Perrierite-(La), (La,Ce,Ca)4(Fe2+,Mn)(Ti,Fe3+,Al)4(Si2O7)2O8, a new mineral species from the Eifel volcanic district, Germany

Non-metamict perrierite-(La) discovered in the Dellen pumice quarry, near Mendig, in the Eifel volcanic district, Rheinland-Pfalz, Germany has been approved as a new mineral species (IMA no. 2010-089). The mineral was found in the late assemblage of sanidine, phlogopite, pyrophanite, zirconolite, members of the jacobsite-magnetite series, fluorcalciopyrochlore, and zircon. Perrierite-(La) occurs as isolated prismatic crystals up to 0.5 × 1 mm in size within cavities in sanidinite. The new mineral is black with brown streak; it is brittle, with the Mohs hardness of 6 and distinct cleavage parallel to (001). The calculated density is 4.791 g/cm³. The IR spectrum does not contain absorption bands that correspond to H₂O and OH groups. Perrierite-(La) is biaxial (-), α = 1.94(1), β = 2.020(15), γ = 2.040(15), 2V _meas = 50(10)°, 2V _calc = 51°. The chemical composition (electron microprobe, average of seven point analyses, the Fe²⁺/Fe³⁺ ratio determined from the X-ray structural data, wt %) is as follows: 3.26 CaO, 22.92 La₂O₃, 19.64 Ce₂O₃, 0.83 Pr₂O₂, 2.09 Nd₂O₃, 0.25 MgO, 2.25 MnO, 3.16 FeO, 5.28 Fe₂O₃, 2.59 Al₂O₃, 16.13 TiO₂, 0.75 Nb₂O₅, and 20.06 SiO₂, total is 99.21. The empirical formula is (La_1.70Ce_1.45Nd_0.15Pr_0.06Ca_0.70)_Σ4.06(Fe _0.53 ²⁺ Mn_0.38Mg_0.08)_Σ0.99(Ti_2.44Fe _0.80 ³⁺ Al_0.62Nb_0.07)_Σ3.93Si_4.04O₂₂. The simplified formula is (La,Ce,Ca)₄(Fe²⁺,Mn)(Ti,Fe³⁺,Al)₄(Si₂O₇)₂O₈. The crystal structure was determined by a single crystal. Perrierite-(La) is monoclinic, space group P2₁/a, and the unit-cell dimensions are as follows: a =13.668(1), b = 5.6601(6), c = 11.743(1) Å, β = 113.64(1)°; V = 832.2(2) ų, Z = 2. The strong reflections in the X-ray powder diffraction pattern are [d, Å (I, %) (hkl)]: 5.19 (40) (110), 3.53 (40) ( $\overline 3 $ 11), 2.96 (100) ( $\overline 3 $ 13, 311), 2.80 (50) (020), 2.14 (50) ( $\overline 4 $ 22, $\overline 3 $ 15, 313), 1.947 (50) (024, 223), 1.657 (40) ( $\overline 4 $ 07, $\overline 4 $ 33, 331). The holotype specimen of perrierite-(La) is deposited at the Fersman Mineralogical Museum, Russian Academy of Sciences, Moscow, Russia, with the registration number 4059/1.     

High-Pressure crystal chemistry of spinel (MgAl2O4) and magnetite (Fe3O4): Comparisons with silicate spinels

High-pressure crystal structures and compressibilities have been determined by x-ray methods for MgAl₂O₄ spinel and its isomorph magnetite, Fe₃O₄. The measured bulk moduli, K, of spinel and magnetite (assuming K′=4) are 1.94±0.06 and 1.86±0.05 Mbar, respectively, in accord with previous ultrasonic determinations. The oxygen u parameter, the only variable atomic position coordinate in the spinel structure (Fd3m, Z=8), decreases with pressure in MgAl₂O₄, thus indicating that the magnesium tetrahedron is more compressible than the aluminum octahedron. In magnetite the u parameter is unchanged, and both tetrahedron and octahedron display the 1.9 Mbar bulk modulus characteristic of the entire crystal. This behavior contrasts with that of nickel silicate spinel (γ-Ni₂SiO₄), in which the u parameter increases with pressure because the silicon tetrahedron is relatively incompressible compared to the nickel octahedron.     

