Patrick H. J. Mercier

Reversed phase equilibrium constraints on the stability of Mg-Fe-Al biotite

Abstract The stability of Mg-Fe-Al biotite has been investigated with reversed phase-equilibrium experiments on four equilibria. Experimental brackets in pure H2O and H2O-CO2 mixtures for the equilibrium: phlogopite + 3 quartz = enstatite + sanidine + H2O (1) are in good agreement with previous experiments in mixed-volatile fluids (Bohlen et al. 1983) and H2O-KCl solutions (Aranovich and Newton 1998), while indicating a reduced stability field for phlogopite compared to previous data in pure H2O (Wood 1976; Peterson and Newton 1989). Aluminum solubility in biotite has been determined in the Fe-, Mg-, and Fe-Mg systems from reversed phase-equilibrium data for the equilibria: 3 eastonite + 6 quartz = 2 phlogopite + 3 sillimanite + sanidine + H2O (2) 3 siderophyllite + 6 quartz = 2 annite + 3 sillimanite + sanidine + H2O (3) over the P-T range ~600.750 °C and 1.1.3.4 kbar. Over the investigated temperatures, the brackets define nominal Al saturation levels of 1.60 ± 0.04 in Mg-biotite, 2.08 ± 0.05 in Fe-biotite, and 1.81 ± 0.03 in biotite with Fe/(Fe + Mg) = 0.43.0.44. The slight decrease in Al with increasing T and decreasing P suggested by the data is less than experimental uncertainties. Compared to biotite on the Phl.Ann join, Al-saturated biotites have a markedly larger stability field, particularly in the Fe-system. This effect has been quantified in the Fe-system with one reversal between 691.709 °C at 2.4 kbar for the equilibrium: biotite + sillimanite + quartz = almandine + sanidine + H2O (4) The combined experimental results place tight constraints on the thermodynamic properties of phlogopite, annite, eastonite, and siderophyllite. The resulting nonzero (ΔH298 = .9.4 kJ/mol, with ΔS = ΔV = 0) energetics for the internal equilibrium: Eastonite + 2/3 Annite = 2/3 Phlogopite + Siderophyllite (5) reflect strong Fe-Al affinity in biotite, which has a marked effect on thermobarometers involving biotite.

Geometrical parameterization of the crystal chemistry of P63/m apatites : comparison with experimental data and ab initio results

Experimental structure refinements and ab initio simulation results for 18 published, fully ordered P6(3)/m (A;{\rm I}_4)(A;{\rm II}_6)(BO4)6X2 apatite end-member compositions have been analyzed in terms of a geometric crystal-chemical model that allows the prediction of unit-cell parameters (a and c) and all atom coordinates. To an accuracy of +/- 0.025 A, the magnitude of c was reproduced from crystal-chemical parameters characterizing chains of ...-A(II)-O3-B-O3-A(II)-... atoms, whereas that of a was determined from those describing (A(I)O6)-(BO4) polyhedral arrangements. The c/a ratio could be predicted to +/-0.2% using multi-variable functions based on geometric crystal-chemical model predictions, but could not be ascribed to the adjustment of a single crystal-chemical parameter. The correlations observed between algebraically independent crystal-chemical parameters representing the main observed polyhedral distortions reveal them as the minimum-energy solution to accommodate misfit components within this flexible structure type. For materials with given composition, good agreement (within +/- 0.5-2.0%) of ab initio crystal-chemical parameters was observed with only those from single-crystal refinements with R </= 4.0%. Agreement with single-crystal work with R > 4.0% was not as good, while the scatter with those from Rietveld refinements was considerable. Accordingly, ab initio cell data, atomic coordinates and crystal-chemical parameters were reported here for the following compositions awaiting experimental work: (Zn,Hg)10(PO4)6(Cl,F)2, (Ca,Cd)10(VO4)6Cl2 and (Ca,Pb,Cd)10(CrO4)6Cl2.

