Fernando Colombo

Nomenclature of the hydrotalcite supergroup: natural layered double hydroxides

Abstract Layered double hydroxide (LDH) compounds are characterized by structures in which layers with a brucite-like structure carry a net positive charge, usually due to the partial substitution of trivalent octahedrally coordinated cations for divalent cations, giving a general layer formula [(M1-x2+Mx3+)(OH)2]x+. This positive charge is balanced by anions which are intercalated between the layers. Intercalated molecular water typically provides hydrogen bonding between the brucite layers. In addition to synthetic compounds, some of which have significant industrial applications, more than 40 mineral species conform to this description. Hydrotalcite, Mg6Al2(OH)16[CO3]·4H2O, as the longest-known example, is the archetype of this supergroup of minerals. We review the history, chemistry, crystal structure, polytypic variation and status of all hydrotalcite-supergroup species reported to date. The dominant divalent cations, M2+, that have been reported in hydrotalcite supergroup minerals are Mg, Ca, Mn, Fe, Ni, Cu and Zn; the dominant trivalent cations, M3+, are Al, Mn, Fe, Co and Ni. The most common intercalated anions are (CO3)2-, (SO4)2- and Cl-; and OH-, S2- and [Sb(OH)6]- have also been reported. Some species contain intercalated cationic or neutral complexes such as [Na(H2O)6]+ or [MgSO4]0. We define eight groups within the supergroup on the basis of a combination of criteria. These are (1) the hydrotalcite group, with M2+:M3+ = 3:1 (layer spacing ~7.8 Å); (2) the quintinite group, with M2+:M3+ = 2:1 (layer spacing ~7.8 Å); (3) the fougèrite group, with M2+ = Fe2+, M3+ = Fe3+ in a range of ratios, and with O2- replacing OH- in the brucite module to maintain charge balance (layer spacing ~7.8 Å); (4) the woodwardite group, with variable M2+:M3+ and interlayer [SO4]2-, leading to an expanded layer spacing of ~8.9 Å; (5) the cualstibite group, with interlayer [Sb(OH)6]- and a layer spacing of ~9.7 Å; (6) the glaucocerinite group, with interlayer [SO4]2- as in the woodwardite group, and with additional interlayer H2O molecules that further expand the layer spacing to ~11 Å; (7) the wermlandite group, with a layer spacing of ~11 Å, in which cationic complexes occur with anions between the brucite-like layers; and (8) the hydrocalumite group, with M2+ = Ca2+ and M3+ = Al, which contains brucite-like layers in which the Ca:Al ratio is 2:1 and the large cation, Ca2+, is coordinated to a seventh ligand of ‘interlayer’ water. The principal mineral status changes are as follows. (1) The names manasseite, sjögrenite and barbertonite are discredited; these minerals are the 2H polytypes of hydrotalcite, pyroaurite and stichtite, respectively. Cyanophyllite is discredited as it is the 1M polytype of cualstibite. (2) The mineral formerly described as fougèrite has been found to be an intimate intergrowth of two phases with distinct Fe2+:Fe3+ ratios. The phase with Fe2+:Fe3+ = 2:1 retains the name fougèrite; that with Fe2+:Fe3+ = 1:2 is defined as the new species trébeurdenite. (3) The new minerals omsite (IMA2012-025), Ni2Fe3+(OH)6[Sb(OH)6], and mössbauerite (IMA2012-049), Fe63+O4(OH)8[CO3]·3H2O, which are both in the hydrotalcite supergroup are included in the discussion. (4) Jamborite, carrboydite, zincaluminite, motukoreaite, natroglaucocerinite, brugnatellite and muskoxite are identified as questionable species which need further investigation in order to verify their structure and composition. (5) The ranges of compositions currently ascribed to motukoreaite and muskoxite may each represent more than one species. The same applies to the approved species hydrowoodwardite and hydrocalumite. (6) Several unnamed minerals have been reported which are likely to represent additional species within the supergroup. This report has been approved by the Commission on New Minerals, Nomenclature and Classification (CNMNC) of the International Mineralogical Association, voting proposal 12-B. We also propose a compact notation for identifying synthetic LDH phases, for use by chemists as a preferred alternative to the current widespread misuse of mineral names.

