Frank C. Hawthorne

Nomenclature of amphiboles : additions and revisions to the International Mineralogical Association's amphibole nomenclature

The introduction of a fifth amphibole group, the Na-Ca-Mg-Fe-Mn-Li group, defined by 0.50 < B(Mg,Fe2+,Mn2+,Li) < 1.50 and 0.50 ≤ B(Ca,Na) ≤ 1.50 a.f.p.u. (atoms per formula unit), with members whittakerite and ottoliniite, has been required by recent discoveries of B(LiNa) amphiboles. This, and other new discoveries, such as sodicpedrizite (which, here, is changed slightly, but significantly, from the original idealized formula), necessitate amendments to the IMA 1997 definitions of the Mg-Fe-Mn-Li, calcic, sodic-calcic and sodic groups. The discovery of obertiite and the finding of an incompatibility in the IMA 1997 subdivision of the sodic group, requires further amendments within the sodic group. All these changes, which have IMA approval, are summarized.

Nomenclature of the tourmaline-supergroup minerals

Abstract A nomenclature for tourmaline-supergroup minerals is based on chemical systematics using the generalized tourmaline structural formula: XY3Z6(T6O18)(BO3)3V3W, where the most common ions (or vacancy) at each site are X = Na1+, Ca2+, K1+, and vacancy; Y = Fe2+, Mg2+, Mn2+, Al3+, Li1+, Fe3+, and Cr3+; Z = Al3+, Fe3+, Mg2+, and Cr3+; T = Si4+, Al3+, and B3+; B = B3+; V = OH1- and O2-; and W = OH1-, F1-, and O2-. Most compositional variability occurs at the X, Y, Z, W, and V sites. Tourmaline species are defined in accordance with the dominant-valency rule such that in a relevant site the dominant ion of the dominant valence state is used for the basis of nomenclature. Tourmaline can be divided into several groups and subgroups. The primary groups are based on occupancy of the X site, which yields alkali, calcic, or X-vacant groups. Because each of these groups involves cations (or vacancy) with a different charge, coupled substitutions are required to relate the compositions of the groups. Within each group, there are several subgroups related by heterovalent coupled substitutions. If there is more than one tourmaline species within a subgroup, they are related by homovalent substitutions. Additionally, the following considerations are made. (1) In tourmaline-supergroup minerals dominated by either OH1- or F1- at the W site, the OH1--dominant species is considered the reference root composition for that root name: e.g., dravite. (2) For a tourmaline composition that has most of the chemical characteristics of a root composition, but is dominated by other cations or anions at one or more sites, the mineral species is designated by the root name plus prefix modifiers, e.g., fluor-dravite. (3) If there are multiple prefixes, they should be arranged in the order occurring in the structural formula, e.g., “potassium-fluor-dravite.”

The crystal chemistry of the M+VO3 (M+ = Li, Na, K, NH4, Tl, Rb, and Cs) pyroxenes

Abstract The crystal structures of M + VO 3 ( M + = K, NH 4 , and Cs) have been refined using three-dimensional counter-diffractometer X-ray data and full-matrix least-squares methods. The structure of these compounds is characterized by a (V 5+ O 2− 3 ) − ∞ chain extending along the c -axis ( Pbcm orientation), with adjacent chains linked by the alkali metal cation. The structure may be considered as a variant of the pyroxene structure, and standard atom nomenclature is proposed in order to facilitate comparison with silicate pyroxenes. Structural variation across this series is discussed in detail and is compared with the analogous M + M 3+ Si 2 O 6 ( M + = Li, Na; M 3+ = Al, Cr, Fe, Sc, In) series.

