Distribution Of Octahedral Cations Research Articles

LITHIUM IN THE SUBSOIL OF UKRAINE Part 4. Mineralogy of lithium-bearing objects: micas

Part 4 of the publication "Lithium in the subsoil of Ukraine" provides a description, mostly typomorphic, of layered silicates of lithium-bearing objects — muscovite, micas of the isomorphic series of annite — trilithionite (formerly known as annite (lepidomelane) — protolithionite — zinnwaldite — cryophyllite — lepidolite), and micas from the biotite group. The mineralogy of mica are characterized in the following order: Volyn chamber pegmatites — Perzhansky ore node — Kamyanomohyla (stone grave) complex of Azov region — rare metal pegmatites of the Azov region and the Inhul megablock. Muscovite is a secondary low-temperature mineral of the chamber pegmatites of Volyn which grew after the crystallization of micas from the isomorphic series of annite-trilithionite. It is represented by the polytypes 2M1 and 3T. Muscovite is common mica in the Perga ore district; in greisens and some metasomatites, it is the main mineral. It contains an elevated amount of iron and belongs to the 1M polytype modification: muscovite (Fe)-1M. Muscovite is also found in the Kamyanomohylskyi complex of the Azov region, primarily in altered granites and greisens formed on them. Its characteristic feature is the elevated iron content and the 2M1 structure. Fuchsite, fibrous muscovite and sericite-1Md are also found here. Muscovite in rare-metal pegmatites is mostly a secondary mineral. Rare pink muscovite, fuchsite and lithium muscovite have been found in the Azov pegmatites. Muscovite is present in relatively small amounts in the Shevchenkivske deposit and is represented by two generations. Muscovite (Li, Fe)-1M is rarely found. The X-ray luminescent properties of muscovite from spodumene pegmatites are of great importance as they are the main typomorphic features of the development of the pegmatitic process. Muscovite is rarely found in petalite pegmatites of the Inhul megablock and is represented by two generations. Its constitution is not fully understood and is controversial. Li-Fe мicas in granites and pegmatites of Volyn are important type-minerals indicating the crystallization of granites and pegmatites from magma with high content of rare elements and volatile components, the degree of differentiation of chamber pegmatites, and the extent of post-magmatic processes. The distribution of octahedral cations in the crystal structure of micas also has typomorphic significance, reflecting the activity of volatile components, the rate of crystallization, and the degree of differentiation of chamber pegmatites. The processes of forming fully differentiated pegmatites are accompanied by a polytypic transformation: 1M → 3T → 1M. Lithium-iron micas in greisens and metasomatites have similar but somewhat different typomorphic significance of the Perzhansky ore district (in which the polytype 2O was discovered) as well as in the Kamyanomohylskyi complex of the Azov region. The article concludes with a discussion of the results of research on rare alkalies in iron-magnesium micas from various types of metasomatic rocks surrounding pegmatites in Polohivske and Stankuvatsko-Lypnyazhske ore fields.

Structural and Crystal-Chemical Features and Rb–Sr Age of Globular Glauconite in the Ust-Il’ya Formation (Lower Riphean, Anabar Uplift)

