Crystal Chemical Study Research Articles

The crystal chemistry and crystal structure of kuksite, Pb3Zn3Te6+P2O14, and a note on the crystal structure of yafsoanite, (Ca,Pb)3Zn(TeO6)2

New discoveries of kuksite, Pb_3Zn_3Te^(6+)P_2O_(14), from the Black Pine mine, Montana, and Blue Bell claims, California, have enabled a detailed crystal-chemical study of the mineral to be undertaken. Single-crystal X-ray structure refinements of the structure indicate that it is isostructural with dugganite, Pb_3Zn_3Te^(6+)As_2O_(14), and joelbruggerite, Pb_3Zn_3(Sb^(5+),Te^(6+))As_2O_(13)(OH,O). Kuksite from the Black Pine mine crystallizes in space group P321, with unit-cell dimensions a = 8.392(1), c = 5.204(1) A, V = 317.39(8) A^3, and Z = 1 (R_1 = 2.91% for 588 reflections [F_o > 4σF] and 3.27% for all 624 reflections), while Blue Bell kuksite has the unit cell a = 8.3942(5), c = 5.1847(4) A, and V = 316.38(4) A^3 (R_1 = 3.33% for 443 reflections [F_o > 4σF] and 3.73% for all 483 reflections). Chemical analyses indicate that solid-solution series exist between kuksite, dugganite, and joelbruggerite. Raman spectroscopic and powder X-ray diffraction data are also presented for samples from both occurrences. The crystal structure of the chemically related species yafsoanite, (Ca,Pb)_3Te_2^(6+)Zn_3O_(12), from the type locality (Delbe orebody, Kuranakh Au Deposit, Aldan Shield, Saha Republic, Russia), has been refined to R_1 = 2.41% for 135 reflections [F_o > 4σF] and 3.68% for all 193 reflections. A garnet-type structure has been confirmed and significantly improves upon the results of an earlier structure determination.

ChemInform Abstract: A Comparative Crystal Chemical Analysis of Ba2CoO4 and BaCoO3.

Abstract Polycrystalline Ba2CoO4 has been synthesized by the ceramic method. X-ray and electron diffraction shows this phase to be monoclinic with a=5.8878(4); b=7.6158(6); c=10.3916(8); β=90.738(2). This compound is isostructural to Ba2TiO4 and the structure has been refined from X-ray powder diffraction data by using the Rietveld method. High resolution images interpreted in comparison to simulated images confirm the structure determined by X-ray diffraction. A crystal chemical study based on the cation arrays allows to establish a new relationship between Ba2CoO4 and 2H BaCoO3. Moreover, the Ba subarray seems to be related to the structure of elemental Ba.

Synthetic dimetaboratesREEAl1.67+0.67x(B4O10)Ox(x ≤ 1) (REE= La, Ce, Nd, Pm, Sm, Eu): crystal-chemical study and comparison with natural counterpart peprossiite-(Ce)

Synthetic dimetaborates<i>REE</i>Al_{1.67+0.67<i>x</i>}(B₄O₁₀)O_<i>x</i>(<i>x</i> ≤ 1) (<i>REE</i>= La, Ce, Nd, Pm, Sm, Eu): crystal-chemical study and comparison with natural counterpart peprossiite-(Ce)

Crystal chemistry of clinopyroxenes from upper-mantle xenolith series in the BalatonBakony volcanic area (CarpathianPannonian region, Hungary)

