Crystal-field spectra for blue and green diopsides synthesized in the join CaMgSi2O6-CaCrAlSiO6     

Ko Ikeda  Kenzo Yagi 《Contributions to Mineralogy and Petrology》1982,81(2):113-118

Synthetic diopsides in the join CaMgSi₂O₆ CaCrAlSiO₆ have been studied by means of crystal-field theory. These diopsides are either blue or pale green in colour. The former forms at lower temperatures and the latter at higher temperatures. The optical spectra and colours can be unequivocally explained based on the assumption that Cr³⁺ions occupy both tetrahedral and octahedral sites in the diopsides. In the blue diopsides Cr³⁺ions are present in two kinds of spin state, i.e., tetrahedrally coordinated low spin and octahedrally coordinated high spin. On heating the blue diopsides, tetrahedral occupancy of chromium decreases sharply due to spin transformation from tetrahedral low spin to octahedral high spin. Above 1,160° C nearly all chromium ions are present in octahedral sites with high spin state and the diopsides become pale green in colour. Some petrogenetic applications of the resultes are discussed.  相似文献   

Structural changes and valence states in the MgCr<Subscript>2</Subscript>O<Subscript>4</Subscript>–FeCr<Subscript>2</Subscript>O<Subscript>4</Subscript> solid solution series     

D.?LenazEmail author  H.?Skogby  F.?Princivalle  U.?H?lenius 《Physics and Chemistry of Minerals》2004,31(9):633-642

The influence on the structure of Fe²⁺ Mg substitution was studied in synthetic single crystals belonging to the MgCr₂O₄–FeCr₂O₄ series produced by flux growth at 900–1200 °C in controlled atmosphere. Samples were analyzed by single-crystal X-ray diffraction, electron microprobe analyses, optical absorption-, infrared- and Mössbauer spectroscopy. The Mössbauer data show that iron occurs almost exclusively as ^IVFe²⁺. Only minor Fe³⁺ (<0.005 apfu) was observed in samples with very low total Fe. Optical absorption spectra show that chromium with few exceptions is present as a trivalent cation at the octahedral site. Additional absorption bands attributable to Cr²⁺ and Cr³⁺ at the tetrahedral site are evident in spectra of end-member magnesiochromite and solid-solution crystals with low ferrous contents. Structural parameters a₀, u and T–O increase with chromite content, while the M–O bond distance remains nearly constant, with an average value equal to 1.995(1) Å corresponding to the Cr³⁺ octahedral bond distance. The ideal trend between cell parameter, T–O bond length and Fe²⁺ content (apfu) is described by the following linear relations: a₀=8.3325(5) + 0.0443(8)Fe²⁺ (Å) and T–O=1.9645(6) + 0.033(1)Fe²⁺ (Å) Consequently, Fe²⁺ and Mg tetrahedral bond lengths are equal to 1.998(1) Å and 1.965(1) Å, respectively.  相似文献   

The crystal structure of CaFe3+SiAlO6 and the crystal chemistry of Fe3+—Al3+ substitution in calcium Tschermak's pyroxene     

Subrata Ghose  Fujio P. Okamura  Haruo Ohashi 《Contributions to Mineralogy and Petrology》1986,92(4):530-535

The crystal structure of a synthetic CaFe³⁺Al-SiO₆ pyroxene (20 kb, 1,375° C) with unit cell dimensions a=9.7797(16), b=8.7819(14), c=5.3685(5) Å, =105.78(1), space group C2/c has been refined by the method of least squares to an R-factor of 0.025 based on 812 reflections measured on an automatic single crystal diffractometer. The octahedral M1 site is occupied by 0.82 Fe³⁺ and 0.18 Al³⁺. Within the tetrahedral T site, Si⁴⁺ (0.50), Al³⁺ (0.41) and Fe³⁺ (0.09) ions are completely disordered, although submicroscopic domains with short-range order are very likely. The octahedral site preference energy of the Fe³⁺ ions with respect to Al³⁺ ions in CaFe³⁺AlSiO₆ is about 10 kcal/mole, which is much higher than that found in Y₃Al _x Fe_5–2O₁₂ garnets. Topologically the structure of CaFe³⁺AlSiO₆ is intermediate between that of diopside and calcium Tschermak's pyroxene, CaAlAlSiO₆. For CaM³⁺ AlSiO₆ clinopyroxenes an increase in the size of the M1 octahedron is accompanied by an increase in the average M2-0, bridging T-0 and 03-03 distances and kinking of the tetrahedral chain.  相似文献   

The influence of ionic radii of cations and covalent forces on the unit cell dimensions of spinels     

Dr. F. H. S. Vermaas  Mr. E. R. Schmidt 《Contributions to Mineralogy and Petrology》1959,6(4):219-232

