The crystal structure of a magnesian cronstedtite-2H2 from Pribram, Czechoslovakia, was refined in space group C1 to a residual of 5.4% with 1832 independent reflections. Tetrahedral ordering between Fe3+ and Si is judged to be complete on the basis of electron density maps, the first confirmation of such ordering in a layer silicate. Octahedral cations are disordered on the M sites. Mean T-O bond lengths do not confirm the tetrahedral ordering, perhaps due to tetrahedral distortion in order to relieve the extreme corrugation of the sheet that would arise from ordering of such different size cations.

In the initial stages of refinement the structure of the crystal under study could not be described adequately on the basis of a single 2H2 polytype. The structure contains two kinds of 2H2 domains that appear shifted by b/3 relative to one another due to a mistake in the interlayer stacking sequence. This mistake of zero shift (the normal 2H2 polytype consists of alternating —b/3 and +b/3 shifts) affects only the tetrahedral sheets. It creates adjacent enantiomorphic domains of 2H2 packets such that domains with small Si tetrahedra sit above larger Fe3+-rich tetrahedra and vice versa. This mechanism to relieve strain may indeed be the cause of the stacking mistakes. A third kind of domain contains regions in which the sense of tetrahedral rotation is reversed, giving rise to split basal oxygens on electron density maps. This multiple domain model best explains extra atomic positions observed on Fourier maps, lack of streaking of k ≠ 3n reflections, and possible nonintegral “satellitic” reflections observed on Weissenberg films.

It is shown that polytypes or stacking sequences of cronstedtite, an Fe-bearing trioctahedral 1:1 phyllosilicate, can be determined using near-atomic high-resolution transmission electron microscopy (HRTEM). By viewing along the [010], [310] and directions (orthohexagonal indexing), the four groups of the standard polytypes can be distinguished. Imaging along the [100], [110] and directions allows determination of the polytypes in each group. The polytypic sequences of groups A and C are intergrown at the monolayer level in cronstedtite from Lostwithiel, England, which is a new insight if compared with previous suggestions that layer stackings characteristic of different groups do not occur together. The HRTEM images also revealed the relationship between the layer polarity and the morphology of the cronstedtite crystals, where the tetrahedral sheet side points towards the top of the truncated pyramidal shape of the crystal.

The crystal structure of cronstedtite-3T from Kutná Hora (Bohemia, Czechoslovakia), space group P31, was refined to Rw(all) = 3.1% for 1336 independent diffractions. There are two and three independent tetrahedral and octahedral positions, respectively, in this structure. The tetrahedra are occupied by 0.75 Si and 0.25 Fe while the octahedra are uniformly occupied by Fe. The refinement process was hindered by two problems: a “strong” superposition structure (all atoms of the octahedral sheets, i.e., ÷ 70% of the total diffraction power contribute almost solely to the family diffractions with mod(h–k, 3) = 0), and a slight disorder of the investigated crystal. The first problem was resolved by a preliminary block-diagonal refinement procedure where the atoms coinciding in the superposition structure were separated into individual blocks. The second problem was resolved by including two scale factors into the final full-matrix refinement: one for family diffractions, the other for the remaining ones which are characteristic for this polytype.

The crystal structure of cronstedfite-1T was refined in space group P31m, using two crystals: a triangular tabular crystal from Herja, Romania and a conical crystal from Lostwithiel, Cornwall, England. The Herja sample has the composition of (Fe2+2.20Fe3+0.80)(Si1.20Fe3+0.78Al0.02)O5(OH)4 and the Lostwithiel sample has the composition of (Fe2+2.32Fe3+0.68)(Si1.32Fe3+0.66Al0.02)O5(OH)4. The results of refinements are as follows: a = 5.512(1) Å, c = 7.106(1) A, R = 3.07%, and 342 independent reflections; and a = 5.503(1), c = 7.104(1) Å, R = 2.24%, and 335 independent reflections for the Herja and Lostwithiel samples, respectively. The structure consists of one tetrahedral and one octahedral sheet. There is one octahedral site, M1, occupied by Fe only, and one tetrahedral site, T1, occupied by Si and Fe in the ratio of 0.617(8):0.383 (Herja) and 0.699(6):0.301 (Lostwithiel). Positions of two hydrogen atoms were determined from a difference map for the Lostwithiel data. The ditrigonalization angle of the tetrahedral sheet is α = −11.5° (Herja) and α = −11.1° (Lostwithiel), and the structures have a Franzini-layer type of B. The crystals studied are affected by ± b/3 stacking faults which produced slight streaking of h - k ≠ 3n reflections.