Kaolin polytypes revisited ab initio

The well known 36 distinguishable transformations between adjacent kaolin layers are split into 20 energetically distinguishable transformations (EDT) and 16 enantiomorphic transformations, hereafter denoted EDT*. For infinitesimal energy contribution of interactions between non-adjacent layers, the lowest-energy models must result from either (a) repeated application of an EDT or (b) alternate application of an EDT and its EDT*. All modeling, quantum input preparation and interpretation was performed with Materials Toolkit, and quantum optimizations with VASP. Kaolinite and dickite are the lowest-energy models at zero temperature and pressure, whereas nacrite and HP-dickite are the lowest-enthalpy models under moderate pressures based on a rough enthalpy/pressure graph built from numbers given in the supplementary tables. Minor temperature dependence of this calculated 0 K graph would explain the bulk of the current observations regarding synthesis, diagenesis and transformation of kaolin minerals. Other stackings that we list have energies so competitive that they might crystallize at ambient pressure. A homometric pair of energetically distinguishable ideal models, one of them for nacrite, is exposed. The printed experimental structure of nacrite correctly corresponds to the stable member of the pair. In our opinion, all recent literature measurements of the free energy of bulk kaolinite are too negative by approximately 15 kJ mol(-1) for some unknown reason.

Colloidal Clay Gelation: Relevance to Current Oil Sands Operations

Abstract Ultrafines are predominantly delaminated colloidal clays with dimensions <0.3 μm that exist naturally in oil sands and are released during conditioning of surface-mined ores. Critical concentrations of these ultrafines and the cations present in process water are capable of forming flocculated structures with a very high water holding capacity. During primary separation of bitumen these ultrafines are detrimental to recovery as a result of increased slurry viscosity as well as through slime coating of released bitumen. Disposition into tailings ponds eventually produces mature fine tailings (MFT) as a result of thixotropic gel formation that entraps coarser solids. The ultrafines concentration of ~3 wt% observed in MFT coincides with the critical gelation concentration determined for suspensions of ultrafines in salt solutions with cationic concentrations representative of that in pond water. This observation accounts for 100% of the water holding capacity of MFT and also explains why virtually no water is released once an MFT gel state has been formed. Here, we review earlier research in this area and identify the harmful effects of ultrafines in some current problematic ores.

Powder X-ray Diffraction Determination of Phyllosilicate Mass and Area versus Particle Thickness Distributions for Clays from the Athabasca Oil Sands

Abstract Bitumen recovery from Athabasca oil sands is known to be affected by the clay component. This effect increases dramatically with decreasing particle sizes of the constituent clay minerals. The present work comprises a mineralogical study of distinct size-fractions of clays from the Athabasca oil sands. Mass and area versus thickness distributions per unit weight of phyllosilicate-mineral component are obtained by Bertaut analysis of measured XRD peak profiles for the first 00ℓ reflection. Distributions for both kaolinite and illite components from distinct size-fractions correlate with particle size. The powder pattern for the smallest size-fraction (< 0.1 μm) is shown to correspond to delaminated illite particles.

Geometrical parameterization of the crystal chemistry of P63/m apatite. II. Precision, accuracy and numerical stability of the crystal-chemical Rietveld refinement

A script developed for crystal-chemical Rietveld refinement of P63/m apatite with TOPAS is implemented in parallel with standard structure refinement. Least-squares standard uncertainty (s.u.) values for directly extracted crystalchemical parameters are nearly an order of magnitude lower than those obtained indirectly by analysis of atom coordinates derived by standard Rietveld refinement. This amazing finding originates partly in the reduction of the number of refinement parameters from 21 to 17 and partly in the fact that cell data now derive from crystal-chemical parameters instead of vice versa. Great precision and accuracy otherwise funneled into unit-cell parameters is then more distributed among mostly crystal-chemical distance parameters. The least-squares s.u. values are supported by analysis of numerous refinements of the same experimental data with added artificial intensity noise. Structural parameters from single-crystal results agree better with those extracted by crystal-chemical refinement. On the basis of singular value decomposition analyses performed using the program SVDdiagnostic [Mercier et al. (2006). J. Appl. Cryst. 39, 458‐465], crystal-chemical and standard Rietveld refinements are shown to have similar numerical stability. Crystal-chemical parameters extracted by direct Rietveld refinement, therefore, are more precise than, more accurate than and numerically as reliable as those derived from analysis of regular crystallographic refinement of the same data.