A detailed crystal chemical study of clinopyroxenes (cpx) from a suite of mantle xenoliths of the Balaton-Bakony volcanic highland (Carpathian-Pannonian region (CPR), Hungary) has been undertaken by means of X-ray single-crystal diffraction and electron microprobe. The study aims to evaluate the response of cpx crystal chemistry to textural changes and increasing deformation, and to estimate equilibrium pressure conditions of texturally heterogeneous spinel-peridotite xenolith series. Moreover, our interest also focussed on cpx crystal chemistry of mosaic and poikilitic textured xenoliths. Results demonstrate that cpx from protogranular, porphyroclastic and some equigranular xenoliths define the ''primary'' compositional trend that reflects structural and textural changes. This cpx-trend also suggests that deformation increase correlates with pressure (and temperature) decrease. Protogranular xenoliths record pressure near to the garnet stability field whereas equigranular xenoliths record lower pressure approaching the plagioclase stability field. Some cpx from equigranular samples show a crystal chemistry similar to that of cpx from poikilitic and mosaic xenoliths which depart from the ''primary'' trend. This suggests that the latter cpx are likely formed at low pressure near to or within the plagioclase stability field. The inverse relationship between pressure and deformation in the main series can be explained geodynamically by the presence of a mantle diapir beneath the region, which could have caused significant deformation and lithosphere thinning in the centre of the CPR.

Compositional Variations, Zoning Types and Petrogenetic Implications of Low-pressure Clinopyroxenes in the Neogene Alkaline Volcanic Rocks of Northeastern Turkey

Clinopyroxene phenocrysts and microphenocrysts in different series of the Neogene alkaline volcanic rocks from the eastern Pontides (NE Turkey) record various stages in the crystallization conditions and evolution history of the alkaline melt as well as its origin. Crystal chemical studies reveal that the clinopyroxenes in each rock series show strong textural and compositional similarities, which all reflect a common petrogenetic affinity. They have relatively high Mg-numbers (0.68-0.95), variable Al_2O_3 (1.3-9.6 wt%), low TiO_2 (

Lead-free piezoelectric ceramics derived from the K0.5Na0.5NbO3–AgNbO3 solid solution system

A lead-free solid solution between potassium sodium niobate and silver niobate, (1−x)K0.5Na0.5NbO3–xAgNbO3 (KNN-AN, with x=0–0.36), has been prepared in the form of ceramics by solid state reaction under O2 atmosphere. The crystal chemical study shows that Ag+ ion diffuses into the KNN lattices to form a solid solution of perovskite structure with x up to 0.30. The substitution of Ag+ ion for (K0.5Na0.5)+ ion results in linear decreases in the ferroelectric Curie temperature (TC), from 420°C in KNN down to 325°C in 0.70KNN-0.30AN, and in the tetragonal to orthorhombic phase transition temperature (TT-O), from 190 down to 150°C. The relative density of the KNN-AN ceramics reaches 94%. The optimum piezoelectric properties are found in the ceramics of 0.82KNN-0.18AN, with a piezocoefficient d33=186pC∕N, an electromechanical coupling factor kp=42.5%, and a TC=355°C. For the ceramics with x=0.24, the remanent polarization reaches a maximum value of Pr=20μC∕cm2, with a reduced coercive field Ec=6.4kV∕cm and a TC=340°C. It is shown that the (1−x)KNN-xAN ceramics exhibit piezoelectric properties which are comparable or superior to the hot-pressed KNN ceramics and other KNN-based systems. In particular, the piezoelectric properties exhibit very a good thermal stability up to high temperature, which is a significant improvement from other KNN-based piezoceramics.

Crystal-chemical study of Rc natural oxides along the eskolaite – karelianite – hematite (Cr2O3–V2O3–Fe2O3) join

AbstractSix natural crystals from the Sludyanka crystalline complex belonging to the eskolaite (Cr2O3)–karelianite (V2O3)–hematite (Fe2O3) solid solution were studied by means of X-ray diffraction and electron microprobe. The Fe3+-poor samples show a general increase in a and c cell parameters with increasing mean cationic radius (MCR), consistent with that shown by the synthetic crystals along the eskolaite–karelianite join. The Fe3+-richer sample deviates significantly from the behaviour shown by the Fe3+-poor ones, similar to synthetic and natural hematites; with increasing MCR, the a and c cell parameters increase linearly along the eskolaite-karelianite join. However, for the samples rich in Fe3+, from karelianite to hematite, a shows a slightly steeper slope whereas the c parameter decreases strongly. The octahedral distortion increases slightly as a function of MCR along the eskolaite-karelianite join, whereas it increases markedly for Fe3+-rich samples. The evolution of the octahedral edges and of the octahedral distortions as a function of MCR are responsible for the behaviour of the unit-cell parameters along the eskolaite-karelianite-hematite join.