Refinement of the crystal structure of bonshtedtite, Na3Fe(PO4)(CO3)

The crystal structure of bonshtedtite, Na₃Fe(PO₄)(CO₃) (monoclinic, P2₁/m, a = 5.137(4), b = 6.644(4), c = 8.908(6) Å, β = 90.554(14)°, V = 304.0(4) ų, Z = 2) has been refined to R ₁ = 0.041 on the basis of 1314 unique reflections. The structure is similar to that of other minerals of the bradleyite group. It is based on the [Fe(PO₄)(CO₃)]^3? layers oriented parallel to (001). The layers are formed by corner-sharing PO₄ tetrahedra and FeO₄(CO₃) complexes, where FeO₆ tetrahedra and CO₃ triangles are edge-shared. The topology of the octa-tetrahedral layer in bonshtedtite is similar to that of the autunite-group minerals, but it differs from the latter in terms of local topological properties.     

Experimental and theoretical study of (Fe2+, Mg)(Al,Fe3+)2O4 spinels: Activity-composition relationships,miscibility gaps,vacancy contents

The compositions of (Fe²⁺, Mg)(Al, Fe³⁺)₂O₄ spinels equilibrated with a l M (Fe²⁺, Mg)Cl₂ aqueous solution at 800°C, 4 kbars were determined. General considerations of reciprocal systems allow derivation of the exchange isotherm between a chloride aqueous solution and (Mg, Fe²⁺)Al₂O₄ spinels. They enable calculation of ΔG of the reaction: FeCl₂ + MgAl₂O₄ = MgCl₂ + FeAl₂O₄ΔG = 2.9 kcal at 800°C, 4 kbars and provide the activity-composition relationships for the binary join FeAl₂O₄-MgAl₂O₄, which shows a substantial positive deviation from ideality. Some tie-lines between coexisting aluminous and ferric spinels were also obtained in the (Fe²⁺, Mg)(Al, Fe³⁺)₂O₄ system.These experimental data are modeled by a Gibbs free energy formulation of the spinel solid solution (Lehmann and Roux, 1984), where the corrective function g₂, necessary to reproduce the deviations from ideality, is artificially split into two parts:

• 1.(1) A homogeneous second degree polynomial in the composition variables, containing only the terms specific to the reciprocal nature of the system, whose coefficients are deduced from ΔG of the exchange reaction: MgAl₂O₄ + FeFe₂O₄ = MgFe₂O₄ + FeAl₂O₄ΔG = 4.5 kcal at 800°C, 4 kbars
• 2.(2) A homogeneous second degree polynomial in the site occupancy fractions, to model the non-ideal behavior of the (Fe²⁺, Mg)Al₂O₄ and (Fe₂₊, Mg)Fe₂O₄ spinels and the miscibility gap along the Fe(Al, Fe³⁺)₂O₄ join.

A model of reciprocal spinel solution involving defect end-members is used to estimate the vacancy contents of the spinels in equilibrium with sesquioxides. In this case, the corrective function necessary to take into account the reciprocal nature of the system is no longer a second degree polynomial, but a rational fraction.     