The bonding in a cronstedtite layer was studied using a ninefold ordered supercell band structure calculation. The tight-binding scheme based upon the extended Hückel method was used to predict the electronic structure. The size of the problem was 162 atoms with 798 valence orbitals. The calculation showed different orbital interactions of oxygen p-orbitals with neighboring atoms with respect to the position in the layer. Substitution of Fe for Si in the terahedra reduced the role of the valence p-orbitals of the central Fe atoms. The d-orbitals of Fe were split in accordance with the rule of t2g-eg splitting. Although the density of states at the Fermi level was high, the partially filled 3d-bands were too narrow to permit normal metallic conduction.

The method for prediction of Gibbs free energies of formation, based on the parameter ΔGO=Mz+(clay) characterizing the oxygen affinity of the cation Mz+, on the smectites, considered as hydrated clay minerals, has been used for micas and brittle micas, and yielded underestimated values. This method of prediction can be improved by a new set of parameters ΔGO=Mz+(clay), characterizing the electronegativity of a cation in a specific site (interlayer, octahedral, tetrahedral in the 10 Å minerals), determined by minimizing the difference between experimental Gibbs free energies and calculated Gibbs free energies of formation from constituent oxides. By considering the crystal structure of 10 Å and 14 Å minerals, and assuming the same electronegativity of cations, ΔGO=Mz+(o), in the octahedral sheets, an attempt is made to determine the electronegativity of cations in the brucitic sheet, ΔGO=Mz+(b). The results indicate that this prediction method compared to other determinations, gives values within 0.25% of the experimentally-estim ated values. The relationships between ΔGO=Mz+(clay) corresponding to the electronegativity of a cation in the interlayer, octahedral, tetrahedral or brucitic sites and known ΔGO=Mz+(aq) were thus determined, allowing the determination of the electronegativity of transition metal ions and trivalent ions in each of the four sites and consequently contribute to the prediction of Gibbs free energies of formation of different micas and chlorites. Examples are given for low-Fe clinochlore whose solubility is measured experimentally and the results appear excellent when compared with experimental values.

The objective of this paper is to describe the new serpentine group mineral, guidottiite, which is analogous to cronstedtite. Guidottiite has an ideal chemical composition of (Mn2Fe3+)(SiFe3+)O5(OH)4. The sample is from the N’chwaning 2 mine, Kalahari manganese field, Republic of South Africa, and apparently forms from hydrothermal solutions. Grains are optically near opaque [average index of refraction 1.765, with variable extinction on the (001)], vitreous, and black, with perfect {001} platy cleavage. A non-separable fibrous substructure exists perpendicular to cleavage that results in a silky luster under optical examination. The average chemical analysis determined from electron microprobe based on four grains with ten analyses each resulted in a structural formula of (Mn1.86Fe0.613+Mg0.54)Σ=3.01(Si1.36Fe0.643+)Σ=2.00O5(OH)4, with calculated density of 3.236 g/cm3. Analysis from another area of the sample showed a slightly different chemical composition and resulted in a formula of (Mn1.70Fe0.963+Mg0.24Σ=2.89(Si1.26Fe0.743+)Σ=2.00O5(OH)4, with calculated density of 3.291 g/cm3. The measured density on a bulk sample (with impurities) was 3.33 g/cm3. Thermal analysis suggested a dehydroxylation temperature of 535°C, a decomposition/recrystallization temperature of 722°C, and weight loss (= H2O loss) of 9.4%. The derived Mohs hardness from nano-indentation is H = 4.25.