Fe-, Cr- and Al-spinels were synthesized and their unit cell sizes determined by means of X-rays. Differential thermal curves show that the magnetic inversion of Fe₂O₃ at 680° C accelerates the formation of the ferrites when the constituent oxides are heated together.A correlation can be made between ionic radii of cations and unit cell dimensions provided the effect of covalent forces in the lattice is taken into account. The values for ionic radii of cations as given byAhrens (1952) permit a better correlation than those ofGoldschmidt.A shrinkage of 0.01 Å in the unit cell size per 0.01 Å decrease in the ionic radius of the divalent cations was determined when spinels with the same cation arrangement in the same group were compared. A shrinkage of 0.027 Å in the unit cell size per 0.01 Å decrease in the ionic radius of the trivalent cations was determined in spinels having the same divalent cation and cation arrangement when the trivalent cations form the same type of bonds.The half-occupation of the 3d orbits in Mn²⁺ and Fe³⁺ causes abnormally high unit cell dimensions in spinels where these ions are incorporated in octahedral sites. This is attributed to the formation of electrovalent bonds by these ions. Variable forces of contraction in the lattice are revealed when the unit cell dimensions are correlated with the ionic radii of cations. The force of contraction can be satisfactorily explained as being due to covalent forces in the spinel structure. The magnitude of this force or the degree of covalence in the bonds increases in the following order of cations where these are situated in tetrahedral sites:The divalent transition element ions, Fe²⁺, Co²⁺ and Ni²⁺; the B-Sub-group element ions Cd²⁺ and Zn²⁺; Fe³⁺ in tetrahedral co-ordination.  相似文献   

Optical absorption investigation of Cr3+ ion-bearing minerals in the temperature range 77–797 K     

M. N. Taran  K. Langer  A. N. Platonov  V. Indutny 《Physics and Chemistry of Minerals》1994,21(6):360-372

The effect of raising temperature on spin-allowed dd-transitions of octahedral Cr³⁺ was studied for various point symmetries of the Cr³⁺-bearing structural sites, i.e. 3 m and 3 with inversion center in spinel and garnets, respectively, or 32, 3, 2 and 1, lacking the inversion centre, in beryl, corundum, diopside and topaz, respectively. For this purpose, crystals of Cr³⁺-bearing spinel, pyrope, andradite, grossular, uvarovite, emerald, ruby, diopside and topaz were analyzed by microprobe, oriented, and measured in polarized radiation (except for the cubic minerals) in the spectral range 30 000 to 11 100 cm^-1 and at temperatures between 77 and 797 K. The evaluation of the intensities, half widths, and energy positions of bands due to Cr³⁺-transitions derived from ⁴ A _2g ⁴ T _2g (F) and ⁴ T _1g (F) as well as of Dq- and B-values derived, had the following results:In all cases, red shift of the above bands and, hence, independent on the site symmetry of Cr³⁺, decreases in the Dq-values were obtained. The dependcies of Dq on T are nearly linear above room temperature and amount between -1.6% in topaz and -5.1% in pyrope in the temperature range studied. From this, values for the local thermal expansion of the Cr³⁺-centered octahedra, _loc, were derived on the basis of the R _M-0 ^-5 -proportionality of 10Dq. Such values are consistently higher than those obtained from X-ray refinements, a method averaging r_m-o for all the respective octahedral positions.  相似文献   

Synthesis of kosmochlor and phase equilibria in the join CaMgSi2O6-NaCrSi2O6     

Ko Ikeda  Kenzo Yagi 《Contributions to Mineralogy and Petrology》1972,36(1):63-72

Kosmochlor (NaCrSi₂O₆) was synthesized by the flux method from melts along the join Na₂O·2 SiO₂-Na₂O·Cr₂O₃·4 SiO₂ at 1000° C in air, and isolated by dissolving the glassy matrix with hydrofluoric and perchloric acids. The join CaMgSi₂O₆-NaCrSi₂O₆ was studied at 1 atmosphere in air by the quenching technique at temperatures between 900° and 1450° C, using mixtures of kosmochlor and diopside crystals or diopside glass as starting materials. The phases are diopside solid solution, kosmochlor, spinel (Mg-chromite), eskolaite (Cr₂O₃) and glass. The maximum solubility of kosmochlor in diopside is 24 wt percent at 1140° C, while diopside is not soluble at all in kosmochlor, resulting in the existence of a wide range of immiscibility. Petrologic significance of the results is discussed.  相似文献   

The red-green change in chromium-bearing garnets     

Harald Carstens 《Contributions to Mineralogy and Petrology》1973,41(3):273-276

The colour of chromium-bearing pyrope-rich garnets may be red or green depending upon the Cr-O distance which varies with the chemical composition. Red garnets become green when the crystal field parameter ₀ for substitutional Cr³⁺ decreases below 17200 cm^–1 corresponding to about 1970 Å.  相似文献   

Crystal structure refinement of hendricksite,A Zn- and Mn-rich trioctahedral potassium mica: A contribution to the crystal chemistry of zinc-bearing minerals     

