Embaie Ferrow

Experimental weathering of biotite, muscovite and vermiculite: a Mossbauer spectroscopy study

Biotite, muscovite and vermiculite were treated in HCl and H2SO4 solutions at pH ranging between 7 and 1. The solid residue was studied using Mossbauer spectroscopy at room temperature. The spectra of the untreated samples, D0Bio, D0Mus and DOVer were fitted assuming quadrupole splitting distribution, QSD. Different fitting models with different combinations of Gaussian components for the Fe sites were tested and the models with chi(2) near 1.0+/-0.1237 and with the least number of free parameters were adopted as working models. The models were then used to fit the experimental products of each group. If a model fails to describe a spectrum of a sample in the group then this is taken to signal a significant crystal chemical change induced by weathering. All three minerals are oxidized during weathering and the correlation between oxidation ratio and pH is modeled using a Weibull function. The rate of oxidation in biotite is higher than that in muscovite and vermiculite, The average CS and QS of the Fe sites in a group remain more or less constant during the experiment. However, the position of the peak QS of Fe-[6](2+) in biotite is shifted to low energies with decreasing pH, a measure of the decreasing covalence character of the Fe/Mg-O bonds. In muscovite the major feature observed is the enhancement of the bi-modal distribution of the QSD of Fe-[6](2+) with decreasing pH. (Less)

Mössbauer effect study of the crystal chemistry of tourmaline

One of the main features of the tourmaline group of minerals is that the six coordinatedY andZ sites, possessing different point group symmetries and sizes, share edges. The ME parameters are, therefore, controlled by the mechanisms by which the tourmaline structure obtains congruency along the common edge between the two octahedra. For example, if the size of theY site in elbaite is increased by replacing Li and Al by Fe2+, then, for small values of Fe2+, size congruency is maintained by simultaneously replacing Al by Fe2+ at theZ site. The ME spectra of Fe-bearing elbaite are therefore characterised by two doublets assigned to Fe2+ in theY andZ sites. For higher values of Fe2+ at theY site, however, the tourmaline structure reduces the misfit further by oxidation of Fe2+ at theY site. The effect of the second mechanism is to increase Fe2+-Fe3+ interaction centres and thereby promote charge delocalisation. These Fe-Fe charge interactions introduce three more peaks with ME parameters intermediate between Fe2+ and Fe3+. On the other hand, the replacement of Fe2+ by Mg2+ reduces the density of Fe2+-Fe3+ centres, blocks Fe-Fe charge interactions and increases the intensity of Fe3+.

Chemistry, reaction mechanisms and micro-structures during retrograde metamorphism of gedrite-biotite-plagioclase bearing rocks from Bergslagen, south-central Sweden

Abstract The Proterozoic rhyolitic volcanics constituting the foot-wall rocks in the Stollberg ore-field, Bergslagen, south-central Sweden, locally contain gedrite altered to chlorite and serpentine, biotite altered to chlorite and plagioclase altered to epidote. The intergrowths between the host gedrite and the chlorite/serpentine inclusions are oriented with the a ∗ of gedrite parallel to the c ∗ of serpentine and chlorite. The biotite has been altered to chlorite by brucitization of both the K-interlayer and talc-like layer. In both cases the net change in volume during chloritization is small. The assumption that Al is conserved during alteration of gedrite and biotite agrees very well with the micro-structures and orientation relations observed by transmission electron microscopy. Normalizing the chlorite to 1.00 mole, the overall chemical change that took place during the retrograde metamorphism of the Stollberg rocks can be written as: 0.84Ged+0.14Bio+0.65Mg+4.76H2O+0.42H=1.00Chl+0.57Alb+0.72Fe+0.01Na+0.12K+0.01Ti+0.05Mn+0.95H4SiO4 The reaction results in ca 9% increase in volume for the solid phases. Thus, a slightly acidic Mg-rich fluid started the reaction and, upon leaving the system, the metasomatic fluid was enriched in Na, Fe, K, and Si.

