The role of beryllium in alloys, Zintl phases and intermetallic compounds

Although beryllium is widely used as alloying component in diverse light-weight alloys, the crystal chemistry of beryllium containing Zintl phases and intermetallic compounds is only scarcely developed and only few phase diagrams, mostly the industrially relevant ones, have been studied in detail. The present review summarizes the crystal chemical data of binary and ternary beryllium intermetallic compounds along with the results of the few documented physical property studies.

Redefinition of coquimbite, AlFe3+3(SO4)6(H2O)12⋅6H2O

Coquimbite, AlFe3+3(SO4)6(H2O)12⋅6H2O, was considered as a pure Fe3+ hydrated sulfate. However, previous mineralogical studies pointed out the occurrence of essential Al, occupying a distinct site in the crystal structure of this mineral. Through the critical re-examination of the available literature and new crystal-chemical data collected on a specimen from the Monte Arsiccio mine, Apuan Alps, Tuscany, Italy, the chemical formula of coquimbite has been revised, taking into account the occurrence of Al. Coquimbite has a homeotypic relationship with paracoquimbite, Fe4(SO4)6(H2O)12⋅6H2O; both mineral species belong to the coquimbite group. On the contrary, aluminocoquimbite, Al2Fe2(SO4)6(H2O)12⋅6H2O, has a different topology and does not belong to that group.

Spanish Bentonites: A Review and New Data on Their Geology, Mineralogy, and Crystal Chemistry

A review and a synthesis of the geological, mineralogical, and crystal chemical data available in the literature on active Spanish bentonitic exploitations were done, and at the same time, new data are provided from a set of representative samples from these deposits. They were located in three different areas with different geological origins: (1) Miocene sedimentary deposits from the Tajo Basin (Madrid–Toledo provinces) in the center of the Iberian Peninsula, where bentonites appear in two different units named for their colors (Green Clays and Pink Clays); (2) samples from Tamame de Sayago (Zamora province) originating from the hydrothermal alteration of granitic Variscan rocks; and 3) Miocene deposits originating from the hydrothermal alteration of volcanic or subvolcanic rocks from the Cabo de Gata volcanic area (Almería Province) in the southern part of Spain, where the three main deposits (Cortijo de Archidona, Los Trancos, and Morrón de Mateo) were studied. The bentonites from the Tajo Basin were formed mainly by trioctahedral smectites, and there were significant mineralogical differences between the Green and Pink Clays, both in terms of the contents of impurities and in terms of smectite crystallochemistry and crystallinity. The smectites from Tamame de Sayago were dioctahedral (montmorillonite–beidellite series), and they appeared with kaolinite, quartz, and mica in all possible proportions, from almost pure bentonite to kaolin. Finally, the compositions of the bentonites from the three studied deposits in Cabo de Gata were quite similar, and zeolites and plagioclases were the main impurities. The structural formulae of the smectites from Cortijo de Archidona and Los Trancos showed a continuous compositional variation in beidellite–montmorillonite, while in Morrón de Mateo, the smectites were mainly montmorillonite, although there was continuous compositional variation from Al montmorillonites to Fe–Mg-rich saponites. The variation in the smectite composition is due to the intrusion of a volcanic dome, which brings new fluids that alter the initial composition of the smectites.

Crystal Chemistry and Properties of Elpidite and Its Ag-Exchanged Forms

Elpidite from the Lovozero alkaline complex, Kola Peninsula, Russia, and Ag-exchanged forms of elpidite from two different localities (Lovozero and Khan Bogdo, Mongolia) were studied by means of single-crystal X-ray diffraction, electron microprobe analysis, thermogravimetry and IR spectroscopy. All studied samples retain the heteropolyhedral framework consisting of double Si6O15 chains (ribbons) and isolated ZrO6 octahedra. Zeolitic cavities in the initial elpidite from Lovozero (space group Pbm2, a = 14.6127(7), b = 7.3383(4), c = 7.1148(3) Å, V = 762.94(6) Å3) are occupied by Na+ cations and H2O molecules. Both Ag-exchanged forms are characterized by evident distortions of the heteropolyhedral framework and a strongly disordered arrangement of extra-framework cations which results in the appearance of the 14-14-14 Å unit cell (a = 14.1755(7), b = 14.6306(9), c = 14.2896(7) Å, V = 2963.6(3) Å3 for the Ag-exchanged form of elpidite from Lovozero and a = 14.1411(5), b = 14.5948(4), c = 14.3035(5) Å, V = 2952.04(17) Å3 for the Ag-exchanged form of elpidite from Khan Bogdo) and space group Cmce. Elpidite from both localities demonstrates a high exchange capacity to Ag. Exchanged Ag+ cations preferably occupy the sites that are close to the Na sites in the initial elpidite. The paper also contains a review of crystal chemical data on elpidite and its laboratory-modified forms.

Lead-antimony sulfosalts from Tuscany (Italy). XVIII. New data on the crystal-chemistry of boscardinite