Dr. J. -L. Robert  Dr. M. Gaspérin 《Mineralogy and Petrology》1985,34(1):1-14

Summary The crystal structure of hendricksite, a trioctahedral mica of biotite type, characterized by high Zn²⁺ and Mn²⁺ contents has been refined by least square methods. The structural formula is: (K_0.89Na_0.10Ba_0.04)(Mg_1.57Zn_0.54Mn _0.40 ²⁺ Fe _0.25 ²⁺ Al_0.07Ti_0.07Cr_0.01)(Si_2.92Al_1.08)O₁₀ (OH)₂. The space group isC2/m and the cell parameters are:a=5.340(2) Å,b=9.524(2) Å,c=10.235(3) Å, =100.07(2)^o, the cell volume isV=497.98 ų. The final unweightedR=0.072. Average cation-anion distances in polyhedra are: T–O=1.659 Å, M(1)–O=2.093 Å, M(2)–O=2.088 Å, A–O_long=3.316 Å and A–O_short=3.004 Å; A is the alkaline cation. The rotation angle of tetrahedra is =6.7°. The analysis of electron densities, of the dimensions and distorsions of polyhedra shows that Zn²⁺ is exclusively in octahedral sites; there is no order between six-fold coordinated cations. A comparison between the structural features of hendricksite and those of the two main end-members of biotites, phlogopite and annite, is presented.The effect of the strong covalence of Zn–O bonds is particularly visible on the dimensions and orientations of the thermal ellipsoids of octahedral sites M(1) and M(2) which contain zinc. In all the published structures of trioctahedral micas, the ellipsoids of cationic sites are uniaxial positive, elongated parallel toc ^*. In hendricksite, this is observed only for the two zinc-free sites (T and A; in the octahedra M(1) and M(2), which contain zinc, the ellipsoids are approximately uniaxial negative, flattened parallel toa, which is a unique situation.Zinc which habitually favours the tetrahedral coordinations with oxygen, enters the octahedra only, i.e. the chemically anisotropic sites, in hendricksite. The strong polarizability of Zn²⁺ is proposed to explain this behaviour.An examination of the behaviour of Zn²⁺ in other compounds shows that this situation is general, zinc favours chemically anisotropic sites and specially those adjacent to OH or H₂O.

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Affinement de la structure cristalline de la hendricksite, mica trioctaédrique potassique riche en Zn et Mn; une contribution à la connaissance cristallochimique des minéraux zincifères

Résumé On a affiné par moindres carrés la structure de la hendricksite, mica trioctaédrique de type biotite, caractérisé par une teneur élevée en Zn²⁺ et Mn²⁺. La formule structurale de ce mica est: (K_0m89Na_0,10Ba_0,04)(Mg_1,57Zn_0,54Mn _0,40 ²⁺ Fe _0,25 ²⁺ Al_0,07Ti_0,07Cr_0,01)(Si_2,92Al_1,08)O₁₀(OH)₂. Le groupe spatial estC2/m et les paramètres de la maille:a=5,340(2) Å,b=9,254(2) Å,c=10,235(3) Å, =100,07(2)°; le volume de la maille estV=497,98 ų. Le résidu final non-pondéré estR=0,072. Les distances cation-anion moyennes dans les polyèdres sont les suivantes: T–O=1,659 Å, M(1)–O=2,093 Å, M(2)–O=2,088 Å, A–O_long=3,316 Å et A–O_court=3,004 Å où A désigne le cation alcalin. L'angle de rotation tétraédrique, =6,7°, est très semblable à celui de la phlogopite. L'analyse des densités électroniques, des dimensions et distorsions des polyèdres montre que Zn²⁺ est exclusivement en coordinance octaédrique et qu'il n'y a pas d'ordre entre les cations hexacoordonnés. On présente une comparaison des caractères structuraux de la hendricksite avec ceux des deux principaux pôles des biotites, la phlogopite et l'annite.L'effet de la forte covalence de la liaison Zn–O est particulièrement visible sur les dimensions et orientations des ellipsoides d'agitation thermique des deux sites octaédriques, sites zincifères. Dans toutes les structures de micas trioctaédriques publiées, les ellipsoides des sites cationiques sont uniaxes positifs, allongés parallèlement àc ^*, ce qui s' observe effectivement dans les deux sites non-zincifères (T et A) de la hendricksite, par contre, dans les octaèdres M(1) et M(2), qui contiennent le zinc, les ellipsoides sont approximativement uniaxes négatifs, applatis parallèlement àa, ce qui est une situation unique.Le zinc, qui se fixe généralement en sites tétraédriques dans les structures de type oxyde, occupe les sites octaédriques, c'est-à-dire les sites chimiquement anisotropes dans la hendricksite. La forte polarisabilité de Zn²⁺ est proposée pour expliquer ce comportement.Un examen du comportement de Zn²⁺ dans d'autres phases montre que cette situation est tout à fait générale, le zinc privilégiant les sites chimiquement anisotropes et en particulier ceux adjacents à OH où H₂O.

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How Cr<Superscript>3+</Superscript> and Fe<Superscript>3+</Superscript> affect Mg–Al order–disorder transformation at high temperature in natural spinels     

F.?MartignagoEmail author  A. Dal?Negro  S.?Carbonin 《Physics and Chemistry of Minerals》2003,30(7):401-408

Three natural Mg(Al_2-yCr_y)O₄ spinels (y 0.07–0.16), highly ordered in terms of Mg–Al, and one Mg(Al_2–yFe³⁺_y)O₄ spinel (y0.08), highly ordered also in terms of Fe³⁺, were studied by means of X-ray single-crystal diffraction. All samples were heated in situ from 25 to 1000 °C in order to follow both thermal expansion and evolution of the structural state of spinel with temperature. Thermal expansion was monitored by means of the variation of cell edge a with temperature, and found to be well represented throughout the temperature range by a regression line a = a₀ (1+T), slightly different at lower and higher temperatures. Thermal expansion coefficient ₁, referring to the lower temperature range (i.e. during pure thermal expansion), was slightly lower than ₂, calculated only over the highest temperatures. The trend showed different slopes for individual crystals. Structural evolution with temperature was studied by means of the variation of oxygen positional parameter u, which is strongly influenced by intersite cation exchange and thus closely correlated with inversion parameter x. In particular, in the three Cr samples, in which Cr resides only in the octahedral site, u parameter variations and hence the order–disorder process, started at about 700 °C. Instead, in the Fe³⁺ sample, this process was triggered at lower temperatures, starting at 550 °C with Fe³⁺–Mg exchange followed at higher temperatures by that of Mg–Al. Cr contents in the Cr samples affected the occupancy of Al in the tetrahedral site at the highest temperatures. In both Mg–Al–Cr and Mg–Al–Fe³⁺ compositions, if CrFe³⁺, parameter u reached the same value only when the Mg–Al exchange was dominant, i.e. at the highest temperatures, but not before. Cation distribution at each temperature was obtained by the bond-length model, applying thermal expansion to pure bond lengths. This method is applied here to complex compositions for the first time.  相似文献   