Galliskiite, Ca4Al2(PO4)2F8·5H2O, a new mineral from the Gigante granitic pegmatite, Córdoba province, Argentina

Abstract Galliskiite, ideally Ca4Al2(PO4)2F8·5H2O, is a new mineral found at the Gigante granitic pegmatite, Punilla department, Córdoba Province, Argentina. It is named for Argentine mineralogist and pegmatite specialist Miguel Ángel Galliski. Galliskiite is triclinic, P1̄, a = 6.1933(7), b = 9.871(1), c = 13.580(2) Å, α = 89.716(3), β = 75.303(4), γ = 88.683(4)°, Z = 2. The strongest lines in the X-ray powder diffraction pattern are [d in Å, (I)]: 7.904 (70), 5.994 (100), 3.280 (58), 3.113 (30), 2.945 (85), 2.887 (44), 2.483 (20), 2.262 (27), 2.150 (23), 1.821 (27), and 1.798 (25). It occurs as crude platy crystals elongated along [001] and flattened on {010}, with frosty surfaces. Simple contact and polysynthetic twinning on {100} by rotation about [010] is ubiquitous. It is colorless and transparent, has white streak and vitreous luster, and is nonfluorescent under ultraviolet radiation. It has a Mohs hardness of 2½, conchoidal to irregular fracture and two fair cleavages at approximately 90°. The measured density is 2.67(3) g/cm3, and the calculated density is 2.670 g/cm3. Galliskiite dissolves slowly in concentrated HCl. The mineral is biaxial (+), α = 1.493(1), β = 1.495(1), γ = 1.520(1), 2Vmeas = 33(5)°, 2Vcalc = 32°; dispersion, r < v; orientation Z ≈ b, X and Z at 40-50° from a and c. No pleochroism is observed. Analysis by electron microprobe (average of 12 analyses given in wt%) provided CaO 34.71, MgO 0.01, FeO 0.10, MnO 0.17, Al2O3 15.92, SiO2 0.06, TiO2 0.01, P2O5 21.94, F 21.35, H2O (calculated by stoichiometry) 15.08, less F≡O 8.99, total 100.39 wt%. The empirical formula, based on 21 (F+O), is (Ca3.98Mn0.02Fe0.01)Σ4.0Al2.01(P1.99Si0.01O8)F7.23(OH)0.77·5H2O. The crystal structure, solved and refined using single-crystal data to R1 = 0.033, consists of double chains of alternating corner-sharing AlF3O3 octahedra and PO4 tetrahedra along the a axis. The chains are joined into a framework via bonds to four distinct Ca atoms. Calcium atoms are also linked by sharing isolated F atoms and H2O molecules. The double-chain motif in the structure of galliskiite is distinct from that in any other known phosphate.

CRYPTIC DIAGENETIC CHANGES IN QUATERNARY ARAGONITIC SHELLS: A TEXTURAL, CRYSTALLOGRAPHIC, AND TRACE-ELEMENT STUDY ON AMIANTIS PURPURATA (BIVALVIA) FROM PATAGONIA, ARGENTINA