Magnesioneptunite, KNa2Li(Mg,Fe)2Ti2Si8O24, a new mineral species of the neptunite group

A new mineral of the neptunite group, magnesioneptunite KNa₂Li(Mg,Fe)₂Ti2Si₈O₂₄, a Mg-dominant analogue of neptunite and manganoneptunite, has been found in the Upper Chegem caldera near Mount Lakargi, Kabardino-Balkaria, the North Caucasus, Russia in a xenolith of altered sandstone located between skarnified carbonate xenoliths and ignimbrite. Magnesioneptunite occurs as nearly isometric grains and aggregates up to 0.1 mm in size in the cores of some grains of a Mg-rich variety of neptunite with Mg/(Fe + Mn) = 0.7?1.0. The chemical composition of magnesioneptunite with a maximum Mg content is as follows, wt %: 3.63 K₂O, 8.21 Na₂O, 1.73 Li₂O, 6.47 MgO, 0.04 MnO, 5.87 FeO, 0.07 Al₂O₃, 18.73 TiO₂, 56.88 SiO₂, 99.62 in total. The empirical formula is (K_0.67Na_0.32Ca_0.01)_Σ1.00Na_2.06Li_1.00 · (Mg_1.39Fe _0.71 ²⁺ )_Σ2.10(Si_7.90Al_0.01)_Σ7.91O₂₄. Grains of magnesioneptunite are dark brown to red-brown, translucent, with vitreous luster. D _calc = 3.15 g/cm³, and the Mohs hardness is 5–6. Cleavage parallel to the (110) is perfect. The new mineral is optically biaxial, positive, α = 1.697(2), β = 1.708 (3), γ = 1.725(3), 2V _meas = 45(15)°. The mineral is associated with quartz, alkali feldspar, rutile, aegirine, and neptunite. Magnesioneptunite and the Mg-rich variety of neptunite were formed as products of ilmenite alteration. Magnesioneptunite is monoclinic, C2/c; unit-cell parameters: a = 16.327(7), b = 12.4788(4), c = 9.9666(4) Å, β = 115.6519(5)°, V = 1830.5(1) ų, Z = 4. The type specimen is deposited at the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow.     

The crystal structure of sonoraite,Fe3+Te4+O3(OH)·H2O

Summary Sonoraite, FeTeO₃(OH)·H₂O, is monoclinic,P 2₁/c, witha=10.984(2),b=10.268(2),c=7.917(2) Å, =108.49(2)°. For 8 formula units per cell the calculated density is 4.179(2) g/cm³; the observed value is 3.95(1) g/cm³. The Supper-Pace automated diffractometer was used to collect 1884 independent reflections which were corrected for absorption. The structure was determined by an automated symbolic addition procedure. It was refined to a residualR of 6.2% using anisotropic temperature factors for the cations and isotropic temperature factors for the oxygen atoms. Chains of octahedra about Fe extend along [101]; edge-sharing pairs of these octahedra are joined by corner sharing. The Fe–Fe distances across the shared edges are 3.05 and 3.20 Å, short enough to suggest magnetic interactions. All but one H₂O are involved in the chains. The Te⁴⁺ ions have a pseudotetrahedral coordination, with three oxygen ions forming one face of the tetrahedron and the lone electron pair of Te occupying the fourth corner. The O–Te–O average bond angle is 95°. The Fe chains are tied together by Te–O bonds in all three dimensions.

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Die Kristallstruktur von Sonorait, Fe³⁺Te⁴⁺O₃(OH).H₂O

Zusammenfassung Sonorait, FeTeO₃(OH)·H₂O, ist monoklin, P 2₁/c, mit den folgenden Zelldimensionen:a=10,984(2),b=10,268(2),c=7,917(2) Å, =108,49(2)°. Mit 8 Formel-Einheiten errechnet man eine Dichte von 4,179(2) g/cm³; die gemessene Dichte beträgt 3,95(1) g/cm³. Das Supper-Pace automatische Diffraktometer wurde zur Sammlung von 1884 unabhängigen Reflexen benutzt, welche für Absorption korrigiert wurden. Die Struktur wurde mit Hilfe eines vollständig automatischen Programms für symbolische Addition bestimmt. Mit anisotropen Temperaturfaktoren für die Kationen und mit isotropen Temperaturfaktoren für die Sauerstoff-Atome wurde ein Residuum von 6,2% erreicht. Ketten von Eisen-Oktaedern erstrecken sich entlang [101]; Oktaeder-Paare mit gemeinsamen Kanten sind über Eckenverknüpfung verbunden. Die Fe–Fe-Abstände über die gemeinsamen Kanten betragen 3,05 und 3,20 Å, kurz genug, um zu magnetischer Wechselwirkung führen zu können. Nur ein H₂O-Molekül ist nicht Teil einer Kette. Die Te⁴⁺-Ionen befinden sich in pseudotetraedrischer Koordination; drei Sauerstoff-Ionen bilden eine Fläche des Tetraeders, die vierte Ecke wird durch das einsame Elektronenpaar von Te besetzt. Der Mittelwert des O–Te–O-Bindungswinkels beträgt 95° Die Fe-Ketten werden durch Te–O-Bindungen dreidimensional verbunden.