For the first time, mineralogical, structural, crystal-chemical and isotope-geochronological data were obtained for a glauconite sample from the Lower Riphean Ust-Il’ya Formation on the Magan River (western slope of the Anabar Massif, northern Siberia). It has been established that glauconite from the lenticular layer of friable glauconitite (glauconite grains ≥80%) contains two populations of mixed-layer crystals, in which mica layers (93%) alternate with either smectite-type (7%) or vermiculite-type layers (7%) with the short-range order factor R = 3. The ratio KAl = (VIAl / (VIFe3+ + VIAl) equal to 0.45 allows identifying the micaceous mineral as glauconite with the unit-cell parameter b = 9.065 A and K2O content of 8.29%. Comparison of new data with the previously obtained mineralogical and crystal-chemical characteristics of Ust-Il’ya glauconites from the Kotuikan River section (2.5 km above the Il’ya River mouth), located 65 km south of the Magan River section, showed both similarities and differences. Analysis of the crystal-chemical heterogeneity of Kotuikan glauconite samples from rocks of different lithological types revealed peculiarities in the distribution of individual mica varieties composing the glauconite globules. Isotopic dating of glauconite from the Magan River section was performed in combination with simulation of the distribution of octahedral cations and comparison of the results obtained with Mossbauer and infrared (IR) spectroscopy data. Such an approach combined with the mineralogical and crystal-chemical analyses contributes to correct interpretation of the stratigraphic significance of isotope data. The results obtained provide grounds for the conclusion that isotopic dates of glauconite from the Magan section (1474 ± 21 Ma) coincides within the error limit with the earlier Rb–Sr and K–Ar dating of the Ust-Il’ya Formation (1485 ± 13 and 1459 ± 20 Ma, respectively) based on the glauconite in the Kotuikan section (Zaitseva et al., 2016). The former value marks the stage of early diagenesis of sediments and is suitable for estimating the age of this formation.

Influence of Octahedral Cation Distribution in Montmorillonite on Interlayer Hydrogen Counter-Ion Retention Strength via First-Principles Calculations

AbstractAlthough multiple types of adsorption sites have long been observed in montmorillonite, a consistent explanation about the chemical structure of these adsorption sites has not yet been established. Identifying the cation interlayer adsorption sites based on the octahedral cation distribution on montmorillonite was investigated in this study by using a Density Functional Theory (DFT) simulation. A clay structural model (H[Al6MgFe]Si16O40(OH)8) with a similar composition to Wyoming SWy-1 montmorillonite was built, where two octahedral Al were respectively substituted by Fe and Mg, and H+ was the charge compensating cation. This model had twenty-one different possible configurations as a function of the distribution of octahedral Al, Fe, and Mg cations. The DFT simulations of 15 of these different configurations showed no preference for the formation of any configuration with a specific octahedral Fe-Mg distance. However, the H+ adsorption energy was separated into three distinct groups based on the number of octahedral jumps from Fe to Mg atoms. The H+ adsorption energy significantly decreased with increasing number of octahedral jumps from Fe to Mg. Assuming an even probability of occurrence of 21 octahedral structures in montmorillonite, the percentages of these three groups are 43, 43, and 14%, respectively, which are very close to the three major sites on montmorillonite from published cation adsorption data. These DFT simulations offer an entirely new explanation for the location and chemical structure of the three major adsorption sites on montmorillonite, namely, all three sites are in the interlayer, and their adsorption strengths are a function of the number of octahedral jumps from Fe to Mg atoms.

Correction to: Influence of Octahedral Cation Distribution in Montmorillonite on Interlayer Hydrogen Counter-Ion Retention Strength Via First-Principles Calculations

Correction to: Influence of Octahedral Cation Distribution in Montmorillonite on Interlayer Hydrogen Counter-Ion Retention Strength Via First-Principles Calculations

Isotopic geochronology and biostratigraphy of Riphean deposits of the Anabar Massif, North Siberia

