈/span〉〈/sup〉Ca〈sub〉4〈/sub〉〈sup〉〈span〉Y〈/span〉〈/sup〉Ce〈sub〉2〈/sub〉〈sup〉〈span〉Z〈/span〉〈/sup〉Fe〈sup〉3+〈span〉T〈/span〉〈/sup〉(Be〈sub〉1.5〈/sub〉□〈sub〉0.5〈/sub〉)[Si〈sub〉4〈/sub〉B〈sub〉4〈/sub〉O〈sub〉22〈/sub〉]〈sup〉〈span〉W〈/span〉〈/sup〉O〈sub〉2〈/sub〉. The type specimen was found in an ejectum collected at Tre Croci (Vetralla, Vico volcanic province, Italy). The empirical formula derived from electron microprobe and LA-ICP-MS analyses, and validated by single-crystal structure refinement is: 〈sup〉〈span〉X〈/span〉〈/sup〉(Ca)〈sub〉4〈/sub〉〈sup〉〈span〉Y〈/span〉〈/sup〉(Ca〈sub〉0.40〈/sub〉REE〈sub〉0.93〈/sub〉(Th,U)0.544+〈sub〉0.13〈/sub〉)〈sub〉Σ2.00〈/sub〉〈sup〉〈span〉Z〈/span〉〈/sup〉(Fe0.503+Al〈sub〉0.23〈/sub〉Mn0.173+Ti0.174+)〈sub〉Σ1.07〈/sub〉〈sup〉〈span〉T〈/span〉〈/sup〉(Be〈sub〉1.04〈/sub〉Li〈sub〉0.04〈/sub〉〈sub〉0.92〈/sub〉)〈sub〉Σ2.00〈/sub〉[Si〈sub〉4.03〈/sub〉B〈sub〉3.89〈/sub〉O〈sub〉22〈/sub〉] (O〈sub〉1.09〈/sub〉(OH)〈sub〉0.38〈/sub〉F〈sub〉0.53〈/sub〉)〈sub〉Σ2.00.〈/sub〉 Ferri-mottanaite-(Ce) is biaxial (–), with α = 1.748(5), β = 1.762(5), γ = 1.773(5) and 2 〈span〉V〈/span〉 (meas.) = 85.9(5)°, 2 〈span〉V〈/span〉 (calc.) = 82.5°. The unit-cell parameters are 〈span〉a〈/span〉 = 19.0548(9), 〈span〉b〈/span〉 = 4.7468(2), 〈span〉c〈/span〉 = 10.2560(5) Å, β = 110.906(2)°, 〈span〉V〈/span〉 = 866.58(7) Å〈sup〉3〈/sup〉, 〈span〉Z〈/span〉 = 2, space group 〈span〉P〈/span〉2/〈span〉a〈/span〉. The strongest reflections in the X-ray powder pattern obtained from single-crystal data [〈span〉d〈/span〉 values (in Å), 〈span〉I〈/span〉, (〈span〉hkl〈/span〉)] are: 2.648, 100, (013,4¯13); 2.857, 50, (411); 1.904, 48, (023,4¯23,6¯21); 2.919, 44, (212); 3.086, 44, (4¯12); 3.246, 43, (4¯10); 3.453, 36, (2¯12); 4.745, 33, (010).〈/span〉