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With 3 Figures     

Oxyphlogopite K(Mg,Ti,Fe)3[(Si,Al)4O10](O,F)2: A new mineral species of the mica group

Oxyphlogopite is a new mica-group mineral with the idealized formula K(Mg,Ti,Fe)₃[(Si,Al)₄O₁₀](O,F)₂. The holotype material came from a basalt quarry at Mount Rothenberg near Mendig at the Eifel volcanic complex in Rhineland-Palatinate, Germany. The mineral occurs as crystals up to 4 × 4 × 0.2 mm in size encrusting cavity walls in alkali basalt. The associated minerals are nepheline, plagioclase, sanidine, augite, diopside, and magnetite. Its color is dark brown, its streak is brown, and its luster is vitreous. D _meas = 3.06(1) g/cm³ (flotation in heavy liquids), and D _calc = 3.086 g/cm³. The IR spectrun does not contain bands of OH groups. Oxyphlogopite is biaxial (negative); α = 1.625(3), β = 1.668(1), and γ = 1.669(1); and 2V _meas = 16(2)° and 2V _calc = 17°. The dispersion is strong; r < ν. The pleochroism is medium; X > Y > Z (brown to dark brown). The chemical composition is as follows (electron microprobe, mean of 5 point analyses, wt %; the ranges are given in parentheses; the H₂O was determined using the Alimarin method; the Fe²⁺/Fe³⁺ was determined with X-ray emission spectroscopy): Na₂O 0.99 (0.89–1.12), K₂O 7.52 (7.44–7.58), MgO 14.65 (14.48–14.80), CaO 0.27 ((0.17–0.51), FeO 4.73, Fe₂O₃ 7.25 (the range of the total iron in the form of FeO is 11.09–11.38), Al₂O₃ 14.32 (14.06–14.64), Cr₂O₃ 0.60 (0.45–0.69), SiO₂ 34.41 (34.03–34.66), TiO₂ 12.93 (12.69–13.13), F 3.06 (2.59–3.44), H₂O 0.14; O=F₂ −1.29; 99/58 in total. The empirical formula is (K_0.72Na_0.14Ca_0.02)(Mg_1.64Ti_0.73Fe_0.30²⁺ Fe_0.27³⁺Cr_0.04)_Σ2.98(Si_2.59Al_1.27Fe_0.14³⁺ O₁₀) O_1.20F_0.73(OH)_0.07. The crystal structure was refined on a single crystal. Oxyphlogopite is monoclinic with space group C2/m; the unit-cell parameters are as follows: a = 5.3165(1), b = 9.2000(2), c = 10.0602(2) ?, β = 100.354(2)°. The presence of Ti results in the strong distortion of octahedron M(2). The strongest lines of the X-ray powder diffraction pattern [d, ? (I, %) [hkl]] are as follows: 9.91(32) [001], 4.53(11) 110], 3.300(100) [003], 3.090(12) [112], 1.895(21) [005], 1.659(12) [−135], 1.527(16) [−206, 060]. The type specimens of oxyphlogopite are deposited at the Fersman Mineralogical Museum in Moscow, Russia; the registration numbers are 3884/2 (holotype) and 3884/1 (cotype).     

