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Natural hardystonite, (Ca3.85Na0.14Pb0.01)(Zn0.85Al0.07Mg0.03Mn0.043+Fe0.023+)Si2.00O7.00, from the type locality was reinvestigated with single crystal X-ray diffraction, microprobe analysis, and electron diffraction. The average structure, space group P (4) over bar2(1)m, a = 7.800(1) Angstrom. c = 5.000(1) Angstrom, was refined to R = 1.91% using 331 independent reflections. Hardystonite exhibits an incommensurate modulated structure. As in synthetic melilite-type compounds, modulation is two-dimensional, with tartan-like appearance: modulation vectors are q(1) = alpha (a(1)* + a(2)*) and q(2) = alpha(-a(1)* + a(2)*). A modulation wavelength lambda = 19.0(4) Angstrom was estimated by centering satellite reflections using a single-crystal diffractometer. TEM-EDX investigations proved the chemical composition of the sample to be slightly inhomogeneous, with stronger and sharper satellites in the regions where the composition approaches the Ca2ZnSi2O7 end-member. Geometrical parameters as well as anisotropic displacement ellipsoids of hardystonite are consistent with those of the other melilite-type compounds having a modulated character.

Cordierite has the ideal formula (Mg,Fe)2Al4Si5O18.x(H2O,CO2), but it must contain some Fe3+ to account for its blue color and strong pleochroism. The site occupation and concentration of Fe3+ in two Mg-rich natural cordierites have been investigated by EPR and 57Fe Mossbauer spectroscopy. In addition, powder IR spectroscopy, X-ray diffraction, and TEM examination were used to characterize the samples. Single-crystal and powder EPR spectra indicate that Fe3+ is located on T11 in natural cordierites and not in the channels. The amount in Mg-rich cordierites is very small with an upper limit set by Mossbauer spectroscopy giving less than 0.004 cations per formula unit (pfu). Fe3+ in cordierite can, therefore, be considered insignificant for most petrologic calculations. Heat-treating cordierite in air at 1,000 °C for 2 days causes an oxidation and/or loss of Fe2+ on T11, together with an expulsion of Na+ from the channels, whereas heating at the Fe–FeO buffer produces little Fe3+ in cordierite. Heating at 1,000 °C removes all class I H2O, but small amounts of class II H2O remain as shown by the IR measurements. No evidence for channel Fe2+ or Fe3+ in the heat-treated samples was found. The blue color in cordierite arises from a broad absorption band (E//b and weaker with E//a) around 18,000 cm−1 originating from charge-transfer between Fe2+ in the octahedron and Fe3+ in the edge-shared T11 tetrahedron. It therefore appears that all natural cordierites contain some tetrahedral Fe3+. The brown color of samples heated in air may be due to the formation of very small amounts of submicroscopic magnetite and possibly hematite. These inclusions in cordierite can only be identified through TEM study.

Exsolved pairs of hornblende and cummingtonite from two localities in southern New England, U.S.A., have been studied using transmission electron microscopy (TEM) to measure the crystallographic orientation of the exsolution lamellae. Both the cummingtonite and the hornblende show multiple generations or stages of exsolution. Electron microprobe analyses gave compositions of the pre-exsolved (averages) and the coarsest exsolved amphiboles. In amphiboles, two orientations of lamellae are usually present and nearly parallel to {lOa} and {lOT}of the host. The observed lamellar orientations are consistent with orientations predicted by the optimal phase-boundary (OPB) theory and are a function of small differences in a, C,and {3of the host and exsolved amphiboles (~a = ahost - alamella, ~{3 = {3host - {3lamella, and ~c = Chost - Clamella)' Because these lattice parameters vary with T and composition, the precise orientations of the lamellae are controlled by these variables. The widths of determined exsolution lamellae varied from microscopic (micrometer size) to submicroscopic (nanometer size). The TEM images show, in some cases, multiple generations of lamellae with lamellar orientations near {lOT} differing up to 4.1°, whereas orientations near {lOa} differed up to 4°. For two pairs of coexisting amphiboles, the T dependence of the lattice parameters from 25 to 600 °C was measured using a Guinier camera. The OPB calculations indicate that for all the samples the different lamellar generations formed between about 780 and 300 °C (:t80 °C, on the basis of maximum errors of the lattice-parameter determinations). Lamellae of the same generation all showed nearly the same width. Exsolution temperatures could not be derived from the "100" lamellae because relatively small variations in ~c, which controls exact orientation of the "100" lamellae, could not be measured accurately enough.

Charoite from the Murun massif in Yakutiya, Russia (Vorob’ev 2008) was investigated using automated electron diffraction tomography (ADT) (Kolb et al. 2007, 2008; Mugnaioli et al. 2010) and precession electron diffraction (PED) (Mugnaioli et al. 2010, 2009), which allowed to determine the structure of charoite for the first time. The structure was solved ab initio in space group P21/m by direct methods using a fully kinematic approach. The least squares refinements with 2878 reflections F(hkl) >4s F converged to unweighted/weighted residuals R 1/wR 2 • 0.173/0.21 (Rozhdestvenskaya et al. 2010).