The structure of Riphean deposits developed on the western slope of the Anabar Massif is described with analysis of their depositional environments, distribution of stromatolite assemblages and organic-walled and silicified microfossils through sections, and evolution of views on stratigraphic significance of some of these assemblages. The investigation included complex mineralogical, geochemical, structural, and isotopic‒geochronological study of globular phyllosilicates (GPS) of the glauconite‒illite series from paleontologically well substantiated Riphean sequences (Ust’-Il’ya and Yusmastakh formations of the Billyakh Group) of the Anabar Massif in the Kotuikan River basin. Isotopic dating of monomineral size and density fractions of GPS from the Billyakh Group was performed in combination with simulation of the distribution of octahedral cations and comparison of the results obtained with Mossbauer spectrometry data. The applied approach is based on an assumption that the formation and transformation of Rb‒Sr and K‒Ar systems in GPS are synchronous with stages in their structural evolution, which are determined by the geological and geochemical processes during depositional history. Such an approach combined with the mineralogical and structural analysis contributes to correct interpretation of stratigraphic significance of isotopic data. The results obtained provide grounds for the conclusion that isotopic dates of GPS from the Ust’-Il’ya (Rb‒Sr, 1485 ± 13 Ma; K‒Ar, 1459 ± 20 Ma) and Yusmastakh (Rb‒Sr, 1401 ± 10 Ma; K‒Ar, 1417 ± 44 Ma) formations mark the stage of early diagenesis of sediments and are suitable for estimating the age of formations in question.

High-angle annular dark field scanning transmission electron microscopic (HAADF-STEM) study of Fe-rich 7 Å–14 Å interstratified minerals from a hydrothermal deposit

AbstractThe distribution of octahedral cations in the two component layers of a 7 Å–14 Å interstratified mineral with a bulk chemical composition (Fe4.122+Mg0.07Mn0.01Al1.69□0.11)(Si2.56Al1.44) O10(OH)8was investigated using high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) in combination with the image simulations. In the 14 Å component layers, comparison between the observed and simulated images revealed that the M4 sites of the interlayer sheets were occupied preferentially by Al together with a small amount of Fe; the other M1, M2 and M3 sites were occupied by dominant Fe and residual Al in equal proportions. Two types of octahedral sheets with disordered and ordered cation distributions were recognized in the 7 Å component layers. The two types of sheets were similar to the octahedral sheet of the 2:1 layer and the interlayer sheet in the 14 Å layer above, respectively. Irregular vertical stacking and lateral contact of the different component layers in structure and chemistry characterized the interstratification, which may be caused by rapid precipitation and accretion of the component layers in hydrothermal environments.

Interpretation of the infrared spectra of the lizardite-nepouite series in the near- and mid-infrared range

A series of 1:1 silicate clays of the lizardite-nepouite series [Si2Mg3−xNixO5(OH4) with x = 0, 0.5, 1, 1.5, 2, 2.5, and 3] was synthesized at 220 °C during 7 days from coprecipitated gels in hydrothermal conditions. A clear relationship was evidenced between the d (06–33) and the Ni/Mg ratio of the synthesized samples following a Vegard’s law and suggested a random distribution of octahedral cations. For the first time, infrared spectra of this series were given in both near and mid-infrared spectral regions (250–7500 cm−1). Notably, the bands due to the OH stretching vibrations and those of their first overtones in the lizardite-nepouite series were attributed. The combination bands observed in the near infrared region for both end-members could be attributed thanks to combinations of two or three middle-infrared features. Some of the observed combination bands are clearly linked to combination of different vibrational groups. Infrared spectroscopy is simple to use and is a powerful tool to study the crystal chemistry of garnierites. More broadly, the improvement of band attributions especially in near infrared contributes to develop the infrared analyses in field geology and remote sensing.

Microwave-Assisted Hydrothermal Synthesis of Ni-Mg Layered Silicate Clays

The synthesis of Ni-containing layered silicate clays from the lizardite-nepouite series (1:1) and kerolite-pimelite series (2:1) with random distribution of octahedral cations was feasible with the microwave-assisted hydrothermal synthesis method. These series were obtained from co-precipitated gels with Si 2 Mg 3-x Ni x O 7 .nH 2 O and Si 4 Mg 3-x Ni x O 11 .nH 2 O (x = 0, 0.5, 1, 1.5, 2, 2.5, and 3) compositions. Syntheses were performed at 220°C during 48 hours under equilibrium water pressure using microwave furnace. X-ray diffraction revealed pure 1:1 and 2:1 synthesized phyllosilicates for the lizardite and the kerolite series, respectively. Fourier transform infrared spectroscopy in both near and mid infrared regions revealed a random distribution of Ni cations in the octahedral sheet of 1:1 and 2:1 silicate clays, that is a good requirement for material applications, especially for the synthesis of metallic catalysts.