Single-crystal X-ray studies of trioctahedral micas coexisting with dioctahedral micas in metamorphic sequences from western Maine

A crystal-chemical study of thirteen biotite (twelve of 1 M polytype and one of 2 M 1 polytype) and four muscovite samples was made. The biotite coexists with the muscovite. Samples are from metamorphic terranes and from granitic and granodioritic bodies occurring in three areas of western Maine. The metamorphic mineral zones identified by mineral compatibilities are, in order of increasing metamorphic grade: the Lower Sillimanite Zone (LSZ), the Upper Sillimanite Zone (USZ), and the K-feldspar + Sillimanite Zone (K + SZ). The muscovite compositions cluster near ideal muscovite and display a small celadonite substitution and a small, but variable, paragonite substitution. The biotite composition has values of VIMg2+/VI(Mg2+ + Fe2+) ranging from 0.26 to 0.54 and significant octahedral Al content (0.48 ≤ VIAl ≤ 0.72 apfu in metamorphic biotite samples, 0.51 ≤ VIAl ≤ 0.67 in those from granites). In trioctahedral micas from western Maine and especially in those with graphite, there are a greater number of interlayer vacancies than in common micas. Interlayer vacancies have an increase in interlayer cation-basal oxygen atom distances and a decrease in tetrahedral flattening angle τ, thus suggesting a reduced interlayer charge. With a few exceptions, tetrahedral rotation angle α is related to crystallization temperature. In particular, α decreases with a temperature increase, and α is also related to octahedral chemical substitutions. Results tentatively suggest, for micas from metamorphic environments, a direct influence of genetic parameters ( T and f O2) on mica crystal structure, and not just chemical composition.

New data on the crystal structures of natural zirconosilicates: structure refinements and ion-exchange behavior

A review of the mineralogical, structural and crystal-chemical studies carried out on a large group of natural zirconosilicates is reported. These zirconosilicates belong to a large number of distinct structural types, and those with mixed frameworks of isolated Zr-centred octahedra and silicate complexes are described according to the degree of condensation of SiO4 tetrahedra.The microporous heteropolyhedral nature of these phases is considered in connection with their ion exchange properties. The application of X-ray diffractometers equipped with CCD detectors, use of the Rietveld method, and electron microprobe and IR spectroscopic data provide new insights into the structural peculiarities of these minerals. The results stimulate the search for new Zr—Si-compounds with potential applications in the areas of catalysis, separation of molecules, ion exchange, etc.

Crystal structure of dimethylgold(III) 8-hydroxyquinolinate and 8-mercaptoquinolinate

A crystal-chemical study of dimethylgold(III) complexes with 8-hydroxyquinoline (CH3)2Au(OR) and 8-mercaptoquinoline (CH3)2Au(SR) (R = C9H6N) was performed. Crystal data for (CH3)2Au(OR): a = 8.7133(17) A, b = 27.875(6) A, c = 8.6688(17) A, β = 102.76(3)°, Z = 8, ρ(calc) = 2.401 g/cm3, space group P21/c, R = 0.0909; for (CH3)2Au(SR): a = 3.5401(7) A, b = 15.689(3) A, c = 19.910(4) A, β = 99.81(3)°, Z = 4, ρ(calc) = 2.361 g/cm3, space group P21/c, R = 0.0712. Both structures are molecular and involve neutral (CH3)2Au(L) molecules, L = C9H6NO or C9H6NS. In the structures, the molecules are arranged in stacks joined by van der Waals interactions. The average Au…Au intrastack distances are 3.57 A and 4.34 A for (CH3)2Au(OR) and 3.5 A for (CH3)2Au(SR).