Non-integral hybrid ions in tourmaline: buffering and geo-thermometry

The tourmaline group of minerals is indeed an enigma. Experimental data from optical spectroscopy, electron microscopy, and Mossbauer spectroscopy reveal a host of physical properties that lack a common structural clarification. For example, tourmaline samples change colour when irradiated with X-ray and gamma-ray radiations some reverting back when heated in air; exhibit simultaneous oxidation and reduction on annealing in an atmosphere of H-2; display different plane group symmetries under TEM; possess the most complicated Mossbauer spectra of all Fe-bearing silicates. In this study, four Brazilian samples were chosen for detailed study by Mossbauer spectroscopy to find out a common structural factor to the physical anomalies reported in the literature. It was found out that the tourmaline group of minerals contain multi-valence elements that are involved in electron exchange between the edge-sharing asymmetric Y and Z crystallographic sites. It is conceivable that the host of physical properties recorded in the literature could be due to the inherent structural misfit between the Y and Z sites and the mechanisms adopted to reduce the strain associated along the shared edges. The complexity of non-integral oxidation states possible - due to electron sharing among the different multi-valence elements present in the structure - further enhances the diverse physical properties observed. Moreover, on heating in air, no net oxidation or reduction takes place in the tourmaline group of minerals over a temperature range as long as there are electron donors and acceptors left in the structure, serving simultaneously as potential single-phase buffers and geo-thermometers.

Mössbauer effect and TEM in mineralogy

Mössbauer effect (ME) provides useful information on oxidation state, co-ordination number, co-ordination state, site occupancies, and magnetic properties of Fe-bearing phases. The information gained by ME together with the information provided by other conventional techniques is used to extract temperature, pressure, and kinetics of rockforming processes. Nevertheless, ME requires that the phases studied are homogeneous over an extremely large volume and that Fe is a major component of the system. Transmission electron microscopy (TEM), on the other hand, provides similar information over a very small volume for a system of any component. However, present TEM spectrometers do not provide sufficient resolution to detect the mixed oxidation state. A complete characterisation of phases in rocks requires, therefore, that conventional techniques be combined with TEM.

The Relation Between c Dimension and Exchange Components in Micas

SummaryA number of micas of varying compositions and polytypism have been selected from the literature for multiple linear regression analysis. The c dimension in micas is found to depend on the sizes of the interlayer cation, di, and tetrahedral cation, dt, as well as on the hydroxyl content, [OH]. The regression equation obtained: cr = 5.415 + 0.071[OH] + 2.098di + 2.335dt with R2 = 90.5%, shows that the three variables affect the c-axis dimension in the order dt > di ≫ [OH]. Addition of 2- and 3-layer polytypes to the regression analysis reduces R2 to 87.2%. Application of the regression analysis to synthetic Al-rich biotites from the literature shows that the amount of [A1IVA1VI]1Y[Fe2+, MgSi]−1y in solid solution is limited and always less than [A1VIO]1z[Fe2+, MgOH]−1z (i.e. 0.35 > y ≪ z). The maximum value of the vector y in solution is slightly higher than that reported for natural Al-rich biotites.ZusammenfassungEine Anzahl von Glimmern unterschiedlicher Zusammensetzung und Polytypie wurde aus der Literatur für eine multiple lineare Regressionsanalyse ausgewählt. Es stellte sich heraus, dass in Glimmern die Gitterkonstante c von den Grössen des Zwischenschicht-Kations di und des tetraedrischen Kations dt abhängt, ferner vorn Hydroxylgehalt, [OH]. Die erhaltene Regressionsgleichung cr = 5,415 + 0,071 [OH] + 2,098di + 2,335dt mit R2 = 90,5% zeigt, dass die drei Variablen die Grösse der c-Achse in der Reihenfolge dt > di ≫ [OH] beeinflussen. Der Einschluss von 2- und 3-Schicht Polytypen in die Regressionsanalysen verkleinert R2 auf 87,2%. Die Anwendung der Regressionsanalysen auf synthetische Al-reiche Biotite aus der Literatur zeigt, der Betrag von [A1IVA1VI]1y[Fe2+, MgSi]−1y beschränkt und immer kleiner als [A1VIO]1z[Fe2+, MgOH]−1z (mit 0,35 < y ≪ z) ist. Der Maximalwert des Vektors y in Lösung ist etwas grösser als jener, der für natürliche Al-reiche Biotite angegeben wurde.