Fe23+Sn3O9,一个未定名的斜钒钛矿族的铁、锡氧化物

Fe,Sn氧化物矿产于四川雪宝顶钨锡矿床中,与之紧密共生的矿物是黑气电石,白云母,石英等,Fe-Sn氧化物呈细小的不规则颗粒嵌在锡石中,粒度大小为0.013 mm～0.052 mm之间,偶有个别大的颗粒(d=0.28 mm)淡红褐色,条痕无色,玻璃-金刚光泽,无解理,(因大小硬度无法测定),Dx=5.93 g/cm3.在偏光镜下淡黄褐色,多色性弱,明显的非均质性,按N=1+dk公式,获得N=2.11,矿物的的化学成份SnO273.70,TiO20.60,Fe 2O3 25.26,总计99.56(7个不同测点的平均值),计算出化学式为Fe 23+Sn3O9.矿物的强X射线值3.702(15,511),3.445(5,901),3.348(100,505),3.264(14,711),3.114(40,712),1.487(10,033),单斜晶系,晶胞参数a=3.375 nm,b=0.457 nm,c=2.001nm,β=93°24',V=3.080 74 nm3,Z=18.     

(Na,Ca)(Ti3+,Mg)Si2O6-clinopyroxenes at high pressure: influence of cation substitution on elastic behavior and phase transition

A compressional study of (Na,Ca)(Ti³⁺,Mg)Si₂O₆-clinopyroxenes was carried out at high pressures between 10⁻⁴ and 10.2 GPa using in situ single-crystal X-ray diffraction, Raman spectroscopy and optical absorption spectroscopy. Compressional discontinuities accompanied by structural changes, in particular, the appearance of two distinct Ti³⁺–Ti³⁺ distances within the octahedral chains at 4.37 GPa, provide evidence for the occurrence of a phase transition in NaTi³⁺Si₂O₆. Equation-of-state parameters are K ₀ = 115.9(7) GPa with K′ = −0.9(3) and K ₀ = 102.7(8) GPa with K′ = 4.08(5) for the low- and high-pressure range, respectively. The transition involves a C2/c–P [`1] \overline{1} symmetry change, which can be confirmed by the occurrence of new modes in Raman spectra. Since no significant discontinuity in the evolution of the unit-cell volume with pressure has been observed, the transition appears to be second-order in character. The influence of the coupled substitution Na⁺Ti³⁺↔Ca²⁺Mg²⁺ on the static compression behavior and the structural stability has been investigated using a sample of the intermediate composition (Na_0.54Ca_0.46)(Mg_0.46Ti_0.54)Si₂O₆. No evidence for a deviation from continuous compression behavior has been found, neither in lattice parameter nor in structural data and the fit of a third-order Birch–Murnaghan equation-of-state to the pressure–volume data yields a bulk modulus of K ₀ = 109.1(5) GPa and K′ = 5.02(13). Raman and polarized absorption spectra have been compared to NaTiSi₂O₆ and reveal major similarities. The main driving force for the phase transition in NaTi³⁺Si₂O₆ is the localization of the Ti³⁺ d-electron and the accompanying distortion, which is suppressed in the (Na,Ca)(Ti³⁺,Mg)Si₂O₆-clinopyroxene.     