Clay mineralogy and chemistry of halloysite and alunite deposits in the Turplu area, Balikesir, Turkey

AbstractA field-mapping and crystal-chemical study of two alunite- and halloysite-rich deposits in the Turplu area, situated northwest of Balıkesir on the Biga Peninsula of northwest Turkey reveals a mineralogically diverse and a potentially economic clay deposit. The mineral assemblage along fault zones is dominated by halloysite and sometimes alunite. The alunite is nearly end-member in composition (a = 6.995 Å, c = 17.195 Å) often occurring with a minor Ca phosphate phase. Of the two deposits studied, the more northerly mine contains more alunite relative to halloysite. Geochemical alteration indices suggest that the northern mine has experienced a slightly greater degree of hydrothermal modification. Halloysite is found in both hydrated and dehydrated states and assumes a tubular morphology. Observations by transmission and scanning electron microscopy are consistent with a model of halloysite dehydration, where the shapes transform from an open-hole tubular morphology to a closed-hole unfurled morphology.Mineral paragenesis includes the effects of initial deposition of volcanic tuffs and andesite on top of karstic terrain. The contact between altered volcanics and underlying limestones is irregular and appears to have provided a mechanism to flush both hydrothermal and meteoric waters through the volcanics. Periods of hydrothermal alteration (hypogene) contemporaneous with extensional and strike-slip faulting have resulted in alunite and halloysite deposits. Hydrothermal alteration is concentrated near the fault zones. Because of subsequent weathering (supergene) away from the fault zones, much of the andesitic volcanic rocks have been altered to a more smectite-rich and kaolinite-bearing assemblage. The deposits continue to be both plastically deformed in the alunite/halloysite regions and to undergo brittle deformation in the saprolitized volcanics. Tectonic deformation has mixed the contacts, such that limestone blocks are entrained into parts of the alteration zones. Gibbsite and gypsum are common weathering products associated with limestone block inclusions. Genetic models for the origins of alunite-halloysite deposits in NW Turkey should consider as possible influencing factors the underlying lithologies, the extent of hydrothermal alteration, and recent weathering by meteoric fluids. In the case of the Turplu deposits, karstic limestones, hydrothermal circulation of sulfate-rich waters, and a post-alteration history of meteoric weathering were all important factors in their formation.

Third update on compounds with krohnkite-type chains: the crystal structure of wendwilsonite [Ca2Mg(AsO4)22H2O] and the new triclinic structure types of synthetic AgSc(CrO4)22H2O and M2Cu(Cr2O7)22H2O (M = Rb, Cs)

As part of a comprehensive crystal-chemical study and classification of minerals and synthetic compounds with krohnkite-type or -like chains of octahedra bridged by tetrahedra, the crystal structures of wendwilsonite from Schneeberg, Saxony, Germany, with composition Ca 2 (Mg 0.90 ,Co 0.10 )(AsO 4 ) 2 ·2H 2 O, and of three new synthetic double-metal dichromate(VI) dihydrates have been determined from single-crystal X-ray diffraction data. Wendwilsonite is monoclinic [space group P 2 1 / c (no. 14), a = 5.806(1), b = 12.923(3), c = 5.628(1) A β = 107.49(3)°, V = 402.75(14) A 3 , Z = 2; R(F) = 1.42%] and adopts type D in the classification of Fleck et al. (2002b); it is isotypic with the three arsenates brandtite, roselite, zincroselite, and the sulphate krohnkite. Infinite krohnkite-type tetrahedral-octahedral chains [(Mg,Co)(AsO 4 ) 2 (H 2 O) 2 ] 2- are separated by Ca 2+ cations. A very strong hydrogen bond (O…O = 2.5294(18)A) links adjacent chains. Hydrothermally synthesised, orange AgSc(CrO 4 ) 2 ·2H 2 O is triclinic [space group P 1 (no. 2), a = 5.586(1), b = 6.054(1), c = 7.432(1)A, α = 110.74(3), β = 90.21(3), γ = 117.34(3)°, V = 204.56(6) A 3 , Z = 1; R (F) = 3.48%] and represents a novel structure type, designated G, among compounds containing krohnkite-type chains. The structure is metrically pseudo-monoclinic and closely related to those of the monoclinic subtypes F1 ( C 2/ c ) and F2 ( C 2/ m ). AgSc(CrO 4 ) 2 ·2H 2 O also represents the first Ag and Sc compound based on a krohnkite-type chain. The structure is based on infinite tetrahedral-octahedral chains [Sc(CrO 4 ) 2 (H 2 O) 2 ] 2- separated by Ag 1+ cations. The Ag atom exhibits a [4+4]-coordination, with the AgO 8 polyhedron closely resembling a strongly sheared cube. Rb 2 Cu(Cr 2 O 7 ) 2 ·2H 2 O and isotypic Cs 2 Cu(Cr 2 O 7 ) 2 ·2H 2 O were both grown at room temperature as orange blades from slightly acidic aqueous solutions. They represent a new, triclinic structure type containing krohnkite-like octahedral-tetrahedral chains based on bridging Cr 2 O 7 groups [space group P 1 (no. 2), a = 7.272(1) / 7.462(2), b = 7.648(2) / 7.819(2), c = 7.772(2) / 7.847(2)A, α = 91.89(3) / 93.19(3), β = 99.72(3) / 99.57(3), γ = 115.79(3) /116.34(3)°, V = 380.84(15) / 400.23(15) A 3 , Z = 1; R(F) = 3.03 / 2.99%]. The Cu atoms show a Jahn-Teller-distorted ’octahedral’ coordination environment, approaching an unusual [2+2+2]-coordination. Hydrogen bonding is weak. A distinct positive correlation between the size of the alkali cation and the Cr-O-Cr angle in the few known alkali metal dichromates with krohnkite-like chains is demonstrated.