The sample is mostly the 2H1 polytype with minor amounts of the 2H2 polytype. Using a predominantly 2H2 crystal, which has better crystallinity, the strongest observed X-ray peaks are: 7.21 Å (Io/Io = 100%), 3.543 (50), 2.568 (39), 1.982 (26), and 2.381 (25). All Gandolfi simulations, even with three crystal remountings, showed preferred orientation effects. Transmission electron microscope (TEM) analysis showed stacking disorder within Group D serpentine polytypes. Thus, a regular alternation of the occupancy of octahedral sets within each layer along the stacking exists, but disorder of the layer displacements of 0 and ±b/3 (b defined here as the orthohexagonal cell) exists. Ordered 2H1 (no layer displacement) and 2H2 (alternating + and —b/3 displacement) domains were also frequently observed. X-ray diffraction analysis showed that even apparent single crystals contain impurity phases, presumably Mn-rich and Ca phases that were detected in the microprobe study. The single-crystal structure refinement used a well (stacking) ordered apparent 2H2 crystal with little to no streaking in the diffraction pattern. Results showed that the crystal has a random interstratification of 2H2 and 2H1. The 2H2 polytype is hexagonal, space group P63, with a = 5.5472(3), c = 14.293(2) Å, and Z = 2, and was refined to R1 = 0.072 and wR = 0.108 from 656 unique reflections. Because the two polytypes in the composite have only small differences in the lower 1:1 layer, a large displacement parameter for the basal oxygen atom results, which was constrained to B = 1.5 Å2 (Ueq = 0.0190) in the refinement. Half of the tetrahedral sites in the 2H1 upper layer superpose over half of the tetrahedral sites in the 2H2 upper layer (T1 sites only) per unit cell. This superposition produces an apparent excess of electron densities of the T1 site relative to the T2 site (T1 = 21.9 electrons, T2 = 15.8). Comparison with the microprobe data indicates that observed tetrahedral bond lengths are generally not affected by this intergrowth. Tetrahedral bond lengths indicated that the tetrahedral sites contain T1 = Si0.678 Fe0.3223+ and T2 = Si0.631Fe0.3693+. This excess of electron densities and other refinement problems associated with the guidottiite single-crystal refinement closely parallel all single-crystal cronstedtite-2H2 refinements to date, suggesting that these refinements also involve random interstratifications of 2H2 and 2H1 polytypes.

Cronstedtite is a member of the kaolin-serpentine group. It yields a wealth of more or less disordered polytypes. The crystals of polytype 1T (space group P31m, a = 5.512, c = 7.106 Å) contain, within coherently scattering blocks, variable concentrations of stacking faults so that domains of the basic 3D periodic structure can be shifted by 1/3 (a2 − a1) or 1/3(a1 − a2). These so-called OD parallel intergrowths have been confirmed by high-resolution transmission electron microscopy. The effect manifests itself in the diffraction pattern so that reflections with h-k = 3n — the family reflections — are always sharp, whereas remaining reflections — the characteristic polytype reflections — may be smeared out parallel to c*. The intensities of the latter are thus underestimated during diffractometer measurements. An analysis of such multiple OD intergrowths reveals that the moduli of structure factors for all characteristic (i.e. non-family) polytype reflections are reduced relative to those calculated for the non-intergrown basic structure, by a common factor. This fact usually leads to the appearance of ghost peaks in Fourier maps and to their erroneous interpretation. The structure of the basic model can, however, be refined much better if two scale factors are assigned to the family and non-family reflections, respectively.

The formation of 2:1 and 1:1 phyllosilicates in Fe–Si–O–H systems occurs in various geological and engineering settings; however, the identification and characterization of these minerals is very challenging due to the limited amount that is accessible, the very small particle size, and often the large degree of heterogeneity of these samples. To overcome these drawbacks, the synthesis of iron-rich phyllosilicates was attempted in this study with an initial Fe/Si molar ratio ranging from 0.50 to 2.33. The synthesis was performed at 150°C under hydrothermal conditions over a period of 7 days. Synthesis products were characterized by X-ray diffraction and fluorescence, by infrared, Raman, and Mössbauer spectroscopies, and by transmission electron microscopy. Results revealed that the stability field of the 2:1 clay mineral was wider than that of the 1:1 clay mineral. The 2:1 clay mineral was less sensitive to redox conditions compared to the 1:1 clay mineral. In addition, a heterogeneity of phases formed (iron oxides, nontronite, cronstedtite, or greenalite) was identified.