ABSTRACT Modern to Pleistocene Amiantis purpurata shells collected in Bahía San Antonio (Patagonia, Argentina) were studied by X-ray diffraction (XRD), optical and electron microscopy, electron microprobe analyses, and microindentation, in order to characterize early diagenetic changes and mechanical resistance. The sole crystalline phase is twinned aragonite showing pseudohexagonal symmetry. The regularity of the crystallographic texture decreases in older samples, but average crystallite size does not increase. The microstructure, which is dominantly crossed lamellar, is progressively replaced by a more randomly oriented grain aggregate. Compositional profiles across the shell show gradients in Sr, Na, S, and Cl, whereas Mg and P are more evenly distributed. Each shell layer has a distinct chemical signature. A marked decrease in the concentration of all of these elements, along with flattening of profiles, is evident as age increases. Vickers microhardness is lowest in modern specimens, showing at the same time the least chipped regions; older shells become harder and more fragile. All of these changes are attributed to postdepositional modifications by dissolution-recrystallization processes mediated by a thin film of water in a vadose environment. Microstructural adjustments are more sluggish than chemical modifications produced by diagenetic processes, whereas microhardness rapidly reaches high values, probably due to the early degradation of organic compounds from the shell. Our study shows that aragonitic shells that retain their primary mineralogical composition have undergone subtle chemical and microstructural changes. A very small amount of calcite was produced during grinding for XRD. Care should therefore be taken when seeking calcite as evidence of diagenetic changes.

Mafic rocks of the Ordovician Famatinian magmatic arc (NW Argentina): New insights into the mantle contribution

We studied the petrogenesis of mafic igneous rocks in the Famatinian arc in the western Sierra Famatina (NW Argentina), an Early Ordovician middle-crustal section in the proto-Andean margin of Gondwana. Mafic rock types consist of amphibolite, metagabbro, and gabbro, as well as pod- and dike-like bodies of gabbro to diorite composition. Field relations together with geochemical and isotopic data for the mafic rocks of the western Sierra de Famatina (at 29°S) define two contrasting suites, which can be correlated with similar assemblages noted in other parts of the orogen. Amphibolite, metagabbro, and gabbro bodies are mostly the oldest intrusive rocks (older than 480 Ma), with the host tonalite and post-tonalite mafic dikes being slightly younger. The older mafic suite is tholeiitic to calc-alkaline and isotopically evolved, except for most of the amphibolite samples. The younger suite is calc-alkaline, typically displaying subduction-related geochemical signatures, and it is isotopically more juvenile. Whole-rock chemical composition and isotopic analyses are compatible with a progressive mixing of different isotopic reservoirs. Pyroxenite (±garnet) was likely the dominant source of the older gabbroic magmas, whereas peridotite dominated in the source of the younger suite, implying that the mafic magma experienced a progressive shift toward more juvenile compositions though time (over 20 m.y.). Pyroxenite-derived melts could have been generated by lithospheric foundering followed by upwelling of primitive melts by adiabatic decompression of mantle wedge peridotite.

The crystal structure of cyanotrichite

Abstract We report the single-crystal average structure of cyanotrichite, Cu4Al2[SO4](OH)12(H2O)2, from the Maid of Sunshine mine, Arizona, USA. Cyanotrichite crystallizes in space group C2/m, with the unitcell parameters a = 12.625(3), b = 2.8950(6), c = 10.153(2) Å and β = 92.17(3)°. All non-hydrogen atoms were located and refined to R1 = 0.0394 for all 584 observed reflections [Fo > 4σFO] and 0.0424 for all 622 unique reflections. The cyanotrichite structure consists of a principal building unit of a three-wide [Cu2Al(OH)6] ribbon of edge-sharing Cu and Al polyhedra ‖ b, similar to that found for camerolaite. The ribbons lie in layers ‖ (001) and between these layers, while SO4 tetrahedra and H2O molecules form rods running ‖ b. A hydrogen-bonding scheme is also proposed. A sample of cyanotrichite from the Cap Garonne mine, Le Pradet, France, showed a 4b superstructure with the following unit cell: space group P2/m, a = 12.611(2) Å, b = 11.584(16) = 4 × 2.896(4) Å, c = 10.190(1) Å and β = 92.29(6)°. The supercell could not be refined in detail, but nevertheless imposes constraints on the local structure in that while the space-group symmetry prevents full order of SO4 and H2O in the 4b supercell, it requires that the sequence of species along any given rod is [-SO4-SO4-(H2O)2-(H2O)2-] rather than [-SO4-(H2O)2-SO4-(H2O)2-].