Fibroferrite: A mineral with a {Fe(OH) (H2O)2SO4} spiral chain and its relationship to Fe(OH)SO4, butlerite and parabutlerite

Summary The mineral fibroferrite has the chemical formula Fe(OH)SO₄·xH₂O; the value forx has not been definitely settled, but as a rule it is found to be near five. Several symmetries are given in the literature.A sample from Saint Felix de Paillères, France, proved to be rhombohedral with space group R3; lattice constants for the hexagonal cell area=24.176,c=7.656 Å. As calculated from the experimental density (=1.95 g·cm^–3)Z=18 for this cell. Intensities were collected on an automated X-ray diffractometer from a thin fiber extended along [00.1]. The structure was determined by Patterson and Fourier methods. Least squares refinement with 818 observed reflections resulted inR=0.076.The structure contains hydroxo-bridged {Fe(OH)(H₂O)₂SO₄} spiral chains built of [Fe(OH)₂(H₂O)₂O₂] octahedra and SO₄ tetrahedra. Hydrogen bonds provide connections between these chains. The spiral chains are a stereoisomer variant of the hydroxo-bridged linear chains of Fe(OH)SO₄, butlerite and parabutlerite. A comparison of these compounds is givenm to understand the relationship between the structure and their water content.

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Fibroferrit: Ein Mineral mit einer {Fe(OH)(H₂O)₂SO₄} Spiralkette und seine Beziehung zu Fe(OH)SO₄, Butlerit und Parabutlerit

Zusammenfassung Das Mineral Fibroferrit hat die chemische Formel Fe(OH)SO₄·xH₂O; der Wert furx scheint nicht endgültig geklärt zu sein, liegt aber meist nahe 5. Verschiedene Symmetrien werden in der Literatur angegeben.Eine Probe von Saint Felix de Paillères, Frankreich, erwies sich als rhomboedrisch mit der Raumgruppe R3; die Gitterkonstanten der hexagonalen Zelle sinda=24,176,c=7,656 Å. Die experimentelle Bestimmung der Dichte (=1,95 g·cm^–3) führt für diese Zelle zuZ=18. Von einer nach [00.1] gestreckten dünnen Faser wurden die Intensitäten auf einem automatischen Röntgendiffraktometer gesammelt. Die Struktur wurde mit Patterson-und Fouriersynthesen gelöst. Eine Verfeinerung nach der Methode der kleinsten Quadrate führte für 818 beobachtete Reflexe aufR=0,076.Die Struktur enthält durch Hydroxil-Gruppen verknüpfte {Fe(OH)(H₂O)₂SO₄}-Spiralketten, die aus [Fe(OH)₂(H₂O)₂O₂]-Oktaedern und SO₄-Tetraedern aufgebaut sind. Die Spiralketten von Fibroferrit sind eine stereoisomere Variante der annähernd linearen Fe–O–S-Ketten von Fe(OH)SO₄, Butlerit und Parabutlerit. Diese Verbindungen werden mit Fibroferrit verglichen, um Beziehungen zwischen Struktur und Wassergehalt zu verstehen.

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With 2 Figures

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Paper presented at the Fifth European Crystallography Meeting, Copenhagen, Denmark 1979.     

The effect of Al3+, Fe3+, and Ti4+ on the configurational heat capacities of sodium silicate liquids

The heat capacities of 29 glasses and supercooled liquids in the Na₂O-SiO₂, Na₂O-Al₂O₃-SiO₂, Na₂O-(FeO)-Fe₂O₃-SiO₂, and Na₂O-TiO₂-SiO₂ systems were measured in air from 328 to 998 K with a differential scanning calorimeter. The reproducibility of the data determined from multiple heat capacity runs on a single crystal MgO standard is within ± 1% of the accepted values at temperatures ≤ 800 K and within ± 1.5% between 800 and 1000 K. Within the resolution of the data, the heat capacities of sodium silicate and sodium aluminosilicate liquids are temperature independent. Heat capacity data in the supercooled liquid region for the sodium silicates and sodium aluminosilicates were combined and modelled assuming a linear compositional dependence. The derived values for the partial molar heat capacities of Na₂O, Al₂O₃, and SiO₂ are 112.35 ± 0.42, 153.16 ± 0.82, and 76.38 ± 0.20 J/gfw · K respectively. The partial molar heat capacities of Fe₂O₃ and TiO₂ could not be determined in the same manner because the heat capacities of the Fe₂O₃- and TiO₂-bearing sodium silicate melts showed varying degrees of negative temperature dependence. The negative temperature dependence to the configurational C _P may be related to the occurrence of sub-microscopic domains (relatively polymerized and depolymerized) that break down to a more homogeneous melt structure with increasing temperature. Such an interpretation is consistent with data from in situ Raman, Mössbauer, and X-ray absorption fine structure (XAFS) spectroscopic studies on similar melts.     