Influence of mineralogical and morphological properties on the cation exchange behavior of dioctahedral smectites

Structure and morphology determine the smectites properties. Thereby, edge sites play a key role for the reactivity and colloidal behavior of the smectite. Even so, the edge surface area represents only 1–3% of the total surface area its contribution is up to 10% of the cation exchange capacity (CEC). Na-saturated dioctahedral smectites <0.2 μm were characterized in detail with respect to their structure (layer charge, octahedral cation distribution, iron content, and layer charge distribution within tetrahedral and octahedral sheets). Furthermore, reduced charge montmorillonite (RCM) of these materials were studied. Particle dimension was estimated from argon adsorption, calculation of specific surface area and direct observation of single layers by atomic force microscopy (AFM), which also revealed morphology and particle size distribution of single layer. CEC was measured both for Na-saturated samples and RCM using the Cu-trien method over a pH range of 4–9. The Wyoming smectite with a mean layer diameter of 277 nm and a low layer charge of 0.26 eq/f.u. differs strongly from the others smectites with a mean layer diameter of about 100 nm and layer charges between 0.30 and 0.37 eq/f.u. Furthermore, the layer stacking is much higher for the Wyoming smectite in powder and RCM. Accordingly the edge properties differ between the four smectites. Based on calculation of edge site density and CEC measurements, we showed that the RCM can be used to study the edge site reactivity of the montmorillonites. The increase of CEC above their pHPZC,edge is equal for Na-saturated montmorillonite and RCM. The present study highlights the diversity in structure and morphology and resulting edge properties of four smectites.

Octahedral cation distribution in glauconites from Southern Urals by combination of crystal-chemical model and quasi-continuous model-independent quadrupole splitting distributions (QSD) fitted to their Mössbauer spectra

Mossbauer spectra of two glauconites, BSH-11 and KUL-1, from the Southern Urals are interpreted by simulation using crystal-chemical modelling and by fitting of quadrupole splitting distribution (QSD). The crystal-chemical modelling is based on X-ray diffraction (XRD) data and includes, as a priori information, individual quadrupole splittings for Fe 3+ and for Fe 2+ that correspond to specific local cation arrangements (LCA) having certain occurrence probabilities. The latter are provided by two-dimensional octahedral cation distribution using integrated infrared optical densities for bands corresponding to the cation pairs bound to the OH groups. Combination of crystal-chemical modelling and quasi-continuous model-independent QSDs fitted to Mossbauer spectra resulted in cation distributions for which R 2+ (Mg, Fe 2+ ) and R 3+ (Al, Fe 3+ ) cations occupy one of two structurally independent cis- sites with equal probabilities. The cation distributions obtained are characterized by Fe-clustering, domain structure, and different trends to homogeneous charge dispersion.

Trans-Vacant and cis-Vacant 2:1 Layer Silicates: Structural Features, Identification, and Occurrence