Structural Aspects of Oxide and Oxysalt Crystals

The goals of theoretical crystallography may be summarized as follows: (1) predict the stoichiometry of the stable compounds; (2) predict the bond topology (i.e. the approximate atomic arrangement) of the stable compounds; (3) given the bond topology, calculate accurate bond lengths and angles (i.e. accurate atomic coordinates and cell dimensions); (4) given accurate atomic coordinates, calculate accurate static and dynamic properties of a crystal. For oxides and oxysalts, we are now quite successful at (3) and (4), but fail miserably at (1) and (2). The current situation in the first two areas is briefly reviewed, prior to discussing in some detail an approach to topological aspects of structure in oxide and oxysalt crystals. The structure of a molecule or crystal may be represented by a graph, in which the vertices represent orbitals, atoms or groups of atoms, and the edges represent orbital interactions or chemical bonds. The topological characteristics of the bond network are contained in the (weighted) adjacency matrix of the graph and the corresponding eigenvalues constitute the spectrum of the graph. Simple graph theory arguments show that molecular (fundamental) building blocks are actually orbital (or energetic) building blocks, showing that there is an energetic basis for the use of fundamental building blocks in the representation

A Rietveld and infrared study of synthetic amphiboles along the potassium-richterite - tremolite join

Abstract Amphiboles were synthesized at 750 ℃, 1 kbar (H2O) for compositions at 20% intervals along the join potassium-richterite-tremolite. Structural variations, site occupancies, and modal analyses of the experimental products (amphibole + minor diopside, quartz, and enstatite) were characterized by Rietveld structure refinement, with final RBragg indices in the range 4-6%, and by infrared spectroscopy in the principal OH-stretching region. Amphibole compositions were determined by (1) site-scattering refinement for the A and M4 sites that are occupied by (K, ⃞) (⃞ = vacancy) and (Na,Ca), respectively; and (2) mass- balance calculations involving the modal analysis and the nominal experimental product composition. These measurements agree within 1% absolute and show close agreement with electron-microprobe compositions for the two samples that we could analyze. Deviations from nominal amphibole composition are up to 19% absolute. The resulting relations between cell dimension and composition are linear. The major change in cell dimensions is a decrease of 0.25 Å in a with increasing tremolite component. The infrared spectra show two principal peaks at 3735 and 3675 cm-1, corresponding to the local arrangements MgMgMg-OH-AK (the Kr band) and MgMgMg-OH-A⃞ (the Kr band), respectively. The relative variation in peak intensity as a function of amphibole composition shows that the molar absorptivities of the two bands are significantly different. The ratio of the molar absorptivities for the two bands is 2.2.

The crystal structure of ianthinite, [U24+(UO2)4O6(OH)4(H2O)4](H2O)5: a possible phase for Pu4+ incorporation during the oxidation of spent nuclear fuel

Abstract Ianthinite, [U 2 4+ (UO 2 ) 4 O 6 (OH) 4 (H 2 O) 4 ](H 2 O) 5 , is the only known uranyl oxide hydrate mineral that contains U 4+ , and it has been proposed that ianthinite may be an important Pu 4+ -bearing phase during the oxidative dissolution of spent nuclear fuel. The crystal structure of ianthinite, orthorhombic, a = 0.7178(2), b = 1.1473(2), c = 3.039(1) nm, V = 2.5027 nm 3 Z = 4, space group P 2 1 cn , has been solved by direct methods and refined by least-squares methods to an R index of 9.7% and a wR index of 12.6% using 888 unique observed [| F | ≥ 5 σ | F |] reflections. The structure contains both U 4+ . The U 6+ cations are present as roughly linear (U 6+ O 2 ) 2+ uranyl ion (Ur) that are in turn coordinated by five O 2− and OH − located at the equatorial positions of pentagonal bipyramids. The U 4+ cations are coordinated by O 2− , OH − and H 2 O in a distorted octahedral arrangement. The Ur φ 5 and U 4+ | 6 (φ: O 2− , OH − , H 2 O) polyhedra l sharing edges to for two symmetrically distinct sheets at z ≈ 0.0 and z ≈ 0.25 that are parallel to (001). The sheets have the β-U 3 O 8 sheet anion-topology. There are five symmetrically distinct H 2 O groips located at z ≈ 0.125 between the sheets of U φ n polyhedra, and the sheets of U φ n polyhedra are linked together only by hydrogen bonding to the intersheet H 2 O groups. The crystal-chemical requirements of U 4+ and Pu 4+ are very similar, suggesting that extensive Pu 4+ ↔ U 4+ substitution may occur within the sheets of U φ n polyhedra in trh structure of ianthinine.