Description: The crystal structure of synthetic BaMg(CO 3 ) 2 whose mineral name is norsethite was re-investigated by single-crystal X-ray diffraction. Complementary in situ high- and low-temperature studies by means of vibrational spectroscopy (Raman, IR), powder X-ray diffraction techniques and thermal analyses were performed. Diffraction images (298 K) revealed weak superstructure reflections caused by the displacement of the O atoms in the earlier considered R 3I m structure model ( a = 5.0212(9), c new = 2 c old = 33.581(6) Å, R 3I c , Z = 6, R 1 = 0.011, sin/ 〈 0.99 Å –1 ). Thermal analyses reveal decarbonatization in two decomposition steps above 750 K, and the heat-flow curves (difference scanning calorimetry) give clear evidence of a weak and reversible endothermal change at 343±1 K. This agrees with a discontinuity in the IR and single-crystal Raman spectra. The changing trend of the c/a ratio supports this discontinuity indicating a temperature-induced structural transition in the range between 343 and 373 K. As the change of the unit-cell volume is almost linear, the character of the transition is apparently second order and matches the mechanism of a subtle displacement of the oxygen atom position. The apparent instability of the R 3I c structure is also evidenced by the remarkably larger anisotropic displacement of the oxygen atom.

Description: Following the characterization of the new amphibole species fluoro-leakeite, ideally A Na B Na 2 C (Mg 2 Al 2 Li) T Si 8 O 22 W F 2 , at Norra Kärr (Sweden), so far considered the type locality of eckermannite, re-examination of the holotype material of eckermannite deposited at the Museum of Natural History in London (BM 1949.151) and of the original sample analyzed by Törnebohm (1906) confirmed that they both are actually fluoro-leakeite. A survey of literature data showed that the only analysis reported for eckermannite is that of sample AMNH 108401 from the Jade Mine Tract, Myanmar. Complete characterization of that sample has led to the approval of a new holotype for eckermannite (IMA-CNMNC 2013-136), ideally A Na B Na 2 C (Mg 4 Al) T Si 8 O 22 W (OH) 2 , which is described in this work. Holotype eckermannite from Myanmar has the empirical unit formula A (Na 0.87 K 0.06 ) =0.93 B (Na 1.89 Ca 0.11 ) =2.00 C (Mg 3.87 Fe 2+ 0.09 Mn 0.01 Fe 3+ 0.38 Al 0.62 ) =4.97 T Si 8.00 O 22 W (F 0.03 OH 1.97 ) =2.00 . It is monoclinic, C 2/ m , with a = 9.8087(7), b = 17.8448(13), c = 5.2905(4) Å, β = 103.660(1), V = 899.8(1) Å 3 ; Z = 2, D calc = 3.02 g/cm 3 . Optics: biaxial (–); α = 1.605, β = 1.630, = 1.634 all ±0.002 ( = 590 nm). The 10 strongest reflections in the X-ray powder pattern [ d values (in Å), I , ( hkl )] are: 2.702, 100, [(31) (151)]; 3.395, 59, (131); 3.128, 56, (310); 2.525, 56, (02); 8.407, 42, (110); 2.574, 36, [(061) (002)]; 3.257, 34, (240); 2.161, 33, (261); 2.966, 33, (060); 4.460, 30, (040). The reason for the rarity of eckermannite compositions are examined and discussed based on considerations on the short-range order of A cations and W anions.

Description: 〈span〉Natural garnets are robust silicate minerals stable over large ranges of temperature and pressure that can provide useful geochemical constraints on petrogenetic conditions. The purpose of this study is to determine how the size of cation sites controls rare earth elements (REE) incorporation in mantle eclogitic garnets. Major and lanthanide element data in a suite of mantle-derived eclogite garnets were combined with new single-crystal structure refinements (SREFs) to examine the effects of major-element chemistry and site dimension on the incorporation of REE into the garnet structure. Several distinct trends are apparent. Pyrope-rich samples similar to mantle lherzolitic garnets are enriched in the smaller heavy rare earth elements (HREE). Almandine-rich garnets are also HREE-enriched, but low rare earth elements (LREE) values are lower than in the pyrope-rich garnets. Intermediate garnet compositions are more depleted in HREE and enriched in LREE (Ce, Nd, and Sm). Finally, Ca-rich garnets (50% grossular component) are depleted in LREE and HREE, but are enriched in MREE. Hence, the X site dimension does exert a crucial role in REE incorporation into the garnet structure. Crystal structure refinements provide further evidence of this influence.〈/span〉