A comparative study of the thermal behaviour of length-fast chalcedony, length-slow chalcedony (quartzine) and moganite

The thermal behaviour of silica rocks upon heat treatment is dependent on the constituent minerals and petrographic texture types. These constituents can be shown to be mainly quartz in the form of two types of chalcedony (Length-fast (LF) chalcedony and Length-slow (LS) chalcedony, the latter also being termed quartzine) and moganite. Even though the thermal behaviour of LF-chalcedony is well understood, major uncertainties persist concerning the high-temperature behaviour of LS-chalcedony and moganite. We present here a comparative study of these three constituents of common silica rocks. Our results show that the chemical reaction is the same in all three, Si–OH + HO–Si → Si–O–Si + H₂O, but that the reaction kinetics and activation temperatures are very different. LS-chalcedony begins to react from 200 °C upwards, that is at temperatures 50 °C below the ones observed in LF-chalcedony, and shows the fastest reaction kinetics of this ‘water’ loss. Chemically bound water (SiOH) in moganite is more stable at high temperatures and no specific activation temperature is necessary for triggering the temperature-induced ‘water’ loss. Moganite is also found to act as a stabilizer in silica rocks preventing them from temperature-induced fracturing. These findings have implications for the study of potential heat treatment temperatures of silica rocks (in industry and heritage studies), but they also shed light on the different structures of SiO₂ minerals and the role of OH impurities therein.     

A proposed mechanism for the growth of chalcedony

The structural disparities that distinguish chalcedony from macrocrystalline quartz suggest that different crystallization mechanisms are operative during the growth of these two forms of silica. Although the paragenesis of chalcedony has provoked marked disagreement among researchers, a review of previous studies supports the idea that chalcedony can precipitate from slightly saturated aqueous solutions at relatively low temperatures (<100° C). These conditions for deposition suggest a model for chalcedony crystallization that involves the assembly of short-chain linear polymers via bridging silica monomers. This assembly occurs through a spiral growth mechanism activated by a screw dislocation withb=n/2 [110], wheren is an integer. The proposed model can account for a number of peculiarities that have been observed in chalcedony at the microstructural scale, such as: (1) the direction of fiber elongation along [110] rather than [001]; (2) the periodic twisting of chalcedony fibers about [110]; (3) the high density of Brazil twin composition planes; (4) the common intergrowth of moganite within chalcedony.     

Crystal structure of xinganite

Xinganite is a new REE-Be-rich silicate discovered in China. Its ideal formula is: (Y, Ce)Be SiO₄ OH. The mineral is of monoclinic system. The intensity data were collected with a single-crystal four-circle diffractometer. The lattice parameters are: a=4.7681 (± 0.00263) Å,b=7.7657 (± 0.00686) Å, c=9.9301 (± 0.00639) Å; α =90°, β=90.171° (±0.0053°), γ=90° space group p2¹/c;,Z=4. The crystal structure has been determined by direct methods and electron density synthesis methods. The least squares refinement gave a final discrepancy indexR=0.086. The crystal structural analysis shows that xinganite is of datolite-type structure.     

Crystal structure of daqingshanite

Daqinshanite, a new mineral recently discovered in China, is a kind of strontium-rare earth-bearing phosphate-carbonate. The ideal formula is: (Sr, Ca, Ba), RF(PO₄)(CO₃)₃. The mineral belongs to the trigonal system. The intensity data were collected with a single-crystal four-circle diffractometer. The lattice parameters are given as:a=10.058(±0.002) Å;c=9.225(±0.003) Å. Space group=R3m,Z=3. The crystal structure was solved by Patterson and electron density synthesis methods. The least-squares refinement led to a final conventional factorR=0.079. Crystal structural analysis shows that daqingshanite is of calcite-type structure and the ordered model of cations is similar to that of huntite (Mg₃Ca(CO₃)₄).     

Conditions of emplacement of chalcedony ore bodies in Northern Armenia

An analysis of the control which fault tectonics on the Shamlug dome exercise on ore mineralization reveals that the largest chalcedony bodies occur along the crest of the dome, inasmuch as the largest tension fault displacements are found here. Alteration of shales to chlorite and/or sericite is restricted to reverse faults with small displacement which came into being in response to compressive stresses; these zones are barren, or, at best, the amount of chalcedony is meager.—M. E. Burgunker.     