Mineralogical and geochemical characterization of the concotto artefacts from firing places of Longola protohistoric settlement (Naples)

A set of concotto samples, a type of fired mixture of various raw materials, coming from firing place of the perifiuvial protohistoric site of Longola (Campania region, southern Italy) was characterized by means of a multi-analytical approach, i.e. polarized light optical microscopy (OM), X-ray powder diffraction (XRD), scanning electron microscopy and energy dispersive spectrometry (SEM-EDS), X-ray fluorescence (XRF), inductively-coupled plasma-mass spectrometry (ICP-MS) and Mossbauer spectroscopy. The specimens generally show a heterogeneous texture, a red-coloured groundmass composed of a mixture of tiny quartz, feldspar, poorly-crystallized goethite, hematite and clay minerals. The crystal fractions show significant amounts of quartz and alkali feldspar and variable percentages of clinopyroxene, leucite, biotite, hematite, magnetite and traces of muscovite. Random secondary phases of calcite, vivianite and gypsum have been identified. The coarser fraction is formed by several types of inclusions, i.e. tephra fragments (pumices, scoriae), volcanic and sedimentary rocks. Illite and very subordinate smectite (montmorillonite) were detected by X-ray diffraction. Trace elements also show a contribution of volcanic raw materials. The Mossbauer analyses at 296 K show the occurrence of paramagnetic Fe(3+) iron (clay minerals) and magnetic Fe(3+) iron (hematite). For the 80 K spectrum, however, two doublets (ferrous and ferric iron in clay minerals) and one sextet (hematite). The concotto samples do not suffer high temperatures, perhaps much lower than 500-600 degrees C in a mainly oxidizing atmosphere. Only sporadically, if any, higher temperatures (> 800 degrees C) might have been reached. (Less)

Characterization of pyrrhotite in Cu-Ni-ore bodies from mines in Botswana by Mössbauer spectroscopy, X-ray diffraction, and thermomagnetometry

Cu-Ni ore bodies from five Botswana mines were investigated, using a combination of X-ray diffraction, thermomagnetic measurements, Mossbauer spectroscopy, and Inductively Coupled Plasma-Atomic Absorption Electro-optic Spectroscopy (ICP-AES). The four dominant phases in the ores were found to be pyrrhotite, magnetite, pentlandite, and chalcopyrite. Mossbauer spectroscopy shows that pyrrhotite is the dominant Fe-bearing phase in the samples with 75 % in B-S1, 84 % in B-S2, and 80 % in B-S4 and was selected for detailed characterization. Our results show that the three first techniques provide an excellent approach to estimate the degree of polytype intermixing in pyrrhotite. Accordingly, the ores can be grouped into three types: A hexagonal (or peak-type), with a 5c stacking arrangement; a monoclinic (or Weiss-type), with a 4c stacking arrangement; and a mixture of monoclinic and hexagonal types. A low-temperature study of the monoclinic form shows that the low-temperature magnetic transition close to 35 K has features similar to the Verwey transition in magnetite induced by Jahn-Teller distortion due to orbital order-disorder transition. (Less)

Some examples of the application of Mössbauer spectroscopy and transmission electron microscopy to micas

I discuss the weaknesses and strengths of Mössbauer spectroscopy and transmission electron microscopy and present the potential of combining them using experimental weathering of biotite as an example. A natural biotite was treated in acid at room temperature and the reaction products were removed by a continuous flow of an external eluent that was in contact with the mineral suspension through a dialysis membrane. The fractionation of Mg/Fe into the fluid was determined by wet chemical analysis, the state of Fe in the solid was monitored by Mössbauer spectroscopy, and the structural changes were characterized by transmission electron microscopy.Weathering is controlled primarily by pH, the type of acid used, and K selectivity of the solid. Once equilibrium has been reached, further weathering does not affect the oxidation state of iron. The tetrahedral layer collapses during the last phase of weathering leaving an amorphous phase rich in Si and Al. Moreover, transmission electron microscopy shows that weathering is a discontinuous processes resulting in quasi-periodic texture. The periodicity is controlled by the concentration of the K in solution.

Fe-57 Mossbauer effect study of bannisterite: a modulated 2 : 1-type phyllosilicate

Abstract Bannisterite is a modulated 2:1-type phyllosilicate having ten octahedral sites with hydroxyl groups in trans, cis and combined trans-cis configurations. Each octahedral site, depending on its proximity to the layer modulation, is exposed to a different degree of distortion. The electric field gradient at the Fe site and the site distortion are used to propose a model for the site assignment and oxidation mechanism in bannisterite. The results show that two models are compatible with the experimental results of fitting the spectrum of bannisterite. However, the model which allows for site distortion accounts better for the observed line overlap of the [6]Fe2+ sites. Accordingly, the octahedral sites with a greater degree of distortion are assigned to the [6]Fe2+ with lower electric quadrupole splitting, and the octahedral sites with lower site distortion are assigned to the [6]Fe2+ with higher electric quadrupole splitting. Moreover, the ease of oxidation decreases in the order trans > trans-cis > cis.