The crystal structure of sarmientite, (AsO4)(SO4)(OH)·5H2O, solved ab initio from laboratory powder diffraction data

Abstract The crystal structure of sarmientite, Fe3+2(AsO4)(SO4)(OH)·5H2O, from the type locality (Santa Elena mine, San Juan Province, Argentina), was solved and refined from in-house powder diffraction data (CuKα1,2 i radiation). It is monoclinic, space group P21/n, with unit-cell dimensions a = 6.5298(1), b = 18.5228(4), c = 9.6344(3) Å, β = 97.444(2)°, V = 1155.5(5) Å3, and Z = 4. The structure model was derived from cluster-based Patterson-function direct methods and refined by means of the Rietveld method to Rwp = 0.0733 (X2 = 2.20). The structure consists of pairs of octahedral-tetrahedral (Fe-As) chains at (y,z) = (0,0) and (½,½), running along a. There are two symmetry-independent octahedral Fe sites. The Fe1 octahedra share two corners with the neighbouring arsenate groups. Both individual chains are related by a symmetry centre and joined by two symmetry-related Fe2 octahedra. Each Fe2 octahedron shares three corners with double-chain polyhedra (O3, O4 with arsenate groups; the O8 hydroxyl group with the Fe1 octahedron) and one corner (O11) with the monodentate sulfate group. The coordination of the Fe2 octahedron is completed by two H2O molecules (O9 and O10). There is also a complex network of H bonds that connects polyhedra within and among chains. Raman and infrared spectra show that (SO4)2- tetrahedra are strongly distorted.

Gayite, a new dufrénite-group mineral from the Gigante granitic pegmatite, Córdoba province, Argentina

Abstract Gayite, ideally NaMn2+Fe3+5 (PO4)4(OH)6·2H2O, is a new member of the dufrénite group found at the Gigante granitic pegmatite, Punilla department, Córdoba province, Argentina. It is named for Hebe D. Gay (b. 1927), Professor Emeritus of Mineralogy of the National University of Córdoba (Argentina). The new mineral is monoclinic, space group C2/c, a = 25.975(3) Å, b = 5.1766(3) Å, c = 13.929(1) Å, β = 111.293(2)°, Z = 4. The strongest lines in the X-ray powder diffraction pattern are [d in Å, (I)]: 12.054 (33), 5.045 (60), 4.147 (37), 3.424 (71), 3.179 (100), 3.004 (33), 2.881 (42), 2.426 (36), 2.109 (39), 1.585 (50). It occurs associated with morinite, natrodufrénite, and quartz in cavities in massive apatite-(CaF), as clusters of tabular crystals up to 130 μm on edge dominated by {100}, with subordinate {201̄} and possibly also {110}, {111}, and {111̄}. Crystals display striations parallel to [010]. The mineral is greenish black with an olive green streak and vitreous luster. Thin tablets are transparent. Gayite is brittle, with perfect {100} cleavage and irregular fracture. Its Mohs hardness is 4 to 5. The measured density is 3.15(5) g/cm3, and the calculated density is 3.241 g/cm3. The mineral dissolves slowly in dilute HCl. Gayite is biaxial (+), α = 1.787(3), β = 1.792(3), γ = 1.806(3), 2Vmeas = 60(5)°, 2Vcalc = 62.1°; moderate dispersion, r < v; strong pleochroism, X (bluishgreen) >> Z (orange) > Y (yellow); orientation Y = b, X ^ a = 48° in obtuse β. Analysis by electron microprobe (average of 28 analyses given in wt%) provided TiO2 0.12, Al2O3 3.10, Fe2O3 41.95, MnO 5.97, MgO 0.08, CaO 0.23, ZnO 0.15, Na2O 3.03, P2O5 32.73, and H2O (calculated by stoichiometry) 10.31, total 97.67 wt%. The empirical formula, based on 24 O, is (Na0.85Ca0..02)Σ0.87(Mn2+0.74Fe2+0.12Mg0.02Zn0.02Ti4+0.01)Σ0.90(Fe3+4.47Al0.53)Σ5.00(P4.03O16)(OH)6·2H2O. The crystal structure (R1 = 6.10%) shows gayite to be a member of the dufrénite group, along with dufrénite, natrodufrénite, matioliite, and burangaite. The structure is a framework consisting of Fe3+O6 octahedra, Mn2+O6 octahedra, and PO4 tetrahedra with channels along the b axis containing Na atoms. The most unusual feature of the structure is an octahedral face-sharing Fe3+-Mn2+-Fe3+ trimer.