Ab initio exploration of layer slipping transformations in kaolinite up to 60 GPa

Abstract Kaolinite transformation into other kaolin layer polytypes under pressure is of interest in many materials or industrial applications as well as in seismology. Assuming that this transformation would involve a layer slipping mechanism similar to that of the reversible dickite to HP dickite transformation observed at 2 GPa, the authors undertake here an exploratory study of corresponding transformations. Neglecting contributions from third neighbour layers and beyond to differences in total energy, it is concluded that 19 stacking models of kaolin layers are prime candidates for the lowest enthalpy under moderate pressure. Ab initio compression of those models up to 60 GPa shows that, although several of the above models come close to kaolinite in enthalpy, kaolinite probably survives compression up to ∼ 12 GPa. Beyond this pressure, a new family of kaolin polytypes with lower enthalpy than kaolinite, resulting from a different layer slipping mechanism, is spontaneously produced by ab initio compression. The silicon coordination transforms gradually from tetrahedra to triangular dipyramids with no drastic change to layer architecture. Corresponding distinguishable transformations between adjacent layers resulting from this new coordination are translations -a/3 and (a+b)/3, which are not possible translations at zero pressure. Numerous low enthalpy new polytypes based on those translations are possible. Compression to 20 GPa of two kaolin polytypes among the 19 models created above have spontaneously resulted, one into the repeated (a+b)/3 polytype with symmetry Cm, and the other one into the repeated -a/3 polytype with symmetry P1. As both models derive from kaolinite by a layer slipping mechanism and have very similar enthalpies, both considerably lower than that of kaolinite, those phases and the polytypes from the same family are prime candidates for post-kaolinite phases beyond ∼ 12 GPa.

Ab initio constrained crystal-chemical Rietveld refinement of Ca10(VxP1 − xO4)6F2 apatites

Extraction of reliable bond distances and angles for Ca10(VxP1-xO4)6F2 apatites using standard Rietveld refinement with Cu Kalpha X-ray powder data was significantly impaired by large imprecision for the O-atom coordinates. An initial attempt to apply crystal-chemical Rietveld refinements to the same compounds was partly successful, and exposed the problematic determination of two oxygen-metal-oxygen angles. Ab initio modeling with VASP in space groups P6(3)/m, P2(1)/m and Pm showed that both these angular parameters exhibited a linear dependence with the vanadium content. Stable crystal-chemical Rietveld refinements in agreement with quantum results were obtained by fixing these angles at the values from ab initio simulations. Residuals were comparable with the less precise standard refinements. The larger vanadium ion is accommodated primarily by uniform expansion and rotation of BO4 tetrahedra combined with a rotation of the Ca-Ca-Ca triangular units. It is proposed that the reduction of symmetry for the vanadium end-member is necessary to avoid considerable departures from formal valences at the AII and B sites in P6(3)/m. The complementarity of quantum methods and structural analysis by powder diffraction in cases with problematic least-squares extraction of the crystal chemistry is discussed.

