Crystallochemical Formula Research Articles

Crystal structure of Li2MgSiO4

The crystal structure of Li2MgSiO4 was established by single-crystal X-ray diffraction analysis. The crystals are monoclinic, a = 4.9924(7) A, b = 10.681(2) A, c = 6.2889(5) A, β = 90.46(1)°, Z = 4, sp. gr. P21/n, V = 335.54 A3, R = 0.062. In a Li2MgSiO4 crystal, four types of independent T(1–4) tetrahedra share vertices to form a three-dimensional framework. Three of these tetrahedra are occupied simultaneously by Li and Mg cations, which corresponds to the crystallochemical formula (Li0.98Mg0.02)(Li0.80Mg0.20) · (Li0.22Mg0.78)SiO4. In slightly distorted SiO4 tetrahedra denoted as T(1), the average Si-O distance is 1.635(2) A. The distortions of other tetrahedra and the average (Li x Mg1 − x )-O distances increase with an increase in lithium content. These distances in the T(2), T(3), and T(4) tetrahedra are 1.955(2), 1.971(4), and 2.019(6) A, respectively. The structure of the new compound is compared with the crystal structures of other Li2 M 2+SiO4 compounds and the luminescence spectra of Cr4+: Li2MgSiO4.

Phosphate minerals from Albian phosphorites in Bulgaria

Results from X-ray diffraction analyses of Albian phosphorites from deposits Sanadinovo, Dekoy and Brushlen indicate presence mainly of fluorapatite and partly of chlorapatite and carbonate fluorapatite (francolite). The microprobe analyses of phosphate grains, coatings, cements, ammonite, wood, and shark tooth determined the following contents (in weight %): 26,65-41,02 P2O5 45,19-57,17 CaO, 2,58-4,64 F, up to 0,80 CI, and silicate components from the admixtures in phosphate minerals. Fifteen crystallochemical formulae were calculated. They are characteristic with Ca9.54-9.58[PO4]5.77-5.78FI1.48-2.81 and Cl0.06-0.30. Generally fluorapaiIte was determimid in 9 samples; and chlorine containing fluorapatite - in 6 samples. The exemplary crystallochemical formulae are: Ca9.54[PO4]5.78F2.75 - fluorapatite Ca9.58[PO4]5.77F1.94CI0.30 - chlorine containing fluorapatite The presence of CO3- and OH in the phosphate minerals was not substantiated by direct data. The studied phosphate minerals show high solubility in lemon acid (30-80%). This indicates defects in the crystal lattices caused by the presence of chlorine and most probably of carbonate group.

CONTINUOUS SOLID-SOLUTION BETWEEN MERCURIAN GIRAUDITE AND HAKITE

Among the minerals of the selenide assemblage at the Niederschlema–Alberoda uranium deposit, Erzgebirge, Germany, members of the mercurian giraudite–hakite solid solution intergrown with berzelianite and galena have been identified as rare and previously unknown phases. They form complexly zoned, anhedral, minute (<350 m) grains embedded in a dolomite matrix. Compositional variability is expressed by the following crystallochemical formula (calculated on the basis of 29 atoms per formula unit): (Cu9.92–9.99Ag0.01–0.08)10.00 (Hg0.92–1.81Cu0.06–1.12Zn0.05–0.10Fe0.00–0.15)1.98–2.06 (As0.69–3.98Sb0.02–3.29)3.91–4.08 (Se10.47– 11.53S1.47–2.61)12.90–13.09. The solid solutions span the range from gir99.5hak0.5 to gir16.2hak83.8, suggesting complete miscibility between mercurian giraudite and mercurian hakite in nature, equivalent to what already has been established for their S-bearing analogues, tennantite and tetrahedrite. The lack of thermodynamic data for both Se-rich species limits reliable inferences on the P–T–X conditions that prevailed during their formation. The assemblage mercurian giraudite–hakite + berzelianite + galena may represent a short-term equilibrium paragenesis of Jurassic age, formed at temperatures between 110 and 150°C under the conditions of low Se and S activities (i.e., –26 < logf(Se2) < –31 and –24 < logf(S2) < –28 at ~110°C), before the bulk of the selenide minerals crystallized. A second, less likely hypothesis calls upon the formation of the mercurian giraudite–hakite solid solutions during an early Cretaceous event, when pre-existing selenide minerals (berzelianite) were partially attacked by infiltrating fluids that introduced the major portion of the As and Sb into the system.