Amorphous gels and oxides corresponding to Ca2Mg5Si8O23 in bulk composition have been reacted to phase assemblages containing tremolitic amphibole using routine hydrothermal methods in the pressure/temperature range from 1–22 kbar and 600–875° C. The products have been characterized by X-ray diffraction, optical microscopy and high-resolution electron microscopy. The nature of the amphibole microstructure and the types of intergrown biopyriboles vary markedly as a function of synthesis temperature and pressure, reflecting different amphibole growth mechanisms. At P(H2O)

Charoite, ideally (K,Sr,Ba,Mn)15–16(Ca,Na)32[(Si70(O,OH)180)](OH,F)4·nH20, is a rock-forming mineral from the Murun massif in Yakutia, Sakha Republic, Siberia, Russia, where it occurs in a unique alkaline intrusion. Charoite occurs as four different polytypes, which are commonly intergrown in nanocrystallme fibres. We report the structure of charoite-96(a =32.11(6),b =19.77(4),c =7.23(1) Å, β = 95.85(9)°,V =4565(24) Å3, space groupP21/m),which was solvedab initioby direct methods on the basis of 2676 unique electron diffraction reflections collected by automated diffraction tomography and refined toR1/wR2=0.34/0.37. The structure of charoite-96 is related to that of the charoite-90, which was also solved recently. Both structures are composed of three different types ofdreiersilicate chains running along [001] and separated by ribbons of edge-sharing Ca- and Na-centred octahedra. In the structure of charoite-96, adjacent blocks formed by three different silicate chains and stacked along thexaxis, are shifted by a translation of 1/2c. The shifts involve a hybriddreierquadruple chain, [Si17O43]18–and a doubledreierchain, [Si6O17]10–. In charoite-90 adjacent blocks are stacked without shifts.

Charoite, ideally (K,Sr,Ba,Mn)15–16(Ca,Na)32[(Si70(O,OH)180)](OH,F)4.0·nH2O, a rare mineral from the Murun massif in Yakutiya, Russia, was studied using high-resolution transmission electron microscopy, selected-area electron diffraction, X-ray spectroscopy, precession electron diffraction and the newly developed technique of automated electron-diffraction tomography. The structure of charoite (a= 31.96(6) Å,b= 19.64(4) Å,c= 7.09(1) Å, β = 90.0(1)°,V= 4450(24) Å3, space groupP21/m) was solvedab initioby direct methods from 2878 unique observed reflections and refined toR1/wR2= 0.17/0.21. The structure can be visualized as being composed of three different dreier silicate chains: a double dreier chain, [Si6O17]10–; a tubular loop-branched dreier triple chain, [Si12O30]12–; and a tubular hybrid dreier quadruple chain, [Si17O43]18–. The silicate chains occur between ribbons of edge-sharing Ca and Na-octahedra. The chains of tetrahedra and the ribbons of octahedra extend parallel to thezaxis. K+, Ba2+, Sr2+, Mn2+and H2O molecules lie inside tubes and channels of the structure. On the basis of microprobe analyses and occupancy refinement of the cation sites, the crystal chemical formula of this charoite can be written as (Z= 1): (K13.88Sr1.0Ba0.32Mn0.36)Σ15.56(Ca25.64Na6.36)Σ32[(Si6O11(O,OH)6)2(Si12O18(O,OH)12)2(Si17O25(O,OH)18)2](OH,F)4.0·3.18H2O.

The presence of structural OH in amphiboles in excess of the usual two OH per formula has been debated for over 40 years (Gier et ah,1964; Leake et ah,1968). However, the reality of the excess-OH phenomenon is still an open question, because accurate water analyses of amphiboles are rarely available. In this study, we review the data available on the chemically simple synthetic system Na2O—MgO-SiO2-H2O (NMSH) and present new results from NMR, infrared spectroscopy, and X-ray-diffraction that allow re-interpretation of previous studies of NMSH amphiboles along the pseudobinary join between the two end-member compositions Na2Mg6Si8O22(OH)2 and Na3Mg5Si8O21(OH)3.We show that there is extensive solid solution involving excess H at 650—750°C, but also document the presence of a wide miscibility gap below 600°C. This miscibility gap is defined by amphiboles very close to the end-member composition Na3Mg5Si8O22(OH)3 coexisting with amphiboles with compositions near the ‘normal’ Na2Mg6Si8O22(OH)2 end member.We also report the characterization of triple-chain silicates (TCS) in the NMSH system and their phase relations with NMSH amphiboles. The upper thermal stability field of the key TCS Na2Mg4Si6O16(OH)2relative to its decomposition to two NMSH amphiboles with a combined equivalent composition has been determined and a pronounced backbend of the transformation boundary documented. Phase relations observed in synthesis experiments suggest that at 550—650°C all TCSs have compositions close to Na2Mg4Si6Oi6(OH)2. Infrared spectroscopy indicates that the TCS synthesized on this composition, studied in detail here, vary from end-member Na2Mg4Si6Oi6(OH)2 to binary solid solutions with less than ∼6 mol.% clinojimthompsonite component. No clear spectroscopic evidence for a ‘Drits’ component NaMg4Si6Oi5(OH)3 (Drits et al.,1975) has been found. Analysis of H2O by vacuum extraction and Karl-Fischer titration indicates large excesses of H2O in all the TCSs studied here that clearly exceed the amounts expected from (OH) groups alone. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) indicate that this excess H2O is structural. We propose that the excess H2O is likely to be molecular H2O located in the ^4-site channels. The observed backbend of the triple-chain decomposition curve is in agreement with a reaction involving dehydration and loss of this molecular H2O. However, the absolute amount of analysed molecular H2O exceeds that expected from the change in Clapeyron slope alone.While demonstrating the reality of excess OH in amphiboles, the evidence presented in this paper also points to interesting avenues for future research on both amphiboles and TCSs, such as understanding the dynamics and enhanced crystal chemistry of excess OH and molecular H2O in pyriboles.