Upper limit of the tetrahedral rotation angle and factors affecting octahedral flattening in synthetic and natural 1M polytype C2/m space group micas

Abstract We have used recently developed quantitative crystal chemical models and a simple structural free-energy model to examine and interpret: (1) previously reported powder X-ray diffraction data for several trioctahedral mica solid solution series (64 synthetic powder samples between the Mg, Co, Ni, and Fe end-members, with different degrees of oxidation, vacancy contents, and Al/Si ratios; indexed as 1M polytype, space group C2/m; supplemented here by 57Fe Mössbauer spectroscopy to obtain accurate iron-site populations of IVFe3+, VIFe3+, and VIFe2+), and (2) 175 previously published single-crystal refinements comprising 138 natural and 37 synthetic 1M mica samples refined in space group C2/m. The crystal chemical models were validated by comparisons between predicted and measured relations between structural parameters, and needed model parameters and their uncertainties were extracted, using the single-crystal refinements. Two main results arise. First, an observed limit value of the b lattice parameter in certain synthetic solid solution series is shown to correspond to an upper limit value for the tetrahedral rotation angle α of αmax = 9.5° for AlSi3 tetrahedral sheets in K-rich micas. This upper limit is also clearly seen in the single-crystal refinement data for those Krich single-crystals that have near-AlSi3 tetrahedral compositions. We argue that the (tetrahedral sheet composition dependent) upper limit of tetrahedral rotation is an intrinsic property of the tetrahedral sheet (presumably corresponding to an intra-tetrahedral-sheet bond-bending limit) rather than arising either from interactions with the interlayer cations or from an octahedral sheet lateral-contraction limit. Second, we find that, except in the extreme cases where one approaches the lower (α = 0°) or upper (α = αmax) tetrahedral rotation limits, the magnitude of the octahedral flattening angle ψ is predominantly determined by octahedral cation stereo-chemical bonding requirements (and other intra-octahedral-sheet properties such as intra-sheet bond bending and intra-sheet electrostatic forces) rather than arising from tetrahedral-octahedral inter-sheet interactions (as generally argued or assumed). In addition, we corroborate a previously reported difference in the crystal chemical behaviors of trivalent octahedral cation (Fe3+, Al3+) and vacancy-bearing trioctahedral micas relative to samples that contain only divalent octahedral cations (e.g., Fe-Mg, Fe-Ni, Mg-Ni, and Co-Mg synthetic series); their b vs. average octahedral metal-oxygen bond-length behaviors are dramatically different, a result that is consistent with our proposed dominant stereo-chemical control of ψ.

Rational ab initio modeling for low energy hydrogen-bonded phyllosilicate polytypes

In a series of recent papers implementing ab initio DFT modeling with VASP, we have explored the kaolin system at zero pressure, kaolinite under pressure up to 60 GPa and the known kaolin phases under moderate pressure at 10 GPa. We summarize here the concepts, conclusions and falsifiable predictions printed in this series of papers, stressing independent and recent experimental results. A new rationalization of the kaolin system results, clarifying its stability diagram, its diagenesis and its solid-state phase transformations. The existence at moderate pressure of two new translations −a/3 and (a + b)/3, not possible at zero or low pressure, leading to five-fold coordination for Si was correctly predicted. Two newly and independently observed kaolinite polytypes (kaolinite II and III) were also correctly predicted. The existence of a still unobserved (SU) kaolinite IV phase is predicted at a pressure not higher than 60 GPa. Finally, transformations of dickite II into SU dickite III and nacrite into SU nacrite II are predicted to occur around 10 GPa. Optimized crystal structures predicted by ab initio modeling for the most likely low enthalpy polytypes are printed, which should simplify their identification when they will be observed, as they did for kaolinite II and III. Concepts developed here for kaolin minerals are generally applicable to hydrogen-bonded phyllosilicate polytypes or other hydrogen-bonded layered systems. This series of papers then implements a new way of expanding knowledge about experimentally difficult systems: inexpensive and relatively fast quantum computations can bring support to experimental results when conflicting reports exist in the literature, as well as produce predictions that are easily falsifiable experimentally, thus pointing to fruitful directions for new experiments. This loop of quantum computation followed by critical experiments results in faster and cheaper scientific progress as seen here on the kaolin system.