Uvarovite: Stability of uvarovite-andradite solid solutions at low pressure     

H. G. Huckenholz  D. Knittel 《Contributions to Mineralogy and Petrology》1976,56(1):61-76

Uvarovite (Ca₃Cr₂Si₃O₁₂) forms a complete solid solution series with andradite (Ca₃Fe ₂ ⁺³ Si₃O₁₂) below 1,137±5 ° C at a total pressure of 1 atm. Pure uvarovite decomposes to pseudowollastonite (CaSiO₃)+eskolaite (Cr₂O₃) at 1,385 ± 10 ° C. The incorporation of Ca₃Fe ₂ ⁺³ Si₃O₁₂ component in the uvarovite structure lowers the thermal stability of the garnet. The breakdown assemblage is garnet_ss (Ca₃(Cr,Fe⁺³ ₂)Si₃O₁₂)+pseudowollastonite (CaSiO₃)+hemeskolaite_ss(Cr,Fe⁺³O₃). Pure andradite decomposes to pseudowollastonite (CaSiO₃)+hematite (Fe₂O₃) at 1,137±5 °C. Andradite thermal stability is increased by incorporation of Ca₃Cr₂Si₃O₁₂ component by 248 °C.At 1,264±5 °C pseudowollastonite+hematite react to liquid defining a thermal minimum of the CaSiO₃-Cr₂O₃-Fe₂O₃ ternary system. This minimum is located at about 64.5 wt.-% CaSiO₃, 0.5 wt.-% Cr₂O₃, and 35.0 wt.-% Fe₂O₃. Uvarovite and andradite bulk compositions start to melt at 1,420 °C and 1,265 ±5 °C, respectively.The unit-cell parameter for uvarovite is 11.999 (2) Å, the refractive index 1.866 (2). The substitution of Cr⁺³ by Fe⁺³ increases a and n almost linearly toward the andradite end member which displays a unit-cell parameter of 12.059 (3) Å and a refractive index of 1.887 (2).  相似文献   

Crystal chemistry of some Mg,Cr, V normal spinels from Sludyanka (Lake Baikal,Russia): the influence of V<Superscript>3+</Superscript> on structural stability     

B.?LavinaEmail author  L.Z.?Reznitskii  F.?Bosi 《Physics and Chemistry of Minerals》2003,30(10):599-605

A suite of vanadian magnesiochromites from the Sludyanka metamorphic complex (South Lake Baikal, Russia) were investigated by means of X-ray single-crystal structural refinements and microprobe analyses. Various morphological types of Cr–V-bearing Mg spinels are located in calc–silicate metamorphic rocks, in an assemblage that also contains other Cr–V minerals such as escolaite–karelianite, uvarovite–goldmanite, Cr–V-bearing clinopyroxene, tourmaline, amphibole, mica, etc. Along the suite there is widespread V–Cr substitution (0.14 V³⁺ 0.95 afu, 1.02 Cr³⁺ 1.80 afu), and minor, variable Al contents. The Mg content of slightly lesser than 1 afu, is almost constant. Cell parameters and octahedral bond distances increase with V³⁺. Unexpectedly, the Mg–O tetrahedral bond distance also increases slowly with V³⁺. This weak dragging effect contributes towards maintaining distortion of the oxygen array with respect to the ideal CCP, thus providing a shielding effect, which reduces V³⁺–V³⁺ repulsion. This leads to the energetic stabilization of the structure, in spite of the increase of bond strain with increasing V³⁺ contents.  相似文献   

Optical spectroscopy study of variously colored gem-quality topazes from Ouro Preto,Minas Gerais,Brazil     

M. N.?TaranEmail author  A. N.?Tarashchan  H.?Rager  St.?Schott  K.?Schürmann  W.?Iwanuch 《Physics and Chemistry of Minerals》2003,30(9):546-555