Atomic Resolution Crystal Structures, EXAFS, and Quantum Chemical Studies of Rusticyanin and Its Two Mutants Provide Insight into Its Unusual Properties,

Rusticyanin from the extremophile Thiobacillus ferrooxidans is a blue copper protein with unusually high redox potential and acid stability. We present the crystal structures of native rusticyanin and of its Cu site mutant His143Met at 1.27 and 1.10 A, respectively. The very high resolution of these structures allows a direct comparison with EXAFS data and with quantum chemical models of the oxidized and reduced forms of the proteins, based upon both isolated and embedded clusters and density functional theory (DFT) methods. We further predict the structure of the Cu(II) form of the His143Met mutant which has been experimentally inaccessible due to its very high redox potential. We also present metrical EXAFS data and quantum chemical calculations for the oxidized and reduced states of the Met148Gln mutant, this protein having the lowest redox potential of all currently characterized mutants of rusticyanin. These data offer new insights into the structural factors which affect the redox potential in this important class of proteins. Calculations successfully predict the structure and the order of redox potentials for the three proteins. The calculated redox potential of H143M ( approximately 400 mV greater than native rusticyanin) is consistent with the failure of readily available chemical oxidants to restore a Cu(II) species of this mutant. The structural and energetic effects of mutating the equatorial cysteine to serine, yet to be studied experimentally, are predicted to be considerable by our calculations.

Vapor pressure and crystal structure of platinum(II) trans-bis-(trifluoroketoiminate)

The temperature dependence of the saturated vapor pressure of the trans-Pt(ktf)2 complex obtained from fluorinated β-ketoimine (CF3-CO-CH2-C(NH)-CH3) was studied by the flow method. The standard thermodynamic parameters ΔH 0 T and ΔS 0 T of sublimation have been determined. Full crystal-chemical study has been performed for the complex. Crystal data for C10H10F6N2O2Pt: a = 5.9790(8) A, b = 7.373(2) A, c = 8.5767(2) A,α = 84.05(2)°, β = 72.43(1)°, γ = 67.14(1)°, V = 332.1(1) A3, Z = 1, d calc = 2.496 g/cm3, triclinic, space group $$P\bar 1$$ . The structure is molecular and consists of isolated trans-Pt(ktf)2 complexes. The Pt atom lies at the symmetry center and has a square planar environment of two oxygen and two nitrogen atoms; the distances Pt-O (1.984 A) and Pt-N (1.969 A) are similar within the limits of 2σ; the OPtN bond angle is 93.9°. Molecular packing in crystal is considered based on structural data; van der Waals energy of the crystal lattice of trans-Pt(ktf)2 was calculated by the atom-atom potential method.