The present study deals with accurate identification of polytypes, twins, and allotwins – oriented crystal associations of more than one polytype. The trioctahedral 1:1 layered silicate cronstedtite was studied using single-crystal X-ray diffraction data collected with a four-circle diffractometer equipped with a CCD detector. The sample from the skarn occurrence, Ouedi Beht, El Hammam, Morocco, was explored. It contains cronstedtite in fibrous massive aggregates in the central part, and euhedral crystals in surrounding veinlets and druses. The reciprocal space (RS) sections created by the diffractometer software and presented here were used to determine the OD (ordered-disordered) subfamilies (Bailey’s group A, B, C, D) and to identify polytypes. The chemical compositions of some crystals were determined thereafter by electron probe microanalysis (EPMA-WDS). Some crystals studied are more or less complicated allotwins. Polytypes were thus separated by cleaving crystals into smaller parts in many cases. All polytypes found belong to subfamilies A or D. The following polytypes of the subfamily A were identified: 2M1 (a = 5.49, b = 9.51, c = 14.40 Å, β = 97.30°, space group Cc), 1M (a = 5.51, b = 9.54, c = 7.33 Å, β = 104.5°, Cm), 3T (a = 5.51, c = 21.32 Å, P31), 6T2 (a = 5.50, c = 42.60 Å, P31). 2M1 and 3T were present as isolated crystals or separated by cleaving, otherwise all these polytypes are parts of allotwins. The 2M1 polytype is sometimes twinned by reticular pseudo merohedry with twin index n = 3 and 120° rotation about the chex axis as the twin operation. Allotwins of 1M + twinned 2M1 polytypes are also present. Another kind of twinning, with rotation by (2n+1)×60° about chex is rare. The subfamily D is represented mostly by 2H1 and 2H2 polytypes, a = 5.50, c = 14.25 Å, space groups P63cm (2H1), P63 (2H2). In addition, several six-layer (a = 5.49, c = 42.80 Å), mostly non-MDO polytypes were separated from allotwins by cleaving. In order to identify them, 24 possible stacking sequences were modeled, diffraction patterns calculated, graphical identification diagrams constructed, and comparisons made with actual RS sections. This simulation revealed that five pairs of sequences provided identical diffraction patterns. Polytypes actually found correspond to the following sequences: 1 (6T1), 5 (proposed Ramsdell’s symbol 6T3), 8+10 (6T5), 11+12 (6T4), 24 (6T6, trigonal polytypes, space group type P3), 22 (6R1), and 23 (6R2, rhombohedral polytypes, space group type R3c and R3, respectively). The RS section corresponding to the hexagonal polytype 6H2 (sequence 14) was also found. However, diffraction patterns geometrically indistinguishable can be produced by the twin with rotation by 180° about chex as a twin operation of the rhombohedral polytype 6R2 (sequence 23). Several aggregates with fiber texture of polytype 2H2 were separated from the central part. Use of EPMA-WDS revealed Fe and Si along with significant amounts of Mn and Mg. Crystals from veins were more Mn- and Mg-rich than these from the central part. Traces of Cl, S, and Al are present also.

The present study presents a generalized procedure to accurately identify trioctahedral 1:1 layer silicates. The reciprocal space (RS) sections were obtained by the precession method by “unwarping” frames that were recorded using diffractometers with area detectors or from electron diffraction tomography (EDT) patterns. Distributions of subfamily reflections along the reciprocal lattice rows [21̄l]* / [11l]* / [1̄2l]* in (2hh̄lhex)* / (hhlhex)* / (h̄2hlhex)* RS planes were used to determine OD (Ordered — Disordered) subfamilies (Bailey’s groups A, B, C, D). The distributions along the [10l]* / [01l]* / [1̄1l]* rows in the (h0lhex)* / (0klhex)* / (h̄hlhex) RS planes allow determination of the polytypes. The use of traditional identification diagrams for the determination of OD subfamilies and polytypes was generalized in order to identify monoclinic and orthorhombic MDO (Maximum Degree of Order) polytypes. In these polytypes, the distribution of characteristic reflections along rows is different in RS sections that are perpendicular and diagonal to the symmetry plane of the polytype. The identification diagram of non-MDO polytype 6T2 is also presented. The method was applied to the identification of single crystals of cronstedtite that were synthesized during interactions of Fe metal with a natural claystone during experiments over a temperature range of 90-60°C. Conical and pyramidal crystals with a maximum size of a few micrometers were studied using electron diffraction tomography (EDT). The following polytypes were identified: 1M, 2M1, an apparently ninetuple polytype that was interpreted as triclinic 3A (group A), 1T (group C), and disordered crystals of group D. The 1M polytype was the most abundant. Some 1M crystals were twinned by reticular merohedry with a 120° rotation along the chex axis as the twin operation. The 2M1 occurred as isolated crystals as well as in mixed crystals. Intergrown 1T and 1M polytypes of the C and A group, respectively, were identified in one mixed crystal. The possible stacking sequence and the 3A polytype identification diagram was presented and discussed. Example RS sections of all polytypes were identified and demonstrated.