Crystal structure of magnetite under pressure

The crystal structure and the unit-cell parameters of magnetite have been studied at room temperature up to a pressure of 4.5 GPa using a diamond anvil cell and a four-circle X-ray diffractometer. The isothermal bulk modulus (K _T ) and its pressure derivative (K' _T ) determined by fitting the pressure-volume data to the Murnaghan equation of state are 181(2) GPa and 5.5(15), respectively. The positional parameter u does not vary significantly over the pressure range of this study. The linear compressibilities of the interatomic distances and the bulk moduli of the polyhedra have been calculated from the pressure dependences of the unit-cell edge a and the u parameter. The Bloch equation has been modified to derive a relationship between the Néel temperature and the parameter u. The modified Bloch equation gives a closer agreement with the experimental results than the Weisz equation.     

The nature of water in chalcedony and opal-C from brazilian agate geodes

The water species (H₂O_(SiOH) and H₂O_(mol)) of length-fast chalcedony and opal-C in Brazilian agates were studied with thermoanalytical, chemical and infrared absorption methods. Specific surfaces were measured with the BET nitrogen adsorption method and the specific densities were determined. Chalcedony and opal-C have fully hydrated crystal surfaces at the open porosity. They contain additional water at inner surfaces, which are closed micropores in the case of opal-C and regions of accumulated defects (e.g. twinlamellae boundaries) in chalcedony. All surfaces are covered with silanole groups, hydrogen-bonded to molecular water. Additional hydroxyl groups, weakly hydrogen bonded to the structural framework within the crystallites, are located at structural point defects. Wall-lining chalcedony ranges from translucent gray to milky white bands corresponding with decreasing total water content, the H₂O_(SiOH)/H₂O_(mol)-ratio, BET-surfaces and increasing density. The H₂O_(SiOH)/H₂O_(mol)-ratio is sensitive to subsequent hydrothermal treatment and indicates a low temperature formation of chalcedony.     

Crystal structure of meteoritic schreibersites: determination of absolute structure

Minerals of the schreibersite–nickelphosphide series (Fe,Ni)₃P crystallize in the non-centrosymmetric space group . As a consequence, they can possess two different spatial arrangements of the constituting atoms within the unit cell, related by the inversion symmetry operation. Here, we present the crystal structure refinements from single crystal X-ray diffraction data for schreibersite grains from iron meteorites Acuña, Carlton, Hex River Mts. (three different crystals), Odessa (two different crystals), Sikhote Alin, and Toluca aiming for the determination of the absolute structure of the examined crystals. The crystals studied cover the composition range from ~58 mol% to ~80 mol% Fe₃P end-member. Unit-cell parameter a and volume of the unit cell V, as well as certain topological structural parameters tightly correlate with Fe₃P content. Unit-cell parameter c, on the other hand, does not show such strong correlation. Eight of the nine crystal structure refinements allowed unambiguous absolute structure assignment. The single crystal extracted from Toluca is, however, of poor quality and consequently the structure refinement did not provide as good results as the rest of the materials. Also, this crystal has only weak inversion distinguishing power to provide unequivocal absolute structure determination. Six of the eight unambiguous absolute structure determinations indicated inverted atomic arrangement compared to that reported in earlier structure refinements (here called standard). Only two grains, one taken from Odessa iron and the other from the Hex River Mts. meteorite, reveal the dominance of standard crystal structure setting.     

Chalcedonic quartz and occurrence of quartzine (length-slow chalcedony) in pelagic sediments

ABSTRACT Chalcedony is the most abundant form of quartz in silicified pelagic sediments from the North Pacific. Varieties of chalcedonic quartz present in chert of deep-sea origin include chalcedony (length-fast and zebraic), quartzine (length-slow), and lutecite.
These occurrences of quartzine in known pelagic sediments emphasize the dangers of using quartzine as an indicator of former evaporitic environments. Quartzine is a diagenetic mineral and does indicate pore fluids rich in sulphate and magnesium. In pelagic sediments, it is always associated with authigenic barite and in many cases with authigenic dolomite. Quartzine should not be used, by itself, as an indicator of any particular environment of deposition.     