Crystal structure of new synthetic Ca,Na,Li-carbonate-borate

In the process of studying the phase formation in the Li2CO3-CaO-B2O3-NaCl system, new Ca,Na, Li-carbonate-borate has been synthesized under hydrothermal conditions. The crystal structure of carbonate-borate with the crystallochemical formula Ca4(Ca0.7Na0.3)3(Na0.7□ 0.3)Li5[B 12 t B 10 Δ O36(O,OH)6](CO3)(OH) · (OH,H2O) was refined to R hkl = 0.0716 by the least squares method in the isotropic approximation of atomic thermal vibrations without the preliminary knowledge of the chemical composition and the formula (sp. gr. R3, a rh = 13.05(2) A, α = 40.32(7)]°, V = 838(2) A3, a h = 8.99(2), c h = 35.91(2) A, V = 2513(2) A3, Z = 3, dcalcd = 2.62 g/cm3, Syntex P\(\bar 1\) diffractometer, 3459 reflections, 2θ-θ method, λMo). The structure has a new boron-oxygen radical [B 12 t B 10 Δ O36(O,OH)6] ∞∞ 15− , a double layer of nine-membered [B 6 t B 3 Δ O15(O,OH)3]7.5−-rings bound by BO3-triangles, and twelve-membered [B 6 t B 6 Δ O19.5(O,OH)3]7.5− rings. This allows one to relate this compound to megaborates with complex boron-oxygen radicals. The structure is built from two types of blocks consisting of Ca,Na,B-and Li,B-polyhedra alternating along the c-axis, which explains the perfect cleavage of the crystals along the (0001) plane.

A Deep-Water Glauconitization Process on the Ivory Coast—Ghana Marginal Ridge (ODP Site 959): Determination of Fe3+-Rich Montmorillonite in Green Grains

AbstractThe mineral and chemical composition of green glauconitic grains from ODP Site 959 (2100 m water depth) located on the northern flank of the Ivory Coast—Ghana Marginal Ridge was studied. Recurrent winnowing of a 20 m thick Pleistocene succession resulted in a low accumulation rate and stratigraphic hiatuses. The green clay material typically occurs as fillings in the chambers of pelagic foraminifers. The amount of green clay present in sediments older than 1 Ma is small, and greater in younger material. Mud composed of smectite, kaolinite, traces of mica, calcite and quartz was the precursor material that filled the chambers of the foraminifers. Processes at the water-sediment interface slowly modified this composition. Kaolinite was dissolved; smectite lost Al but gained Fe, K and layer charge. In that matrix, the nanocrystals of neoformed smectite are observed. The infrared (IR) spectra showed OH-stretching and bending vibrations due to groups incorporating Fe3+. The spectra are in agreement with the crystallochemical formulae of Fe3+-rich montmorillonite as determined by point-by-point analyses on the neoformed crystallites and on the surrounding matrix. The layer charge in this Fe3+-rich montmorillonite is almost wholly octahedral as shown in crystallochemical formulae and documented independently by a new IR method. The tetrahedral charge appeared when the Fe content increased by &gt; 1.2 Fe per formula unit. With the maturation process, the increased role of the closed layers is observed, with the color of grains becoming greener. We have documented for the first time glauconitization proceeding at a depth of 2100 m at a temperature near 3°C. The most important factors of the process are: accumulation of terrigenous clayey material in the foraminiferal chambers, Fe supply from a nearby continent, and a lengthy residence at the water-sediment interface in the zone of the winnowing and low sediment accumulation rate.