Boscardinite, ideally TlPb4(Sb7As2)∑9S18, has been described recently as a new homeotypic derivative of baumhauerite, found at Monte Arsiccio mine, Apuan Alps, Tuscany, Italy. New findings of boscardinite in different mineral associations of this deposit have allowed the collection of new crystal-chemical data. Electron-microprobe analysis of the crystal used for the single-crystal X-ray diffraction study gave (in wt.%): Ag 1.81(5), Tl 12.60(21), Pb 17.99(12), Hg 0.14(5), As 9.36(12), Sb 33.60(27), S 23.41(30),Cl 0.06(1), total 98.97(100). On the basis of ∑Me= 14 apfu, it corresponds to Ag0.42Tl1.52Pb2.14Hg0.02(Sb6.82As3.08)∑9.90S18.04Cl0.04. With respect to the type specimen, these new findings are characterized by a strong Pb depletion, coupled with higher Tl contents, and a significant As enrichment. The single-crystal X-ray diffraction study of this (Tl,As)-enriched boscardinite confirms the structural features described for the type sample. The unit-cell parameters area= 8.1017(4),b= 8.6597(4),c= 22.5574(10) Å, α = 90.666(2), β = 97.242(2), γ = 90.850(2)°,V= 1569.63(12) Å3, space groupP̄1. The crystal structure was refined down toR1= 0.0285 on the basis of 6582 reflections withFo> 4σ(Fo). Arsenic is dominant in threeMeS3sites, compared to one in type boscardinite. The main As-enrichment is observed in the sartorite-type sub-layer. Owing to this chemical peculiarity, (Tl, As)-rich boscardinite shows alternation, alongb, of Sb-rich sites and As-rich sites; this feature represents the main factor controlling the 8 Å superstructure. The chemical variability of boscardinite is discussed; the Ag increase observed here gets closer to stoichiometric AgTl3Pb4(Sb14As6)∑20S36(Z= 1), against possible extension up to AgTl2Pb6(Sb15As4)∑19S36for type boscardinite.

New crystal-chemical data for marécottite

Marécottite, ideally Mg3[(UO2)4O3(OH)(SO4)2]2(H2O)28, a triclinic, Mg-dominant member of the zippeite group, was described originally from a small uranium deposit at La Creusaz in Wallis (Switzerland). It has recently been found at Jáchymov (Czech Republic), where it forms exceptional crystals, up to 0.3 mm across. According to an electron microprobe study of these crystals, marécottite from Jáchymov is chemically similar to the material from the La Creusaz deposit. However, the Jáchymov crystals exhibit more cation substitution (Zn2+ and Mn2+ for Mg2+). The chemical composition of marécottite from Jáchymov corresponds to the empirical formula [(Na0.05K0.07)Σ0.12(Mg1.83Zn0.41Mn0.41Cu0.15Ni0.08)Σ2.88Al0.07]Σ3.07(UO2)8[(SO4)3.77(SiO4)0.21]Σ3.98O6(OH)1.84·28H2O (the mean of four representative spots; calculated on the basis of eight U atoms and 28 H2O per formula unit and 1.84 OH for charge balance). According to single-crystal X-ray diffraction, marécottite from Jáchymov is triclinic, P1, a = 10.8084(2), b = 11.2519(3), c = 13.8465(3) Å, α = 66.222(2), β = 72.424(2), γ = 70.014(2)o, V = 1421.57(6) Å3 and Z = 1. The crystal structure was refined from a highly redundant dataset (30,491 collected reflections) to R1 = 0.0367 for all 7042 unique reflections. The refined structure confirms the previously determined structure for the crystal from the La Creusaz deposit. An extensive network of hydrogen bonds is an important feature that keeps the whole structure together, but the positions of H atoms had not been determined previously. The H-bond scheme proposed based on a detailed bond-valence analysis and the role of different types of molecular H2O in the structure is discussed.

Manganiferous minerals of the epidote group from the Archaean basement of West Greenland

The chemical compositions and crystal structures of Mn3+-containing minerals from the epidote group in Greenland rocks are investigated and described in detail. They occur in hydrothermally altered Archaean mafic sequences within the gneissic complex of the North Atlantic craton of West Greenland. The Mn-containing minerals have a characteristic red to pink colour. A detailed microchemical study shows a significant inter- and intra-sample variation in Mn content. The samples from different parageneses can be classified as Mn-bearing epidote and Mn-bearing clinozoisite. The intra-sample variation in the content of Al, Fe and Mn is on a very fine scale, but still allows for identification of a negative correlation between Mn and Fe. Textures indicate different stages of growth. Crystal chemical data are compared with literature data and illustrate the basic systematic differences between the influence of Fe and Mn on the crystal structure of the epidote group minerals.

Mineralogic evidences of a mid-Paleozoic tectono-thermal event in the Zonguldak terrane, northwest Turkey: implications for the dynamics of some Gondwana-derived terranes during the closure of the Rheic Ocean

The Zonguldak terrane is a Gondwana-derived continental microplate along the Black Sea coast in northwest Anatolia. It includes a Cadomian basement, with oceanic- and island-arc sequences, unconformably overlain by siliciclastic rocks of Ordovician to Middle Silurian age. After a period of deformation and erosion, late Lower Devonian (Emsian) quartzites and shallow-marine limestones unconformably cover Middle Silurian (Wenlock) graptolitic shales. Along several cross sections across the unconformity plane, the mineralogical characteristics of the Paleozoic sedimentary rocks in the Zonguldak terrane are studied to check whether this regional unconformity is only of epeirogenic nature or the result of a thermal event. In addition to the appearance of kaolinite in Devonian units, crystal-chemical data of illites show a sudden jump at the unconformity plane. The b cell dimension values of illites of Ordovician–Silurian units are somewhat higher than those of Devonian–Carboniferous units and show a drastic drop between the Silurian and Devonian units. The new mineralogic data indicate that the pre-Emsian rocks in the Zonguldak terrane experienced a thermodynamo event, prior to the Emsian transgression. This Caledonian-time event is also reported in east Moesian terrane but not noticed in the neighboring Istanbul–Zonguldak and in the west Moesian – Balkan – Kreishte terranes. By this, it is suggested that Zonguldak and east Moesian terranes behaved independently from the Istanbul–Balkan terranes during the closure of the Rheic Ocean. They very likely docked to Laurussia during Emsian by strike-slip faults and remained thereon at its platform margin, where the Middle–Late Devonian shallow-platform conditions were followed by fluvial (lagoon and delta) conditions and deposition of coal during Late Carboniferous.