AbstractA comprehensive study of clay minerals should include determination of the vacancy pattern of the dioctahedral sheet. The purpose of this report is to consider the advantages and limitations in various diffraction and non-diffraction methods for the determination of the layer types in clay minerals. Identification of trans-vacant (tv) and cis-vacant (cv) clay minerals reported here is based on powder X-ray diffraction (XRD) patterns calculated for different polytypes consisting of either tv or cv layers, on the simulation of experimental XRD patterns corresponding to illite or illite fundamental particles in which tv and cv layers are interstratified, and on the semi-quantitative assessment of the relative content of the layer types in the interstratified structures by generalized Méring’s rules. A simple and effective method for identification of tv and cv layers in dioctahedral 2:1 layer silicates employs thermal analysis and is based on different dehydroxylation temperatures for tv and cv illite and smectite layers.Crystal chemical analysis of various dioctahedral 2:1 layer silicates consisting of tv and cv layers indicates that compositional control is present in the distribution of octahedral cations over trans- and cis-sites. In dioctahedral smectites the formation of tv and cv layers is related to the layer composition and local order-disorder in the distribution of isomorphous cations. Dioctahedral 1M micas with abundant Fe3+ and Mg occur only as tv varieties. In contrast, 1M-cv illite, as well as cv layers in illite fundamental particles of I-S, can form only as Fe- and Mg-poor varieties. In illites and illite fundamental particles of I-S consisting of tv and cv layers, cv layers prevail when the amounts of Al in octahedra and tetrahedra are &gt;1.55 and &gt;0.35 atoms per O10(OH)2, respectively.The main factors responsible for the stability of cv and tv illites have been established. Monomineral cv 1M illite, its association with tv 1M illite, and interstratified cv/tv illite occur around ore deposits, in bentonites, and in sandstones mostly as a result of different types of hydrothermal activity. The initial material for their formation should be Al-rich, and hydrothermal fluids should be Mg- and Fe-poor.Tv and cv smectites of volcanic origin differ in terms of octahedral cation composition and distribution of isomorphous octahedral cations. Mg-rich cv smectites have random distribution of isomorphous octahedral cations, whereas in Mg-bearing tv smectites octahedral Mg cations are dispersed so as to minimize the amount of Mg-OH-Mg arrangements.

Octahedral cation distribution in palygorskite

The OH speciation of 18 palygorskite samples from various localities were evaluated by near infrared spectroscopy (NIR) and compared to the corresponding octahedral composition derived from independent, single-particle analytical electron microscopy (AEM). NIR gives evidence for dioctahedral-like (AlAlOH, AlFe3+OH, Fe3+Fe3+OH) and trioctahedral-like (Mg3OH) species. Therefore, palygorskite can be approximated by the formula yMg5 Si8O20(OH)2·(1 – y)[xMg2Fe2·(1 – x)Mg2Al2]Si8O20(OH)2, where x is the Fe content of the dioctahedral component, and y is the trioctahedral fraction. The values of x estimated from the NIR data are in excellent agreement with the Fe/(VIAl + Fe) ratio from AEM (R2 = 0.98, σ = 0.03), thus suggesting that all octahedral Al and Fe in palygorskite participate in M2M2OH (dioctahedral-like) arrangements. Furthermore, y values from AEM can be compared to NIR (R2 = 0.90 and σ = 0.05) after calibrating the relative intensity of the Mg3OH vs. (Al,Fe)2OH overtone bands using AEM data. The agreement between the spectroscopic and analytical data are excellent. The data show that Fe3+ for Al substitution varies continuously in the analyzed samples over a broad range (0 < x < 0.7), suggesting that fully ferric dioctahedral palygorskites (x = 1) may exist. On the other hand, the observed upper trioctahedral limit of y = 0.50 calls for the detailed structural comparison of Mg-rich palygorskite with sepiolite.

Crystal-Chemical Factors Responsible for the Distribution of Octahedral Cations Over trans- and cis-Sites in Dioctahedral 2:1 Layer Silicates