Stratigraphic condensed deposition and diagenetic evolution of green clay minerals in deep water sediments on the Ivory Coast–Ghana Ridge

Green grains concentrations were studied through the 22 m Pleistocene sedimentary column recovered from Ocean Drilling Project Site 959 (in 2100 m water depth) located on a small plateau on the northern flank of the Côte d'Ivoire–Ghana marginal ridge. The green clay material usually fills the chambers of pelagic foraminifers. Taking into account the variance in color and mineralogic composition of the bulk green grains fraction observed in each level, a methodological approach was especially performed: mineralogical and chemical analyses were done successively on the bulk green material, then on hand-picked white, pale green, medium green and dark green grains, and lastly, surface and spot analysis of nanno-structures were obtained using an energy dispersive microprobe coupled with a scanning electron microscope. Green grains concentrations are evidenced showing both rising up of green/white ratio and increasing abundance of cracked dark green grains. These irregular concentrations are not consistent directly with a general climatic forcing as evidenced by δ 18O isotopic curve, but are the result of recurrent bottom-current activity. Thus, winnowing process is the cause of the long lasting ionic exchange and mineralogical evolution at the water sediment interface. Clay infilling composed of smectite (35–40%), kaolinite (45–50%), traces of mica, calcite and quartz was a precursor material. Along with dissolution of kaolinite, crystallochemical modifications of smectites were observed both to the sequential concentration scale (bulk samples) and to the individual grain scale (from white to dark green): increment of Fe content associated decrease of Al and increase of layer charge and K content. In the layers with the highest green grain concentrations and in agreement, with crystallochemical formulae, Fe 3+-rich montmorillonite displays octahedral charge, tetrahedral charge appears only when Fe content increased >1.2 Fe per formulae. With the maturation process the growing of the closed layers is observed (mixed layer smectite–glauconite 80/20). Nearly similar evolution patterns were recorded at various levels (0.01, 0.85, 2.2 Ma) of the Pliocene–Pleistocene accumulation, however the nontronite way is evidenced in the lowest green grains concentrations and remind the previously described shelf processes. Independently of the morphologically induced winnowing, in these marine tropical deposits, the glauconitization process was especially favored by abundant iron supply and was not affected by low temperature (∼3°C). It is suggested that in deep-water environment, bottom-current controlled iron deposition and disponibility, strongly influencing the mineralogic evolution. A too lengthy dispersal of iron microparticles played an ultimate role in the development of the montmorillonite way rather than the nontronite way. This would be the most commonly found in shallow-water environment.

Characterization of magnetite in silico-aluminous fly ash by SEM, TEM, XRD, magnetic susceptibility, and Mössbauer spectroscopy

Spinel magnetite contained in a silico-aluminous fly ash (originating from la Maxe's power plant, near Metz in the east of France) issued from bituminous coal combustion has been studied by scanning and transmission electron microscopy linked with energy dispersive spectroscopy, X-ray diffraction, susceptibility measurements, and Mössbauer spectroscopy. The results show that in this magnetite Mg is strongly substituted for Fe and the chemical formula is closer to MgFe 2O 4 than Fe 3O 4. Magnetite also contains Mn, Ca, and Si elements, but at a lower proportion. The results are compatible with the chemical formula Fe 2.08Mg 0.75Mn 0.11Ca 0.04Si 0.02O 4 and crystallochemical formula [Fe 2+ 0.92Ca 2+ 0.06Si 4+ 0.02] tetra[Fe 3+Fe 2+ 0.16Mg 2+ 0.73Mn 2+ 0.11] octaO 4, showing the cation distribution on octahedral and tetrahedral sites of the spinel structure. The reason Mg element is not incorporated in soluble surface salt and in glass composition of the silico-aluminous fly ashes is now understood.

Nomenclature of the micas

AbstractEnd-members and species defined with permissible ranges of composition are presented for the true micas, the brittle micas, and the interlayer-deficient micas. The determination of the crystallochemical formula for different available chemical data is outlined, and a system of modifiers and suffixes is given to allow the expression of unusual chemical substitutions or polytypic stacking arrangements. Tables of mica synonyms, varieties, ill-defined materials, and a list of names formerly or erroneously used for micas are presented. The Mica Subcommittee was appointed by the Commission on New Minerals and Mineral Names of the International Mineralogical Association. The definitions and recommendations presented were approved by the Commission.

Nomenclature of the Micas

End-members and species defined with permissible ranges of composition are presented for the true micas, the brittle micas, and the interlayer-deficient micas. The determination of the crystallochemical formula for different available chemical data is outlined, and a system of modifiers and suffixes is given to allow the expression of unusual chemical substitutions or polytypic stacking arrangements. Tables of mica synonyms, varieties, ill-defined materials, and a list of names formerly or erroneously used for micas are presented. The Mica Subcommittee was appointed by the Commission on New Minerals and Mineral Names of the International Mineralogical Association. The definitions and recommendations presented were approved by the Commission.