Carlhintzeite, Ca2AlF7·H2O, from the Gigante granitic pegmatite, Córdoba province, Argentina: description and crystal structure

Abstract Carlhintzeite, Ca2AlF7·H2O, has been found at the Gigante pegmatite, Punilla Department, Córdoba Province, Argentina. It occurs as colourless prismatic crystals up to 0.8 mm long, ubiquitously twinned on {001}. Electron microprobe analyses provided the empirical formula Ca1.98Al1.02F6.24(OH)0.76·H1.62O. A crystal fragment used for the collection of structure data provided the triclinic, C1̅ cell: a = 9.4227(4), b = 6.9670(5), c = 9.2671(7) Å, α = 90.974(6), β = 104.802(5), γ = 90.026(6)º, V = 558.08(7) Å3 and Z = 4. The crystal structure, solved by direct methods and refined to R1 = 0.0322 for 723 Fo > 4σF reflections, is made up of linkages of AlF6 octahedra, CaF8 polyhedra and CaF6(H2O)2 polyhedra. The AlF6 octahedra are isolated from one another, but share polyhedral elements with Ca polyhedra. Most notably, the Al1 octahedron shares trans faces with two CaF8 polyhedra and the Al2 octahedron shares trans edges with two CaF6(H2O)2 polyhedra. The linkage of the Ca polyhedra alone can be described as a framework in which edge-sharing chains along b are cross-linked by edge-sharing. Edge-sharing chains of Ca polyhedra along b in the carlhintzeite structure are similar to those along c in the structures of gearksutite, CaAlF4(OH)·(H2O), and prosopite, CaAl2F4(OH)4.

Increasing data completeness in synchrotron tts-microdiffraction experiments for δ-recycling phasing of low-symmetry compounds

Abstract Successful phasing of synchrotron through-the-substrate microdiffraction data by δ-recycling direct-methods largely depends on the number of missing intensities caused by the limited sample rotation range [J. Rius, Direct phasing from Patterson syntheses by δ recycling. Acta Cryst. A 2012, 68, 77–81]. Particularly, for the unfavorable triclinic system, dataset completeness resulting from a single series of consecutive ϕ-scans covering a total ϕ interval of ±35° is around 41%. This value is not enough for the routinary solution of a crystal structure by δ-recycling but can be increased by ~29% by applying the orthogonal χ strategy consisting of merging the information of two series of orthogonal ϕ-scans collected at the same microvolume of the polished thin section. Test calculations using simulated and experimental tts-data of the triclinic mineral axinite confirm that, with the help of the orthogonal χ strategy, crystal structures can be solved routinely. Since data in the ±35 ϕ-interval are normally accessible even for relatively thick glass-substrates (1–1.5 mm), a crystal structure can be determined from a single microvolume. For high-symmetry phases, due to the Laue symmetry redundancy, a single series of ϕ-scans normally suffices for the application of δ-recycling. However, when for experimental causes this series is incomplete, the orthogonal χ strategy also provides a simple way to increase the completeness which besides allowing solving the structure, is also beneficial for the subsequent refinement.