AbstractCrystal chemical analysis of various dioctahedral 2:1 phyllosilicates consisting of trans-vacant (tv) and cis-vacant (cv) layers and interstratified cv and tv layers shows that there is compositional control over the distribution of octahedral cations over trans and cis sites. Fe3+ and Mg-rich dioctahedral micas (celadonite, glauconite, leucophyllite and most phengite) occur only as tv varieties. Similarly, the occurrence of tv illites and tv illite fundamental particles in illite-smectite (I-S) does not depend significantly on the cation composition of the 2:1 layers. In contrast, compositional restrictions exist to control the occurrence of pure cv1M illite, which can form only as Fe- and Mg-poor varieties. Similarly, proportions of cv and tv layers in illite fundamental particles depend on the amount of Al in octahedral and tetrahedral sheets of the 2:1 layers.Simulations of atomic coordinates and interatomic distances for periodic tv1M and cv1M illite structures allow us to reveal the main structural factors that favor the formation of cv layers in illite and I-S. It is shown that in contrast to the tv1M structure, interlayer K in cv1M illite has an environment which is similar to that in 2M1 muscovite. This similarity along with a high octahedral and tetrahedral Al content probably provides stability for cv1M illite in low-temperature natural environments. Because of structural control, the occurrence of monomineral cv1M illite, its association with tv 1M illite, and interstratified cv-tv illite fundamental particles is confined by certain physical and chemical conditions. These varieties are most often formed by hydrothermal activity of different origin. The initial material for their formation should be Al-rich and the hydrothermal fluids should be Mg- and Fe-poor. They occur mostly around ore deposits, in bentonites and in sandstone sedimentary rocks.The factors governing the formation of tv and cv layers in dioctahedral smectite are probably related to the layer composition and local order-disorder in the distribution of isomorphous octahedral cations, because there is no influence from fixed interlayer cations. In particular, the occurrence of Mg-OH-Mg cation arrangements is more favorable for the formation of cv montmorillonite layers.

福島県石川山花崗岩ペグマタイト産鉄コルンブ石の化学組成

Chemical evolution of ferrocolumbites from the Ishikawa-yama granitic pegmatites, Fukushima, Japan, has been revealed by electron microprobe analysis. The compositions of ferrocolumbites are different among sample locations, which are within about 1 km distant from each other. Three discernible trends in compositional variations in ferrocolumbites have been observed: (1) a homogeneous crystal trend, with low Ta/(Ta + Nb); (2) a trend with oscillatory zoning related to Ta and Nb; (3) a trend with patchy zoning related to Fe and Mn. Oscillatory zoning of ferrocolumbite is intimately related to crystal growth in magmatic process. Ferrocolumbite with patchy zoning shows parallel extinction and doesn’t appear to have the complementary distribution of octahedral cations due to some coupled substitutions on the plots of partition coefficient (each zone/the assumed average) against ionic radius. These facts and the curved boundaries of patchy zoning suggest that patchy zoning of ferrocolumbite has been produced by topotaxy in the open system.

Interpretation of infrared spectra of dioctahedral smectites in the region of OH-stretching vibrations

AbstractDioctahedral smectite samples of a wide range of compositions (beidellites, montmorillonites, nontronites, Fe-rich montmorillonites and Al-rich nontronites) were studied by infrared (IR) spectroscopy. A special sample-preparation technique was used to eliminate the contribution of molecular water. The OH-stretching regions of the spectra were decomposed and curve-fitted, and the individual OH-stretching bands were assigned to all the possible types of OH-bonded cation pairs that involve Al, Mg and Fe. The integrated optical densities of the OH bands were assumed to be proportional to the contents of the specific types of OH-linked cation pairs with the absorption coefficients being the same for all individual OH bands. Good agreement between the samples’ octahedral cation compositions calculated from the IR data and those given by crystal-chemical formulae was obtained for a representative collection of samples in terms of a unique set of individual OH-band positions that vary within narrow wavenumber intervals. This has allowed us to minimize the ambiguity in spectra decomposition caused by the poor resolution of smectite spectra and confirmed the validity of the resulting band identification.The bands associated with specific OH-bonded cation pairs in the spectra of smectites are, on the whole, shifted to greater wavenumbers with respect to the corresponding bands in micas. In addition to OH bands that refer to the smectite structure, AlOHAl and AlOHFe bands of the pyrophyllite structural fragments were identified. The band-position variation ranges overlap in a few cases (AlOHFe and MgOHMg; AlOHAl of smectite and AlOHFe of pyrophyllite-like component).Unambiguous interpretation of the OH-stretching vibrations was found to be possible only for smectite samples with known chemical compositions, so that IR data cannot be used for quantitative determination of octahedral cation composition of mixtures of dioctahedral 2:1 phyllosilicates. In the case of the studied monomineral smectites with known chemical compositions, IR data provided information on the short-range order/disorder in the distribution of octahedral cations along cation-OH-cation directions. This information can be employed, in conjunction with the data of other spectroscopic and diffraction techniques, in the analysis of short-range octahedral cation distribution.