Nomenclature of the Micas

AbstractEnd members and species defined with permissible ranges of composition are presented for the true micas, the brittle micas and the interlayer-cation-deficient micas. The determination of the crystallochemical formula for different available chemical data is outlined, and a system of modifiers and suffixes is given to allow the expression of unusual chemical substitutions or polytypic stacking arrangements. Tables of mica synonyms, varieties, ill-defined materials and a list of names formerly or erroneously used for micas are presented. The Mica Subcommittee was appointed by the Commission on New Minerals and Mineral Names (“Commission”) of the International Mineralogical Association (IMA). The definitions and recommendations presented were approved by the Commission.

Oxidation mechanisms in Al-Mg-Fe spinels. A second stage: α-Fe 2 O 3 exsolution

A two-stage model of oxidation was devised to explain the observed variations in crystallographic parameters in two artificially oxidized natural spinels. In the first stage, oxygen is added to the crystal boundary as cations are preserved, with Fe rising in total valence and vacant sites being formed. In the second stage, oxygen is preserved and α−Fe2O3 intergrowth occurs, at the expense of the oxygen of the parent spinel structure. On the basis of this model, crystallochemical formulae were calculated and cations partitioned in the various conditions. It was found that, both before and after oxidation, the spinel site population varies continuously in the direction of an increase in random charge distribution, depending on the increase of heat to the crystals. This trend was found to be reversible. Cation vacancies produced during oxidation are distributed between tetrahedral site T and octahedral site M.

Zinccopperite—A New Variety of Zinc‐Copper Intermetallic Compounds Discovered in a Porphyry‐Copper Deposit

Abstract: Zinccopperite (tentatively named) is a rare native alloy mineral discovered in quartz monzonite—porphyry in the Xifanping area, Yanyuan County, Sichuan Province. It is a new variety of zinc—copper alloy mineral found for the first time in the porphyry‐copper deposit in China. Its intergrown minerals are K—feldspar (mainly perthite), albite—oligoclase, quartz and biotite; and the associated minerals include pyrite and chalcopyrite. It is characterized by a golden reflection colour, being isotropic (isometric), with the grain size ranging from 10 to 50 μm, microhardness VHN10 = 190 kg/mm2, and reflectance RVM = 67.97%. Electron microprobe (Model JXA—733) analysis shows Cu = 59.15%–62.55% and Zn = 36.32%–39.85%. The crystallochemical formula is Cu6.27‐7.0Zn4.0, simplified as Cu7Zn4.

Ferrimagnetic spinels in hydrothermal and thermal treatment of MnxFe2−2x(OH)6−4x

Products of hydrothermal treatment of the initial amorphous system MnxFe2−2x(OH)6−4x for 0≤x1 in 0.1x intervals, and products of their further thermal treatment, were examined by chemical analysis, X-ray, IR, and DTA techniques supported by magnetic measurements. After hydrothermal growth for lowx, hematite and goethite phases occurred. Although the goethite phase was still identifiable atx=0.6, formation of a solid solution with the isostructural groutite was not found. The ferrimagnetic spinel phase, which resists heating up to 400‡C, was present at 0.5≤x≤0.9. At higher temperatures, it transformed into the rhombohedral hematite type phase or into the cubic bixbyite phase. AtT≥900‡C, a ferrimagnetic spinel structure reappeared up tox=0.8. For x=0.9, the low- and high-temperature forms of the hausmannite phase occurred, forx= 1 passing from one form into another through Mn5O8 and partritgeite.For a primary mixture Mn0.5Fe(OH)4, corresponding to the manganese ferrite structure, the lattice parameter of which passes from 8.43 å through 8.33 å to 8.50 å, the probable crystallochemical formula was suggested.