The present study provides an example of the accurate identification of polytypes of trioctahedral 1:1 layered silicates from single-crystal X-ray diffraction data collected with the aid of a four-circle diffractometer equipped with an area detector. Single crystals of the mineral cronstedtite from the Nagybörzsöny gold ore deposit, northern Hungary, were studied. The chemical composition of some crystals was determined by electron probe microanalysis (EPMA). The precession-like images of the reciprocal space (RS) sections created by the diffractometer software and presented in the study were used to determine the OD (ordered-disordered) subfamilies (Bailey’s groups A, B, C, D) and particular polytypes. With one exception, all crystals studied belong to subfamily A. The rare polytype 1M, a = 5.51, b = 9.54, c = 7.33 Å, β = 104.5°, space group Cm is relatively abundant in this occurrence. Another polytype 3T, a = 5.51, c = 21.32 Å, space group P31 was also found. Both polytypes occur separately or in mixed, mostly 1M dominant crystals. Some 1M polytype crystals are twinned by order 3 reticular merohedry with a 120° rotation along the chex axis as the twin operation. A rare 1M+3T mixed crystal with 1M part twinned also contains a small amount of subfamily C. A possible presence of the most common 1T polytype of this subfamily cannot be confirmed because of overlap of the characteristic reflections with those of 3T. Several completely disordered crystals produced diffuse streaks instead of discrete characteristic reflections on the RS sections. The EPMA revealed Fe, Si, traces of Mg, Al, S, and Cl. One black crystal originally considered to be cronstedtite was identified as (111) twinned sphalerite. Some crystals of cronstedtite are covered partially by a honey-brown crust or small crystals of siderite.

The crystal structure of cronstedtite-2H2 was refined in a hexagonal cell, space group P63, Z = 2, using two acicular crystals from Wheal Maudlin, Cornwall, England, and from Pribram, Czech Republic. The Wheal Maudlin sample has the chemical composition $\left({\text{Fe}}_{2.291}^{2+}{\text{Fe}}_{0.709}^{3+}\right)\left({\text{Si}}_{1.298}{\text{Fe}}_{0.707}^{3+}{\text{Al}}_{0.004}\right){\text{O}}_5{\left(\text{OH}\right)}_4$ and the Příbram sample has the composition $\left({\text{Fe}}_{2.269}^{2+}{\text{Fe}}_{0.731}^{3+}\right)\left({\text{Si}}_{1.271}{\text{Fe}}_{0.724}^{3+}{\text{Al}}_{0.005}\right){\text{O}}_5{\left(\text{OH}\right)}_4$. The results of refinements are as follows: a = 5.500(1), c = 14.163(2) Å, V = 371.08(8) Å3, R = 3.83%, from 381 independent reflections, and a = 5.4927(1), c = 14.1481(2) Å, V = 369.70(4) Å3, R = 4.77%, from 1088 independent reflections for the Wheal Maudlin and Příbram samples, respectively. The best Fovs.Fc agreement was achieved when the structure was interpreted as merohedral twin; several possible twinning laws are discussed. The cronstedtite layer consists of one tetrahedral sheet and one octahedral sheet. There is one octahedral (M1) position, occupied by Fe only, and two tetrahedral (T1, T2) positions in the structure. Refinement of occupancy of tetrahedral sites led to values Si:Fe = 0.45:0.55(1) (Wheal Maudlin) and 0.432:0.568(8) (Příbram) in T1, and Si: Fe = 0.99:0.01(1) (Wheal Maudlin) and 0.888:0.112(7) (Příbram) in 72. Whereas the size of T1 is reasonable (average dT1-O = 1.693 Å (Wheal Maudlin), 1.691 Å (Příbram)), T2 is unusually large: (dT2-O= 1.740 Å (Wheal Maudlin), 1.737 Å (Příbram)) with respect to the small or almost zero Fe content. As an explanation, an alternative structure model comprising a certain amount of vacancies in T2 is presented. The tetrahedral rotation angle α is highly positive (+12.1° and +12.5° for the Wheal Maudlin and Příbram samples, respectively), and the layer belongs to the Franzini type A. Distortion parameters of octahedra and tetrahedra are given for both samples. One hydrogen atom engaged in the hydrogen bond was located in the Wheal Maudlin sample.