Variously colored gem-quality topazes from Ouro Preto, Minas Gerais, Brazil, were studied by optical absorption spectroscopy and photoluminescence methods. In the near infrared range (750–2500 nm) the absorption spectra display an identical pattern of narrow intense absorption lines caused by overtones and combination vibrations of OH groups, which do not relate to the coloration of the topazes studied. Their colors were found to be caused by combination of three sets of absorption features, (1), (2), and (3) in the visible and near-UV range, which are due to different color center. (1) denotes a pair of broad split bands with maxima 18 000 and 25 000 cm^–1 caused by electronic spin-allowed dd transitions of Cr³⁺ ions. They cause a light rose to deep violet color and characteristic pleochroism of Cr³⁺-containing topazes. Photoluminescence evidences of at least three different types of Cr³⁺ complexes which, most probably, differ by ligand surroundings, O₄F₂, O₄F(OH) and O₄(OH) (2) Corresponds to the intense weakly polarized UV absorption edge. Two different parts, the thermally stable one, caused by ligand-to-metal charge transfer, and the thermally unstable one, caused by some defect center(s), contribute to the edge. (3) denotes a system of two broad unstructured bands with maxima around 19 000 cm^–1 (X>Y Z) and 24 000 cm^–1 (Y Z X). They cause the unique orange color and characteristic pleochroism of Brazilian Imperial topazes. Combinations of (1) and (3) absorption features cause various yellow-rose colors of the samples. Investigations of natural irradiated and thermally treated topazes show that the color centers (1) and (3) transform to each other at annealing and X- or gamma irradiation. The color of natural orange-red Imperial topazes is assumed to be caused by Cr⁴⁺, stabilized by other impurity ions and/or defect irradiation EPR centers. At T=300 °C Cr⁴⁺ reduces to Cr³⁺, the color of Imperial topazes changes to pale rose, caused by spin-allowed bands of Cr³⁺. In artificially irradiated crystals the (3)-center, Cr⁴⁺, may be induced according to the reaction 2Cr³⁺ Cr⁴⁺ + Cr²⁺, which involves chromium pairs in adjacent Al sites of the structure. Such artificially induced color is unstable at room temperature and in daylight. The process of the decay of (3)-centers may be described as a recombination Cr⁴⁺+Cr²⁺ 2Cr³⁺ that results in vanishing of the (3)-bands accompanied by the appearance or increase in Cr³⁺ dd bands, the original orange color turning to a pale rose.  相似文献   

From the green color of eskolaite to the red color of ruby: an X-ray absorption spectroscopy study     

Émilie Gaudry  Philippe Sainctavit  Farid Juillot  Federica Bondioli  Philippe Ohresser  Isabelle Letard 《Physics and Chemistry of Minerals》2006,32(10):710-720

The best known cause for colors in insulating minerals is due to transition metal ions as impurities. As an example, Cr³⁺ is responsible for the red color of ruby (α-Al₂O₃:Cr³⁺) and the green color of eskolaite (α-Cr₂O₃). Using X-ray absorption measurements, we connect the colors of the Cr _x Al_2−x O₃ series with the structural and electronic local environment around Cr. UV–VIS electronic parameters, such as the crystal field and the Racah parameter B, are related to those deduced from the analysis of the isotropic and XMCD spectra at the Cr L_2,3-edges in Cr_0.07Al_1.93O₃ and eskolaite. The Cr–O bond lengths are extracted by EXAFS at the Cr K-edge in the whole Cr _x Al_2−x O₃ (0.07≤x< 2) solid solution series. The variation of the mean Cr–O distance between Cr_0.07Al_1.93O₃ and α-Cr₂O₃ is evaluated to be 0.01₅ Å (≈1%). The variation of the crystal field in the Cr _x Al_2−x O₃ series is discussed in relation with the variation of the averaged Cr–O distances.  相似文献   

Titanian andradite in a metapyroxenite layer from the Malenco ultramafics (Italy): implications for Ti-mobility and low oxygen fugacity     

Othmar Müntener  Jörg Hermann 《Contributions to Mineralogy and Petrology》1994,116(1-2):156-168

Ti-andradite (melanite) has been found in a metapyroxenite layer in the upper part of the Malenco ultramafics(Italy), coexisting with clinochlore, diopside and magnetite. Field observations, as well as major and trace elementbulk-rock composition, strongly suggest a cumulate origin for the layer. Textural relationships indicate thatTi-andradite formed during two different metamorphic stages. Under peak metamorphic conditions (400–450°C, 5±2 kbar)Ti-andradite grew in an assemblage of diopside, clinochlore, magnetite and rare ilmenite and perovskite. Later, retrograde brittle deformationinduced formation of veins containing the paragenesis Ti-andradite, vesuvianite, diopside, chlinochlore, magnetite and accessory perovskite.The Ti-andradite varies considerably in TiO₂ (0.11–9.62 wt%), Fe₂O₃(14.3–30.5 wt%), Al₂O₃ (0.65–3.90 wt%), Cr₂O₃(>0.18–0.98 wt%) and SiO₂ (32.1–36.1 wt%); this is mostly, but not entirely, due to distinct zoning.Ti-andradite contains 0.32 to 0.66 wt% H₂O as determined by infrared spectroscopy and 0.83 to 1.76 wt% FeO. The CaO shows almost no variation (34.1±0.7 wt%) and Ca completely fills the dodecahedral site. Single crystal site refinements indicate that no tetrahedral Ti or Fe replaces Si. Titanium incorporation is attributed to similar degrees of substitution along the exchange vectors Ti³⁺ Fe³⁺, Ti⁴⁺ Al^IV Al _-1 ^VI Si_-1 and (Fe²⁺, Mn²⁺, Mg²⁺)Ti⁴⁺ 2Fe _-1 ³⁺ . The presence of mixed valence states of both Fe and Ti suggests a low oxygen fugacity during crystallization of Ti-andradite. Mass balance calculations indicate an isochemical origin of the first generation of Ti-andradite in the clinopyroxenite layer. Its occurrence is restricted to antigorite-free mineral assemblages containing clinochlore of 0.95X _Al>1.1. The hydrothermal crystallization of Ti-rich andradite in veins demonstrates Ti mobility in aqueous fluids under moderate P-T conditions. The zonation patterns indicate disequilibrium conditions during vein crystallization. As no fluorine-, carbonate- and phosphate-bearing minerals were found, OH^- is most probably the ligand complexing Ti.  相似文献   