The 2M1 dioctahedral mica polytype: A crystal chemical study

AbstractThe structure of dioctahedral true micas such as muscovite and celadonitic muscovite (2M1 polytype, space group C2/c) is mostly affected by variations of the octahedral Al (VIAl) content. Crystals with greater Mg, Fe substitutions (i.e. celadonitic muscovite) reduce the dimensional difference between the larger trans-oriented M1 site and smaller cis-oriented M2 octahedral site. The octahedral anionic position O4 is displaced from the center of the hexagon, defined by 031 and 032 oxygen atoms (i.e. ‘octahedral hexagon’), both on and off the (001) plane. The distance between interlayer cation A and O4 is smaller in more substituted species, thus providing different orientations of the O4−H vector, as a function of VIAl. Octahedral distances (&lt;M2−O3&gt; and &lt;M2−O4&gt; are expressed as a function of cell parameters and VIA1 content, thus allowing an approximate estimate of site dimensions. These approximations are useful when a detailed structural refinement is not available. In celadonitic muscovite, the octahedral hexagon mean edge (&lt;O31−O32&gt;Hex) is not significantly affected by VIA1 content. The VIA1 increase produces both a decrease in cell lateral dimensions and a distorted ‘octahedral hexagon’. The decrease in a and b is consistent with a decrease of &lt;O31−O32&gt;Hex, whereas the distortion of the’ octahedral hexagon’ is consistent with an increase of (&lt;031–032&gt;Hex), because an irregular hexagon produces a longer mean edge than a regular hexagon of equal area.The tetrahedral mean basal edge (VI&lt;O−O&gt;bassal) is reduced as celadonitic substitution progresses. The tetrahedral rotation angle, α was thus found to increase from celadonite to muscovite. However, in muscovite with VIAl content between 1.8 and 2.0 atoms per formula unit (a.p.f.u.), α approaches a saturation value, thus showing a proportional increase of tetrahedral and octahedral sheet lateral dimensions. Furthermore, α variation allows a coarse approximation of the threshold VIAl content, below which celadonitic substitution may not progress.

Magnetic Semiconducting Chalcogenide Spinels: Preparation and Physical Chemistry

Systematic crystal-chemical studies of chalcogenide spinels and related phases are used to develop a novel, unified approach to analyzing the nonstoichiometry of close-packed phases in the series atacamite–spinel–halite with allowance for alternatively occupied tetrahedral and octahedral nonatacamite sites. The main physicochemical approaches to the synthesis and crystal growth of magnetic semiconducting chalcogenide spinels are considered. The phase diagrams of systems containing magnetic semiconducting phases are mapped out. To choose flux growth conditions, the liquidus relations in spinel–solvent systems and the kinetics of mass transport in chemical vapor transport systems are investigated. The influence of high temperatures and pressures on the structure of polycrystalline chalcogenide spinels is analyzed. The purity of the starting materials is shown to have a significant effect on the physical properties of chalcogenide spinels. Physical property measurements demonstrate that the magnetic semiconducting chalcogenide spinels have considerable potential for practical application.

Crystal-chemical studies of trace heavy elements zoning in garnet from Sanpou mine, Okayama

Crystal-chemical studies of trace heavy elements zoning in garnet from Sanpou mine, Okayama

X-ray study of volatile Ni(II), Cu(II), and Pd(II) complexes of β-ketoimine pivalyltrifluoroacetone

Synthesis of volatile complexes based on β-ketoimine pivalyltrifluoroacetone, C(CH3)3C(NH)CH2COCF3, is described. The general formula of the complexes is M(L)2, where M = Cu, Ni, Pd. Complexes of this kind with Ni and Pd were obtained for the first time. The Cu and Pd complexes were found to be isostructural. A comprehensive crystal-chemical study showed that all structures are molecular and built of trans-complexes. The central atom has a square plane environment. The average M-O and M-N distances are nearly equal in all compounds: 1.84 A, 1.92 A, and 1.98 A for Ni, Cu, and Pd complexes, respectively; the mean values of the O-M-N chelate angles are 93.4°, 91.9°, and 92.7°, respectively.