The cooling of steel containers in radioactive-waste storage was simulated in a step-by-step experiment from 90 to 40ºC. Among newly formed clay minerals observed in run products, cronstedtite was identified by a number of analytical techniques (powder X-ray diffraction, transmission electron microscopy, and scanning electron microscopy). Cronstedtite has not previously been recognized to be so abundant and so well crystallized in an iron—clay interaction experiment. The supersaturation of experimental solutions with respect to cronstedtite was due to the availability of Fe and Si in solution, as a result of the dissolution of iron metal powder, quartz, and minor amounts of other silicates. Cronstedtite crystals are characterized by various morphologies: pyramidal (truncated or not) with a triangular base and conical with a rounded or hexagonal cross-section. The pyramidal crystals occur more frequently and their polytypes (2M1, 1M, 3T) were identified by selected area electron diffraction patterns and by automated diffraction tomography. Cronstedtite is stable within the 90-60ºC temperature range. At temperatures of ⩽ 50ºC, the cronstedite crystals showed evidence of alteration.

Cronstedtite from Lostwithiel, Cornwall, England, in which two polytypic groups (A and C) are present within the same crystal, has been investigated using various techniques to reveal the distribution of the two groups and the relationship between polytypic structure and chemical composition. X-ray precession photographs from cleaved fragments of pyramidal crystals revealed the variable proportions of the two groups from the top to the base. Near the top, the crystal consists entirely of group C, with 1T as the dominant polytype. Near the base, both groups A and C are present. Back-scattered electron images from cross-sections parallel to the pyramidal axis showed mosaic contrast near the base, suggesting that two compositionally-different domains of several tens of microns in size are present, whereas the contrast was uniform near the top. Electron microprobe chemical analysis indicated the compositions $\left({\text{Fe}}_{2.31}^{2+}{\text{Fe}}_{0.69}^{3+}\right)\left({\text{Si}}_{1.31}{\text{Fe}}_{0.69}^{3+}\right){\text{O}}_5{\left(\text{OH}\right)}_4$ and $\left({\text{Fe}}_{2.16}^{2+}{\text{Fe}}_{0.84}^{3+}\right)\left({\text{Si}}_{1.16}{\text{Fe}}_{0.84}^{3+}\right){\text{O}}_5{\left(\text{OH}\right)}_4$. Electron back-scattered patterns (EBSPs) confirmed that the domain with Si-rich composition belongs to group C and that with Si-poor composition to group A. This is the first evidence that specific chemical compositions are related to the polytypic structures in cronstedtite. Transmission electron microscopy revealed that intergrowth of groups A and C at the monolayer level is also present as stacking disorder in both domains near the base, whereas such intergrowth was not observed in the region near the top.