Diamondiferous peridotite xenoliths from the Argyle (AK1) lamproite pipe,Western Australia     

A. L. Jaques  H. St. C. O'Neill  C. B. Smith  J. Moon  B. W. Chappell 《Contributions to Mineralogy and Petrology》1990,104(3):255-276

This paper describes a suite of peridotite xenoliths. some carrying diamonds at high grades, from the richly diamondiferous early Proterozoic (1180 Ma) Argyle (AK1) lamproite pipe, in northwestern Australia. The peridotites are mostly coarse garnet lherzolites but also include garnet harzburgite, chromite — garnet peridotite, a garnet wehrlite, and an altered spinel peridotite with extremely Cr-rich chromite. In all cases the garnet has been replaced by a kelyphite-like, symplectic intergrowth of Alrich pyroxenes, Al-spinel and secondary silicates. The peridotites have refractory compositions characterized by high Mg/(Mg+Fe) and depletion in lithophile elements (Al₂O₃ and CaO < 1%, Na₂O0.03%) and high field strength cations such as Ti, Zr, Y, and Yb. Olivines have high Mg/(Mg+Fe) (Mg ^91–93 ) and, like olivine inclusions in diamonds from the Argyle pipe, contain detectable amounts of Cr₂O₃ (0.03%–0.07%) but have very low CaO contents (typically 0.04%–0.05%). Enstatites (Mg ^92–94 ) have comparatively high Cr₂O₃ (0.2%–0.45%) and Na₂O (up to 0.18%) but very low Al₂O₃ contents (0.5%–0.7%). Diopsides (Mg ^92–94 , Ca/(Ca+Mg+Fe)=0.37–0.43) are Cr-rich (0.7%–1.9% Cr₂O₃) and have low Al₂O₃ (0.7%–2.2%) and Na₂O (0.5%–1.6%) contents. Many have high K₂O contents, typically 0.1%–0.4% but up to 1.3% K₂O in one xenolith. The chromite coexisting with former garnet is Mg-and Cr-rich [Mg/(Mg+Fe²⁺)=0.68–0.72, Cr/(Cr+Al)=0.72–0.79] whereas chromite in the spinel peridotite is even more Cr-rich (65% Cr₂O₃, Cr/(Cr+Al)=0.85, resembling inclusions in diamond. One highly serpentinized former garnet peridotite contains a Cr-rich (up to 13% Cr₂O₃) titanate resembling armalcolite but containing significant K₂O (1%–2.5%), CaO (0.6%–2.2%), ZrO₂ (0.1%–0.8%), SrO (0.1%–0.3%), and BaO (up to 0.58%): this appears to have formed as an overprint of the primary mineralogy. Temperatures and pressures estimated from coexisting pyroxenes and reconstructed garnet compositions indicate that the garnet lherzolites equilibrated at 1140°–1290° C and 5.0–5.9 GPa (160–190 km depth), within the stability field of diamond. Oxygen fugacties within the diamond forming environment are estimated from spinel-bearing assemblages to be reducing, with f _O2 between MW and IW. The presence of significant K in the diopsides from the peridotite xenoliths and in diopsides from heavy mineral concentrate from the Argyle pipe implies metasomatic enrichment of the subcontinental lithosphere within the diamond stability field. The P-T conditions estimated for the Argyle peridotites demonstrate that diamondiferous lamproite magmas incorporate mantle xenoliths from similar depths to kimberlites in cratonic settings, and imply that Proterozoic cratonized orogenic belts can have lithospheric roots of comparable thickness to beneath Archaean cratons. These roots lie at the base of the lithosphere within the stability field of diamond. The xenoliths, the calcic nature of chrome pyropes from heavy mineral concentrate, and the diamond inclusion assemblage indicate that the lighosphere beneath the Western Australian lamproites is mostly depleted lherozolite rather than the harzburgite commonly found beneath Archaean cratons. Nevertheless, the dominance of eclogitic paragenesis inclusions in Argyle diamonds indicates a significant proportion of diamondiferous eclogite is also present. The form, mineral inclusion assemblage, and the C-isotopic composition of diamonds in the peridotite xenoliths suggest that disaggregated diamondiferous peridotites are the source of the planar octahedral diamonds which constitute a minor component of the Argyle production. These diamonds are believed to have formed from mantle carbon in reduced, refractory peridotite (Iherzolite-harzburgite) in contrast to the predominant strongly ¹³C-depleted eclogitic suite diamonds which contain a recycled crustal carbon component. The source region of the lamproites has undergone long-term (2 Ga) enrichment in incompatible elements.  相似文献   

Fe,Ti ordering and octahedral distortions in acentric neptunite: Temperature dependent X-ray and neutron structure refinements and Mössbauer spectroscopy     

Martin Kunz  Thomas Armbruster  George A. Lager  Arthur J. Schultz  Rick J. Goyette  Werner Lottermoser  Georg Amthauer 《Physics and Chemistry of Minerals》1991,18(3):199-213