Computer simulation of octahedral cation distribution and interpretation of the Mossbauer Fe2+ components in dioctahedral trans-vacant micas

Cation distributions (CDs) in the representative collection of trans-vacant dioctahedral celadonites, glauconites and ferriillites were investigated using Mossbauer and IR spectroscopies and the original approach. The approach is based on individual quadrupole splittings Δ i pred for Fe 3+ in possible local cation arrangements around Fe 3+ , and on a computer simulation of two-dimentional CDs using the IR data in the OH stretching vibration region. The resulting CDs were next used to make correlations between Δ j of Fe 2+ derived from computer fits to the corresponding spectra and cation composition of local cation arrangements around Fe 2+ with their occurrence probabilities. Basing on this correlations, a total of eight individual Δ tent for Fe 2+ referred as “tentative” have been derived. The order of local cation arrangements in terms of increasing quadrupole splitting was found to be the same both for Fe 3+ and Fe 2+ and implies a direct dependence of the Fe 2+ quadrupole splitting on the structural distortion at Fe 2+ site. The set of Δ i tent for Fe 2+ combined with Δ i pred for Fe 3+ and with the new CD simulation program provide an additional means for controlling the CD reconstruction.

Potential problems in the interpretation of powder X-ray diffraction patterns from fine-dispersed 2M1 and 3T dioctahedral micas

XRD patterns were calculated for 2 M 1 and 3 T dioctahedral mica structural models in which trans -vacant ( tv ) and cis-vacant ( cv ) layers were interstratified and arranged, independent of octahedral cation distribution, as in the periodic 2 M 1 and 3 T mica polytypes. The XRD patterns of the interstratified cv/tv2M 1 and cv/tv 3T models were shown to be similar to those of the tv2M 1 and tv3T mica structures, respectively. Because both the positions and shapes of hkl reflections in the XRD patterns of the tv3T and cv/tv3T as well as of the tv2M 1 and cv/tv2M 1 , respectively, are the same qualitative X-ray identification of the tv2M 1 , cv/tv2M 1 , tv3T and cv/tv3T mica varieties should be a complex problem, especially when the amount of cv layers is either rather low (10–15 %) or high (30–40 %) with significant tendency to segregation of the layer types. The diffraction features of cv/tv2M 1 and cv/tv3T mica structures make it possible to use the Rietveld technique for the structural refinement. Correct application of this technique to the cv/tv2M 1 and cv/tv3T structures should reveal partial occupancy of trans -octahedra because of the ability of diffraction to average structural parameters. Pavese et al. (2001) found partial occupancies of trans -octahedra in 3T and 2M 1 powder phengite samples. To explain these results the authors postulated the ± b/3 stacking disorder. However, the calculations have shown that such disorder strongly modifies the XRD patterns. Therefore, in the paper an alternative, crystal-chemically more justified hypothesis is suggested according to which the occupancy of trans -sites in 3 T and 2M 1 phengites results from coexistence of tv and cv layers in their structures. Diffraction features a dioctahedral mica sample given as a tv3T standard in textbooks (Brindley, 1980; Bailey, 1984) are quite similar to those characteristic for a 3T structure in which tv and cv 2:1 layers are interstratified.