Crystal structure of columbite under high pressure

The structural evolution of two columbites under pressure, one ferrocolumbite from Raode (Africa) and one manganocolumbite from Kragero (Norway), has been determined by single-crystal X-ray diffraction. Structural investigations at high pressure have been carried out on samples which were preliminarily annealed to attain the complete cation-ordered state. For each crystal, five complete datasets have been collected from room pressure up to ca. 7 GPa. Structure refinements converged to final discrepancy factors R ranging between 5.2 and 5.8% for both the crystals. Structure refinements of X-ray diffraction data at different pressures allowed characterisation of the mechanisms by which the columbite structure accommodates variations in pressure. A and B octahedral volumes in both samples decrease linearly as pressure increases, with a larger compression of the larger A site. The difference in polyhedral bulk moduli of the A sites for the two samples does not appear to relate directly to the octahedral sizes, the A site being more compressible in the Fe-rich sample than in the Mn-rich one. By far the most compressible direction in both the analysed samples is along b. The cations are in fact free to move along this direction, thus allowing the octahedral chains to slide over each other; this effect is particularly evident in the manganocolumbite sample which shows a steep shortening of interchain A–B distances along b.     

The Chinamora batholith, Zimbabwe: structure and emplacement-related magnetic rock fabric

The origin of dome-and-keel structural geometries in Archean granite–greenstone terrains appears to lack any modern analogues and is still poorly understood. The formation of these geometries is investigated using structural and anisotropy of magnetic susceptibility (AMS) data for the Chinamora batholith in Zimbabwe. The roughly circular-shaped batholith is surrounded by ca. 2.72–2.64 Ga greenstones. The batholith granitoid suites have been divided on the basis of their ages and fabric relationships into four distinct units: (i) banded basement gneisses; (ii) granodioritic gneisses; (iii) equigranular granites; and (iv) central porphyritic granites. In the gneissic granites a partial girdle (N–S) of poles to the magnetic foliation is developed that has been folded around a consistent, flat lying magnetic lineation plunging at shallow angles to the E or W. In the equigranular granites, the magnetic lineation generally plunges to the NW. The magnetic foliation has a variable strike, no clear trends can be distinguished. The AMS measurements of the porphyritic granite revealed a NW–SE striking foliation and showed subhorizontal magnetic lineations. The magnetic foliation is subparallel to the macroscopic foliation. Wall rocks are moderately inclined and show radial or concentric lineations, triaxial strain ellipsoids and kinematics that demonstrate off-the-dome sliding and coeval pluton expansion. The results of the observations do not point to a single emplacement process. Neither the observed structural data nor the magnetic fabric support a model envisaging spherically ‘ballooning’. It is argued that pluton diapirism played a major part in the formation of the fabrics in the gneisses, whereas the fabrics in the porphyritic granites reflect emplacement as laccolith-like sheets.     

Crystal structure and cation distribution in some natural magnetites

Summary The crystal structures of two natural magnetites were refined. Both turned out to have equipoints 8a and 16d (Fd3m) fully occupied and hence different from type I and type II defect structures investigated by Fleet (1981, 1982). It was possible to improve the cation radii for Fe²⁺ ad Fe³⁺ in octahedral and tetrahedral coordinations for pure and almost pure magnetites obtaining very good agreement between observed and calculated values of the two independent geometric structure parameters, i.e. cell edge and oxygen coordinate. The present results lead to an estimate of inversion parameter i in (Fe _1–i ²⁺ Fe _i ³⁺ )(Fe _i ²⁺ Fe _2t-i ³⁺ )O₄, of about 0.90, equal for all the four pure magnetites, independently of type of structure and also of cooling history.