Single-crystal X-ray and neutron structure refinements carried out on neptunite (KNa₂Li(Fe, Mg, Mn)₂Ti₂Si₈O₂₄) from San Benito, California at various temperatures (neutrons: 15 K and 293 K; X-rays: 110 K, 293 K and 493 K) indicate that this mineral crystallizes in the acentric space group Cc (T=293K: a=16.427 Å, b=12.478 Å, c=9.975 Å, = 115.56°, Z=4, V=1844.53 ų) due to ordering of octahedrally coordinated metals (Ti, Fe, Mn, Mg). In the neptunite structure, Ti and (Fe, Mn, Mg) octahedra share edges to form chains that run along [110] and [110]. These chains are, in turn, linked through shared corners along [001]. The resulting octahedral framework is interwoven by a similar [Si₈O₂₂] tetrahedral framework. Li, Na and K occupy 6-, 8- and 10- coordinated sites within the framework. The metal-containing polyhedra show strong distortions at all temperatures. In particular, Ti exhibits a strong off-center displacement (0.25 Å) within its octahedron, leading to four Ti-O distances of 2.0 Å, one of 2.2 Å and one of 1.7 Å. The displaced Ti position is in good agreement with a position that minimizes differences between ionic bond strengths and is interpreted as an energy minimum in an ionic potential model. Mössbauer spectra collected at 77 K, 293 K and 400 K indicate all Fe to be present as octahedral Fe²⁺. Although two distinct Fe positions were found in the structure, 77 K and 293 K spectra display only one quadrupole doublet. Two Fe sites can only be resolved in the 400 K spectrum. It is suggested that the temperature dependence of octahedral edge distortions is responsible for the separation of the Mössbauer doublets.  相似文献   

The crystal chemistry of birefringent natural uvarovites. Part III. Application of the superposition model of crystal fields with a characterization of synthetic cubic uvarovite     

M. Andrut  M. Wildner 《Physics and Chemistry of Minerals》2002,29(9):595-608

Synthetic, flux-grown uvarovite, Ca₃Cr₂ [SiO₄]₃, was investigated by optical methods, electron microprobe analysis, UV-VIS-IR microspectrometry, and luminescence spectroscopy. The crystal structure was refined using single-crystal X-ray CCD diffraction data. Synthetic uvarovite is optically isotropic and crystallizes in the “usual” cubic garnet space group Ia3¯d [a=11.9973 Å, Z=8; 21524 reflections, R1=2.31% for 454 unique data and 18 variables; Cr–O=1.9942(6), Si–O=1.6447(6), Ca–O_a=2.3504(6), Ca–O_b= 2.4971(6) Å]. The structure of Ca₃Cr₂[SiO₄]₃ complies with crystal-chemical expectations for ugrandite group garnets in general as well as with predictions drawn from “cubically averaged” data of non-cubic uvarovite–grossular solid solutions (Wildner and Andrut 2001). The electronic absorption spectra of Cr³⁺ in trigonally distorted octahedra of synthetic uvarovite were analyzed in terms of the superposition model (SM) of crystal fields. The resulting SM and interelectronic repulsion parameters are =9532 cm^?1, =4650 cm^?1, power law exponent t ₄=6.7, Racah B₃₅=703 cm^?1 at 290 K (reference distance R ₀=1.995 Å; fixed power law exponent t ₂=3 and spin-orbit parameter ζ=135 cm^?1). The interelectronic repulsion parameters Racah B ₅₅=714 cm^?1 and C=3165 cm^?1 were extracted from spin-forbidden transitions. This set of SM parameters was subsequently applied to previously well-characterized natural uvarovite–grossular solid solutions (Andrut and Wildner 2001a; Wildner and Andrut 2001) using their extrapolated Cr–O bond lengths to calculate the energies of the spin-allowed bands. These results are in very good agreement with the experimentally determined band positions and indicate the applicability of the superposition model to natural 3d ^N prevailing systems in geosciences. Single-crystal IR absorption spectra of synthetic uvarovite in the region of the OH-stretching vibration exhibit one isotropic absorption band at 3508 cm^?1 at ambient conditions, which shifts to 3510 cm^?1 at 77 K. This band is caused by structurally incorporated hydroxyl groups via the (O₄H₄)-hydrogarnet substitution. The water content, calculated using an integral extinction coefficient ?=60417 cm^?2 l mol^?1, is c _H2O=33 ppm.  相似文献   

Thermodynamic properties,low-temperature heat-capacity anomalies,and single-crystal X-ray refinement of hydronium jarosite, (H<Subscript>3</Subscript>O)Fe<Subscript>3</Subscript>(SO<Subscript>4</Subscript>)<Subscript>2</Subscript>(OH)<Subscript>6</Subscript>     

J.?MajzlanEmail author  R.?Stevens  J.?Boerio-Goates  B. F.?Woodfield  A.?Navrotsky  P. C.?Burns  M. K.?Crawford  T. G.?Amos 《Physics and Chemistry of Minerals》2004,31(8):518-531