Ferri-ottoliniite and ferriwhittakerite, two new end-members of the new Group 5 for monoclinic amphiboles

Mineralogical and crystal-chemical descriptions are provided for two end-members of the new Group 5 introduced by Leake et al. (2004) in the nomenclature of amphiboles. This new classification scheme recognizes the occurrence of compositions with B-site populations intermediate between (Ca,Na) amphiboles and (Mg,Fe,Mn,Li) amphiboles. Its implications for the classification and nomenclature of Li-rich amphiboles are also discussed in this paper. Holotype ferri-ottoliniite [ideally A ◻ B (NaLi) C (Mg 3 Fe 2 3+ ) Si 8 O 22 (OH) 2 ] has a = 9.535(3), b = 17.876(6), c = 5.294(2) Å, β = 102.54°(1), V = 880.9 Å 3 , and unit formula A (K 0.07 Na 0.38 ) B (Na 0.70 Li 1.24 Ca 0.06 ) C (Mg 1.35 Fe 2+ 0.92 Mn 2+ 0.13 Zn 0.31 Fe 3+ 1.71 Al 0.10 Ti 0.06 Li 0.42 ) Si 8 O 22 (OH 1.51 F 0.47 ). Holotype ferriwhittakerite [ideally A Na B (NaLi) C (Mg 2 Fe 2 3+ Li) Si 8 O 22 (OH) 2 ] has a = 9.712(9), b = 17.851(23), c = 5.297(2) Å, β = 103.63°(5), V = 892.5 Å 3 , and unit formula A(K 0.13 Na 0.64 ) B (Na 1.27 Li 0.62 Ca 0.11 ) C (Mg 1.47 Fe 2+ 0.58 Mn 2+ 0.12 Zn 0.40 Fe 3+ 1.48 Al 0.10 Ti 0.12 Li 0.73 )Si 8 O 22 (OH 1.30 F 0.72 ). The root names have been chosen to acknowledge the contribution given to crystal-chemical studies of minerals, and particularly of amphiboles, by L. Ottolini (Pavia, Italy) and E.J.W. Whittaker (Oxford, U.K.)

Crystallisation and genesis of peralkaline magmas from Pantelleria Volcano, Italy: an integrated petrological and crystal-chemical study

Pantelleria, the type locality of pantellerite, is a Late-Pleistocene magmatic system with a bimodal association of alkali basalts and peralkaline rocks showing a large compositional gap at SiO 2=50–67 wt.%. Major and trace element data show a strong change in all element patterns with two distinct liquid lines of descent for the mafic and the felsic end-members for many elements (Al 2O 3, FeO tot, Ba, and Cr). The mineral chemistry data also show compositional gaps for many mineral phases, similar to those observed in the whole-rock data. Trace element models show that a trachytic melt can be generated from a basaltic parental magma either by a two step fractional crystallisation process, or by 30% partial melting of gabbroic cumulate. Pantelleritic magmas are derived by fractional crystallisation of mainly K-feldspar from a trachytic end-member. The crystal-chemical parameters of the clinopyroxene crystals show two clearly distinct patterns for the mafic and felsic rocks, which are unlikely to be generated by continuous crystal fractionation process from basalt to pantellerite. Clinopyroxene crystals in the mafic rocks show trends comparable with patterns observed for some Italian potassic suites (e.g., Sabatini volcano), with an increase of Al in the T site and Ca in M2 with differentiation. On the other hand, crystals in the peralkaline felsic rocks follow the patterns of the African Na-rich products (e.g., Nyambeni, Boseti) with Si increasing in T and Na in M2. Chemical parameters of the host rocks plotted vs. structural data of the clinopyroxene crystals divide mafic and felsic rocks into two well distinguished groups of undersaturated and oversaturated magmas, but do not explain the transition between the two groups. The data suggest that simple fractional crystallisation by itself cannot explain the generation of peralkaline magmas in Pantelleria starting from parental basalts. Other processes such as partial melting of gabbroic cumulates are more viable and have to be considered in the genesis of peralkaline magmas.