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Kristallstruktur und kationverteilung von einigen natürlichen magnetiten

Zusammenfassung Die Kristallstrukturen von zwei natürlichen Magnetiten wurden verfeinert. Es stellte sich heraus, daß ihre Gitterplätze 8 a und 16d (Fd 3 m) voll besetzt sind, und sich somit von den von Fleet (1981, 1982) bestimmten Defektstrukturen unterscheiden. Die Kationradien für Fe²⁺ und Fe³⁺ in Oktaeder- und Tetraeder-Koordinationen wurden für reine und fast reine Magnetite verbessert. Dabei wurde eine gute Korrelation zwischen beobachteten und berechneten Werten der beiden unabhängigen Parameter in der Struktur, d.h. Gitterkonstante und Sauerstoff-Ortsparameter gefunden. Die vorliegenden Ergebnisse bringen einen Umkehrparameter i in (Fe _1–i ²⁺ Fe _i ³⁺ )(Fe _i ²⁺ Fe _2–i ³⁺ )O₄ von ungefähr 0.90, der in allen reinen Magnetiten gleich und unabhängig von dem Strukturtyp sowie dem Abkühlverhalten ist.

     

Machatschkiite: Crystal structure and revision of the chemical formula

Summary Single crystal X-ray investigation shows that machatschkiite crystallizes in space groupR3c witha _hex =15.127(2) Å andc _hex =22.471(3) Å. The crystal structure was determined by direct methods and Fourier syntheses; the refinement by least squares methods led toR=0.04 for 645 independent reflections. Our X-ray results, supplemented by a partial electron microprobe analysis, indicate that the chemical formula of machatschkiite is close to Ca_6–x Na _x (AsO₄)(AsO₃OH)₃(PO₄)_1–x (SO₄) _x ·15H₂O (x0.3) withZ _hex =6. The atomic arrangement of machatschkiite represents a new structure type and seems to be the first example of a crystal structure in which three oxygens of an AsO₄ group are acceptors of each one hydrogen bond from three surrounding AsO₃(OH) groups.

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Machatschkiit: Kristallstruktur und Revision der chemischen Formel

Zusammenfassung Röntgenographische Einkristalluntersuchungen zeigten, daß Machatschkiit in der RaumgruppeR3c mita _hex =15,127(2) Å undc _hex =22,471(3) Å kristallisiert. Die Kristallstruktur wurde mit direkten Methoden und mit Fourier-Synthesen bestimmt; die Verfeinerung nach der Methode der kleinsten Quadrate führte für 645 Reflexe aufR=0.04. Unser Röntgenbefund, der durch eine partielle Elektronenstrahlmikroanalyse ergänzt wird, weist darauf hin, daß die chemische Formel des Machatschkiites mit guter Näherung Ca_6–x Na _x (AsO₄)(AsO₃OH)₃ (PO₄)_1–x (SO₄) _x ·15H₂O (x0,3) mitZ _hex =6 lautet. Die Atomanordnung des Machatschkiites stellt einen neuen Strukturtyp dar und ist anscheinend das erste Beispiel, in welcher drei Sauerstoffe einer AsO₄-Gruppe Akzeptoren von je einer Wasserstoffbindung dreier benachbarter AsO₃OH-Gruppen sind.

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With 4 Figures

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Dedicated to the memory of Prof. Dr.F. Machatschki, Editor of Tschermaks Mineralogische und Petrographische Mitteilungen 1948–1968.     

Crystal structure refinement and electron density distribution in diaspore

Diaspore from Dilln, Hungary, AlOOH, is orthorhombic with space group Pbnm, a=4.4007(6), b=9.4253(13), c=2.8452(3) Å, and Z=4. The crystal structure and electron distribution have been refined from 791 graphite-monochromatized MoKα data (maximum 2θ=130°) to R=0.035 (R _w =0.029). Difference maps show substantial electron density ascribed to covalent bonding in the hydroxyl group, O(2)-H, but no residual density is observed along the Al-O(1,2) bonds. An analysis of the charge distribution implies net charges of +1.47(26), ?1.08(16), ?0.59(13) and +0.20(5) for Al, O(1), O(2) and H respectively. Semi-empirical molecular orbital calculations of the Hückel type agree with the experimentally determined atomic charge distribution and also allow a rationalization of the observed bond length variations.