Crystals of hydronium jarosite were synthesized by hydrothermal treatment of Fe(III)–SO₄ solutions. Single-crystal XRD refinement with R₁=0.0232 for the unique observed reflections (|F_o| > 4_F) and wR₂=0.0451 for all data gave a=7.3559(8) Å, c=17.019(3) Å, V^o=160.11(4) cm³, and fractional positions for all atoms except the H in the H₃O groups. The chemical composition of this sample is described by the formula (H₃O)_0.91Fe_2.91(SO₄)₂[(OH)_5.64(H₂O)_0.18]. The enthalpy of formation (H^o_f) is –3694.5 ± 4.6 kJ mol^–1, calculated from acid (5.0 N HCl) solution calorimetry data for hydronium jarosite, -FeOOH, MgO, H₂O, and -MgSO₄. The entropy at standard temperature and pressure (S^o) is 438.9±0.7 J mol^–1 K^–1, calculated from adiabatic and semi-adiabatic calorimetry data. The heat capacity (C_p) data between 273 and 400 K were fitted to a Maier-Kelley polynomial C_p(T in K)=280.6 + 0.6149T–3199700T^–2. The Gibbs free energy of formation is –3162.2 ± 4.6 kJ mol^–1. Speciation and activity calculations for Fe(III)–SO₄ solutions show that these new thermodynamic data reproduce the results of solubility experiments with hydronium jarosite. A spin-glass freezing transition was manifested as a broad anomaly in the C_p data, and as a broad maximum in the zero-field-cooled magnetic susceptibility data at 16.5 K. Another anomaly in C_p, below 0.7 K, has been tentatively attributed to spin cluster tunneling. A set of thermodynamic values for an ideal composition end member (H₃O)Fe₃(SO₄)₂(OH)₆ was estimated: G^o_f= –3226.4 ± 4.6 kJ mol^–1, H^o_f=–3770.2 ± 4.6 kJ mol^–1, S^o=448.2 ± 0.7 J mol^–1 K^–1, C_p (T in K)=287.2 + 0.6281T–3286000T^–2 (between 273 and 400 K).  相似文献   

The crystal structure of scotlandite,PbSO3     

Doz.Dr. F. Pertlik  Prof. Dr. J. Zemann 《Mineralogy and Petrology》1985,34(3-4):289-295

Summary The crystal structure of scotlandite —a=4.505(2),b=5.333(2),c=6.405(6) Å, =106.24(3)^o; space groupP2₁/m; cell content 2 {PbSO₃} — was determined from singlecrystal X-ray diffractometer data. Scotlandite is isotypic with molybdomenite, PbSeO₃. Lead is coordinated to nine oxygen atoms with Pb-O_av=2.75 Å, and possibly further to one sulphur atom with Pb–S=3.46 Å. The average S–O distance in the pyramidal SO₃ group is 1.52 Å. The structural relationships to cerussite, PbCO₃, are discussed.

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Die Kristallstruktur des Scotlandits, PbSO₃

Zusammenfassung Die Kristallstruktur des Scotlandits —a=4,505(2),b=5,333(2),c=6,405(6) Å, =106,24(3)^o; RaumgruppeP2₁/m; Zellinhalt 2 {PbSO₃} — wurde aus Einkristall-Röntgendiffraktometerdaten bestimmt. Scotlandit ist mit Molybdomenit, PbSeO₃, isotyp. Blei wird von neun Sauerstoffatomen mit Pb–O_av=2,75 Å und möglicherweise zusätzlich von einem Schwefelatom mit Pb–S=3,46 Å koordiniert. Der durchschnittliche S–O-Abstand in der pyramidalen SO₃-Gruppe mißt 1,52 Å. Die strukturellen Beziehungen zu Cerussit, PbCO₃, werden diskutiert.

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With 2 Figures  相似文献   

A thermodynamic model of Fe–Cr spinels     

Viktor?A.?KurepinEmail author 《Contributions to Mineralogy and Petrology》2005,149(5):591-599

A new thermodynamic model for multi-component spinel solid solutions has been developed which takes into account thermodynamic consequences of cation mixing in spinel sublattices. It has been applied to the evaluation of thermodynamic functions of cation mixing and thermodynamic properties of Fe₃O₄–FeCr₂O₄ spinels using intracrystalline cation distribution in magnetite, lattice parameters and activity-composition relations of magnetite–chromite solid solutions. According to the model, cation distribution in binary spinels, (Fe_1-x²⁺ Fe_x³⁺)[Fe_x²⁺Fe_2-2y-x³⁺Cr_2y]O₄, and their thermodynamic properties depend strongly on Fe²⁺–Cr³⁺ cation mixing. Mixing of Fe²⁺–Fe³⁺ and Fe³⁺–Cr³⁺ can be accepted as ideal. If Fe²⁺, Fe³⁺ and Cr are denoted as 1, 3 and 4 respectively, the equation of cation distribution is –RT ln(x²/((1–x)(2–2y–x)))= G₁₃^* + (1–2x)W₁₃+y(W₁₄–W₁₃–W₃₄) where G₁₃^* is the difference between the Gibbs energy of inverse and normal magnetite, W_ij is a Margules parameter of cation mixing and G₁₃^*, J/mol =–23,000+13.4 T, W₁₄=36 kJ/mol, W₁₃=W₃₄=0. The positive nonconfigurational Gibbs energy of mixing is the main reason for changing activity–composition relations with temperature. According to the model, the solvus in Fe₃O₄–FeCr₂O₄ spinel has a critical temperature close to 500°C, which is consistent with mineralogical data.  相似文献