The problem to be solved is whether Al is a necessary component of Fe-Mg chlorites. Very unusual Al-depleted and Fe-enriched trioctahedral chlorites with the empirical formulae Na0.05Ca0.05(Fe2+3.01Mg2.01Ti0.14Fe3+0.04)Σ6.00[(Si3.53Fe3+0.41Al0.06)Σ4.00O10](OH)8·nH2O (Sample 1) and Na0.05Ca0.01(Fe2+3.26Mg1.97Fe3+0.75Mn0.01Ti0.01)Σ6.00[(Si3.16Fe3+0.75Al0.09)Σ4.00O10](OH)8 (Sample 2) have been discovered in Al-depleted dolomite carbonatites of the Kovdor complex of ultramafic, alkaline rocks and carbonatites, Kola Peninsula, Russia. The presence of substantial amounts of Ti in Sample 1 is another unusual feature of this mineral. In both samples, chlorites are intimately intergrown with cronstedtite-1T which is an indication of a low stability of chlorite structure in the absence of aluminum in the tetrahedral sheet. The crystal structure of chlorite in Sample 1 was solved by the Rietveld method. The mineral is triclinic (IIb-4-module), space group C-$1$, a = 5.4153(4), b = 9.3805(7), c = 14.5743(12) Å, α = 90.137(5)°, β = 96.928(5)°, γ = 90.043(6)°, V = 734.95(10) Å3, and Z = 2. A problem to be solved is how stable are Al-free chlorites belonging to the clinochlore–chamosite solid-solution series and whether their existence in natural mineral assemblages is possible. The results obtained indicate that even though Al-depleted chlorites belonging to the clinochlore–chamosite solid-solution series exist in Nature as metastable phases, these minerals are extremely rare and much less stable than Al-poor serpentines.

The formation conditions and stability fields of Fe-serpentines are still poorly understood in both terrestrial (natural or anthropic) and extraterrestrial environments. Knowledge of the effects of physical-chemical parameters on compositional and structural features of Fe-serpentines is lacking, and only a few thermodynamic parameters of these minerals are available in the literature. To fill these gaps, the synthesis of these minerals, while controlling all the physicochemical experimental parameters, was undertaken. Two hydrothermal syntheses were carried out at 90°C to investigate the effect of two different starting mineralogical mixtures on the nature of Fe-serpentines. The run products were identified by several analytical techniques (powder X-ray diffraction, transmission, and scanning electron microscopy). Berthierine crystals were synthesized from a starting mixture of kaolinite and metal iron. The berthierines synthesized show different morphologies and iron contents (~3–38 at.%). From a starting mineralogical mixture composed of quartz and metal iron, cronstedtite crystallizes. Most of the crystals are 1M polytypes. Magnetite is always associated with both berthierine and cronstedtite. Lepidocrocite was observed only in the experiment with kaolinite. These experimental results demonstrated that Fe enrichment in serpentines depends on the silicate precursor (kaolinite or quartz) of the starting mixture. The results are also in agreement with the geochemical equilibrium predicted by thermodynamic modeling, i.e. the formation of berthierine and cronstedtite in association with magnetite at the expense of metal iron and silicates.

Transmission electron microscope samples were prepared of ALH 78045 and ALH 88045, two clay-and phyllosilicate-bearing Antarctic meteorites, using argon ion milling and focused ion beam (FIB) techniques. ALH 78045 contains clay- and phyllosilicate-filled veins that have formed by terrestrial weathering of olivine, orthopyroxene and metal. Very narrow (∼10 nm) intragranular clay-filled veins could be observed in the TEM samples prepared by argon ion milling, whereas differential thinning and lack of precision in the location of the electron-transparent areas hindered the study of wider (5 — 15 μm) phyllosilicate-filled intergranular veins. Using the FIB instrument, electron-transparent slices were cut from specific parts of the wider veins and lifted out for TEM study. Results show that these veins are occluded by cronstedtite, a mixed-valence Fe-rich phyllosilicate. This discovery shows that silicates can be both dissolved and precipitated during terrestrial weathering within the Antarctic ice. ALH 88045 is one of a small number of known CM1 carbonaceous chondrites. This meteorite is largely composed of flattened ellipsoidal aggregates of serpentine-group phyllosilicates. To determine the mineralogy and texture of phyllosilicates within specific aggregates, TEM samples were prepared by trenching into the cut edge of a sample using the FIB instrument. Results show that Mg-rich aggregates are composed of lath-shaped serpentine crystals with a ∼0.73 nm basal spacing, which is typical of the products of low temperature aqueous alteration within asteroidal parent bodies. Results of this work demonstrate that the FIB has enormous potential in a number of areas of Earth and planetary science.