Crystal structures and metal uptake capacity of 10Å-manganates: An overview

Marine and synthetic 10Å-manganates and terrestrial todorokites have similar tunnel structures. Each tunnel wall consists of 3 edge-shared [(Mn 4+, Me (2+,3+), Mn 2+)(O 2−, OH −) 6]octahedral strings along the tunnel, where Me (2+,3+) represents transition cations such as Fe 3+, Co 2+, Ni 2+ and Cu 2+. A ceiling or floor is composed of n edge-shared [Mn 4+O 2− 6] octahedral strings along the tunnel. A tunnel is filled mainly by n−2 edge-shared [Me 1+,2+)(OH −, H 2O) 6] octahedral strings, where Me (1+,2+) stands for alkali and alkaline earth cations such as Na +, Ca 2+ and Ba 2+. The n numbers are invariant along the direction normal to the ceilings except where lattice defects occur, but vary randomly along the direction normal to the walls. The interlayer strength of the phases increases with the substitution of Me (2+,3+) for Mn 2+ ions, the oxidation of Mn 2+ to Mn 4+ ions and the concomitant change of their OH − to O 2− ligands in the walls. The ideal lattice of these phases appears to be orthorhombic, but some crystals may be monoclinic mainly because of inhomogeneous cation distributions in the walls and wall distortion. The crystallochemical formulae for the phases can be generalized as Me <(n̄ −2) (1+,2+)(Mn 4+, Me (2+,3+), Mn 2+) 3Mn ñ 4+O 2̃n 2− < (O 2−, OH −) 6 (OH −) 2( n − 3) · 4H 2O where n ̄ denotes the mean of n numbers and is equal to or larger than 3. The upper limit in Me (2+,3+) Mn molar ratios is 3 n ̄ . The presence of Mn 2+ ions or an easy collapse of interlayers upon drying indicates unsaturation with trace cations.

Crystallochemical and Structural Classification of Layer Silicates and Sepiolite.

The complex intergrated system of classification and identification of phyllosilicates and their structures has been outlined in this work. The identification of mineral species is developed. This is facilitated by the new projection system of chemical composition, as given by a crystallochemical formula, onto a field with orthogonal axes chosen for octachedral divalent cations, R2+, and Si (X, Y, respectively), and oblique axes for octahedral trivalent cations, R3+ and vacancies, (V, Z, respectively). The stoichiometry of compositions is controlled, at any point of the projection field by the crystallochemical formulae, valid equally well for di- and tri-octachedral species.A notation common for all the silicate structures is based on the known Ramsdell symbols, which are combined with those related to the Subfamily and MDO group within homo-, meso-, hetero-octahedral Families. The symbols used have physical meaning as they correspond with the number of layers in the unit cell, symmetry system, and structural XZ and YZ projections.

Palagonite Reconsidered: Paracrystalline Illite-Smectites From Regoliths on Basic Pyroclastics

AbstractPoorly crystalline authigenic alteration products of basic pyroclastics from the Golan Heights, Israel, were investigated by XRD, DTA, TGA, FTIR and chemical analysis. Modeling XRD patterns with the use of NEWMOD code provided a way to identify these clays as random interstratified illite/smectites (I/S) with ∼70% of illitic interlayers. Their characteristic features were very poor basal reflections, distinct hk bands, high CEC and low (∼2%) K2O content. Crystallite thickness distribution was found to follow Ergun's model with a weight-average thickness of 2.7–2.8 layers. A new method was proposed to calculate the proportion of kaolinite and 2:1 minerals in their mixtures and the average crystallochemical formula of 2:1 minerals in the presence of kaolinite. The method starts from data of chemical analysis and TGA and assumes that the anionic frameworks of kaolinite and 2:1 minerals are exactly O10(OH)8 and O10(OH)2 respectively. The number of OH-groups per ten oxygens not bonded to H in the empirical formula of the mixture is used to evaluate the proportion of kaolinite. Formation of I/S in well-drained environments under humid mediterranean climatic conditions was attributed to long dry seasons. Interstitial water composition was shown to be consistent with authigenic formation of I/S.