Reversible hydration in synthetic mixite, BiCu6(OH)6(AsO4)3·nH2O (n≤3): hydration kinetics and crystal chemistry

 The presence of zeolitic water, with a reversible hydration behaviour, was determined by structural and kinetic studies on synthetic mixite BiCu₆(OH)₆(AsO₄)₃·nH₂O (n≤3). X-ray diffraction and infrared-spectroscopic investigations were performed on single crystals. Isothermal thermogravimetric experiments were carried out to determine the reaction kinetics of the de- and rehydration processes. The single-crystal structure refinement of a fully hydrated crystal yielded five partially occupied Ow positions (Ow=oxygen atom of a H₂O molecule) within the tube-like channels of the hexagonal [BiCu₆(OH)₆(AsO₄)₃] framework. For the partially dehydrated form, with n≈1, at least two of these sites were found to be occupied significantly. In addition, the structural investigations allowed two different intra-framework hydrogen bonds to be distinguished that are independent of the extra-framework water distribution and are responsible for the stability of the self-supporting framework. The kinetic analysis of the rate data in the 298–343K temperature range shows that the dehydration behaviour obeys a diffusion-controlled reaction mechanism with an empirical activation energy of E _a ^dehyd=54±4 kJ mol^–1. A two-stage process controls rehydration of which the individual steps were attributed to an initial surface-controlled (E _a ^hyd-I=6±1 kJ mol^–1) and subsequent diffusion-controlled reaction mechanism (E _a ^hyd-II=12±1 kJ mol^–1). The estimated hydration enthalpy of 42±5 kJ mol^–1 supports the distribution model of molecular water within the channels based on a purely hydrogen-bonded network. Received June 26, 1996 / Revised, accepted November 11, 1996     

Temperature-dependent magnetic susceptibility behaviour of spinelloid and spinel solid solutions in the systems Fe2SiO4–Fe3O4 and (Fe,Mg)2SiO4–Fe3O4

The magnetic behaviour and Curie temperatures (T _C ) of spinelloids and spinels in the Fe₃O₄–Fe₂SiO₄ and Fe₃O₄–(Mg,Fe)₂SiO₄ systems have been determined from magnetic susceptibility (k) measurements in the temperature range –192 to 700 °C. Spinelloid II is ferrimagnetic at room temperature and the k measurements display a characteristic asymmetric hump before reaching a T _C at 190 °C. Spinelloid V from the Mg-free system is paramagnetic at room temperature and hysteresis loops at various low temperatures indicate a ferri- to superparamagnetic transition before reaching the T _{C .} The T _C shows a non-linear variation with composition between –50 and –183 °C with decreasing magnetite component (X _Fe3O₄). The substitution of Mg in spinelloid V further decreases T _C . Spinelloid III is paramagnetic over nearly the total temperature range. Ferrimagnetic models for spinelloid II and spinelloid V are proposed. The T _C of Fe₃O₄–Fe₂SiO₄ spinel solid solutions gradually decrease with increasing Si content. Spinel is ferrimagnetic at least to a composition of X _Fe3O₄=0.20, constraining a ferrimagnetic to antiferromagnetic transition to occur at a composition of X _Fe3O₄<0.20. A contribution of the studied ferrimagnetic phases for crustal anomalies on the Earth can be excluded because they lose their magnetization at relatively low temperatures. However, their relevance for magnetic anomalies on other planets (Mars?), where these high-pressure Fe-rich minerals could survive their exhumation or were formed by impacts, has to be considered.