A Crystal-Chemical Study of Natural and Synthetic Anionic Clays

AbstractA comparative crystallochemical study was performed on natural and synthetic hydrotalcite-like compounds with similar compositions. The nature of the brucite-like sheet stacking was addressed by means of powder X-ray diffraction. From the resulting electron diffraction patterns it was possible to establish the order-disorder of the cations in the brucite-like sheet. The results show that a natural sample from Snarum is an intergrowth of hydrotalcite (3R1 polytype) and manasseite (2H1 polytype) at a ratio of 77:23 (wt.%). An aluminian serpentine is associated with the hydrotalcite and manasseite minerals. The structure of a synthetic sample, Mg:Al = 2:1, was determined as space group R3¯m. For a few crystals in this sample, the octahedral cation distribution is compatible with the observed supercell (a = a′ √3). A second synthetic sample showed the presence of stacking faults and was described as a random layer sequence of two polytypes (3R and 2H).

Structural and chemical heterogeneity of illite-smectites from Upper Jurassic mudstones of East Greenland related to volcanic and weathered parent rocks

Illite-smectites (I-S) in one Upper Jurassic mudstone core from East Greenland were investigated to determine their structural and crystal-chemical features and to find the relation between these features and source rocks. The phase composition and layer sequences were determined by X-ray diffraction (XRD), the distribution of octahedral cations over trans - and cis -octahedra by thermal analysis, the structural formulae by XRD, Mossbauer spectroscopy, and total chemical analysis, and the short-range order in isomorphous cation distribution by infrared (IR) and Mossbauer spectroscopies. For all samples (except one having maximum degree of ordering for R = 1), simulation of the experimental XRD patterns led to two different I-S models having indistinguishable diffraction patterns. For the first, single-phase model, expandability ( w S) is 0.45–0.60. For the second, two-phase model, two randomly interstratified I-S having w S equal to 0.40 and 0.85, respectively, are present in different proportions in different samples. The single-phase model was selected. A new approach for simulating the two-dimensional distributions of the isomorphous octahedral cations using IR and Mossbauer parameters revealed a tendency for Fe segregation into edge-shared octahedra that may form zigzag chains. Almost identical IR and Mossbauer parameters found for the I-S having different amounts of trans -vacant ( tv ) and cis -vacant ( cv ) layers (ranging from 0.08 to 0.80) demonstrate that these parameters are largely determined by local cation environments around Fe3+ and OH groups. Different levels in the Upper Jurassic Kimmeridgian core contain I-S having different structures. I-S of hemipelagic mudstones at the bottom (37 m depth) and in the middle (13 m depth) of the core, with a high proportion of cv layers and of smectite layers ( w S ~ 0.60), probably formed from volcanic material. The other four samples have a high proportion of tv layers and probably formed by weathering of micaceous material. One of these I-S, from a mudstone turbidite (at 27 m depth), having maximum degree of ordering for R = 1, probably originated from one type of parent rock, and three mudstones (at depths of 4, 12, and 13 m) with segregated I-S, probably originated from a second rock type.

Fe, Mg and Al distribution in the octahedral sheet of montmorillonites. An infrared study in the OH- bending region

AbstractTen montmorillonites of different origins with variable Fe contents were analysed using transmission IR spectroscopy. Special attention was devoted to the OH-bending region to obtain information about the distribution of octahedral cations. For low to medium Fe contents (≤0.56 per Si8 formula unit), a linear relationship between the position of the δAlFeOH band and Fe content was observed. Such correlation might be explained by changes in the cis-trans occupancy of Fe in the octahedral sheet. Deconvolution of the OH-bending domain allows us to discriminate three components (δAlAlOH, δAlMgOH and δAlFeOH) which are correlated with cation abundances derived from chemical analysis. The relative area of each band can then be compared with theoretical areas calculated assuming a fully random distribution of cations in the octahedral sheet. Using such treatment, eight of the 10 montmorillonites studied presented a nearly randomized octahedral distribution. The two samples from Wyoming were clearly different as they exhibited a strong ordering tendency.