Electronic absorption spectra of chromium-bearing sapphirine

Violet, non-pleochroic and greenish-blue, pleochroic chromium-substituted sapphirines were found in corundum-bearing spinel-websterite xenolites from the Yakutian kimberlite pipes Noyabrskaya (N) and Sludyanka (Sl), respectively. The crystallochemical formulae of sapphirine crystals from such xenolites were determined by EMP to be (Mg3.40Fe0.23Al3.25Cr0.16)[6] Al 1.00 [6] [O2/Al4.53Si1.47O18] (N) and (Mg2.53Fe0.55 Mn0.04Ti 0.03 4+ Al3.55Cr 0.08 3+ )[6]Al 1.00 [16] [O2/Al4.28Si1.73O18] (Sl). Single crystal spectra in the range 35000–6000 cm1- showed a slightly polarization dependent absorption edge near 3200 cm1- (N) or 30000 cm1- (Sl) and unpolarized bands at 25300 and 17300 cm1-, typical of spin-allowed transitions, derived from 4A2g→4T1g and 4A2g→4T2g, of Cr3+ in octahedral sites, with point symmetry C1, of the structure. Another weak band at 23000 cm−1 in the sapphirine-N spectra is attributed to low symmetry splitting of the excited 4T1 (F)-State of Cr3+. These assignments lead to crystal field parameters Dq=1730cm−1 and B= 685cm−1 of Cr3+ in sapphirine. Crystallochemical and spectroscopic arguments suggest that Cr3+ subsitutes for Al in the M(1) or M(8) sites of the sapphirine structure. In addition to Cr3+-transitions, spectra of Sl exhibit weak dd-bands of Fe2+ at 10000 and 7700 cm1-, which are unpolarized in consistency with the C1 site symmetry of the octahedra in the structure. Spectra of Sl show also prominent, broad bands (Δv1/2∼-5000 cm1-) at 15000 and 11000 cm1-, which occur in E//Y(//b) and E//Z(//c=12°) only and exhibit an intensity ratio αY∶αz close to 1∶3. This result, the large half width, as well as band energy — MM distance considerations suggest that these bands originate from Fe2+[6]-Fe3+[6] charge-transfer transitions in wall octahedra M(1)M(2), M(6)M(7) etc., forming MM vectors of 30° with the c-axis. The lack of Fe2+-Fe3+ charge-transfer bands in sapphirine N might indicate a lower oxygen fugacity during the formation of the websterite from the Noyabrskaya pipe compared to that from the Sludyanka pipe.

Thermogravimetric analysis of a talc mixture

A commercial talc also containing chlorite, dolomite and magnesite was subjected to thermogravimetric analysis to determine its mineral composition. Knowledge of the reactions and the decomposition temperatures, as well as the crystallochemical formulae of the phases present, has permitted the application of a system of equations that has provided mineral composition results in good agreement with standard analysis obtained via elemental chemical analysis.

Crystallochemical classifications of phyllosilicates based on the unified system of projection of chemical composition: I. The mica group

AbstractA unified system of vector representation of chemical composition is proposed for the phyllosilicates based on projection of the composition, as given by crystallochemical formula, onto a field with orthogonal axes chosen for octahedral divalent cations, R2+, and Si (X, Y, respectively), and oblique axes for octahedral trivalent cations, R3+, and vacancies, □, (V, Z, respectively). Point coordinates for each set of axes were used to define the direction and length of the unit vectors for phyllosilicates belonging to different groups. Parallel to these fundamental directions the composition isolines were drawn in the projection fields. Applied to micas, this system enables control of the chemical composition by the general crystallochemical formula covering all varieties of Li-free dioctahedral and trioctahedral micas:where z (number of vacancies) = (y-x+ m)/2; m (layer charge) =1; u+y+z = 3. There is a similar formula for vacancy-free lithian micas:where w = m — x+y;m=1; u+y+w = 3, and for Li-free brittle micas:where z = (y — x+m)/2; m = 2; u+y+z = 3. Projection fields were used to classify micas.

Crystallochemical classifications of phyllosilicates based on the unified system of projection of chemical composition: II. The chlorite group

AbstractThe classification and nomenclature of chlorites have been critically reviewed. A new classification based on the unified projection system of chemical composition has been proposed in which for the first time all trioctahedral, di-trioctahedral and dioctahedral species are included. The chemical composition of chlorites is controlled by the general crystallo-chemical formula: This is superior to the known formulae for the trioctahedral and dioctahedral chlorites. It has been shown that the chlorite end-member compositions of Bayliss (1975) project in one point of the projection field leaving many end-member compositions unnamed. As they correspond to the names used previously, a proposal is made to restore these names.