SIMS matrix effects in the analysis of light elements in silicate minerals: Comparison with SREF and EMPA data

Abstract Matrix effects in secondary-ion mass spectrometric (SIMS) analysis of light elements (H, Li, Be, B, and F) have been investigated in phenacite, kornerupine, danburite, axinite, spodumene, tourmaline, hambergite, and mica, all of which were epoxy-mounted in a known crystallographic orientation relative to the primary-ion beam. As reference chemical information, we used data from electron microprobe analysis (EMPA) and from single-crystal structure-refinement (SREF) on the same crystals used for SIMS. Quantification of secondary-ion intensities into concentrations was done using Si as the reference matrix element. The results indicate that matrix effects due to crystallographic orientation are <10% relative, or below analytical uncertainty for most analyzed elements. In dioctahedral mica, there is a difference in H/Si ion yield (IY) of ~25% relative when the crystal is analyzed parallel and orthogonal to the main cleavage (which is perpendicular to the c axis). The magnitude of this effect is significant and higher than our SIMS accuracy for H in micas: ±10% relative. Among the analyzed elements, Be is affected least by matrix effects, even when present as a major element. The most significant chemical effects on SIMS analysis of H, Li, F, and B in silicates seem to be related to the Fe (+Mn) content of the matrix: the light-element IY decreases as the Fe (+Mn) content increases, as previously seen in tourmaline, axinite, and kornerupine. Silicon and Al seem to have complementary and opposite effects on IY with respect to Fe and Mn. The agreement between SIMS and SREF is close for most light elements when they are present as major constituents. The results of our study also show that analytical problems are still present for B by EMPA, and this technique may not be adequate to measure B accurately in some minerals.

Understanding the weakly bonded constituents in oxysalt minerals

The crystal structure of a mineral may be divided into two parts: (1) the structural unit, an array of high-bond-valence polyhedra that is usually anionic in character, and (2) the interstitial complex, an array of large low-valence cations, simple anions and (H2O) groups that is usually cationic in character. Interstitial complexes link the structural units with weak cation-anion and hydrogen bonds into a continuous structure, and the breakdown of a structure is usually controlled by the strengths of the weak bonds that link the structural units together. The interstitial complex is (usually) a complex cation, and can be characterized by its Lewis acidity, a measure of the electrophilic character of the complex. The structural unit is (usually) a complex oxyanion, and can be characterized by its Lewis basicity. The interaction between the structural unit and the interstitial complex can be examined using the principleof correspondence of Lewis acidity-basicity. If one examines a series of structures with the same structural unit, it is evident that the average coordination of the O atoms of the structural unit varies slightly from one structure to another, producing a range of Lewis basicity for this specific structural unit. In this way, a specific structural unit can be stable over a range of Lewis basicity (i.e., over a specific pH range). The formula of an interstitial complex may be written in the following way: {[m]M+a[n]M2+b · [l]M3+c(H2O)d(H2O)e(OH)f(H2O)g}(a+2b+3c–f)+, where [m], [n] and [l] are coordination numbers, a, b and c are the numbers of monovalent, divalent and trivalent cations, d is the number of transformer (H2O) groups, e is the number of (H2O) groups bonded to two interstitial cations or one interstitial cation and one hydrogen bond, f is the number of interstitial (OH) groups, and g is the number of (H2O) groups not bonded to any cation. The number of transformer (H2O) groups strongly affects the Lewis acidity of the interstitial complex, and the variation in Lewis acidity of a generalized interstitial complex can be graphically represented as a function of the number of transformer (H2O) groups. Where the Lewis acidity of a generalized interstitial complex overlaps the range of Lewis basicity of a specific structural unit, the principle of correspondence of Lewis acidity-basicity is satisfied and a stable structural arrangement is possible. Detailed predictions of the compositions of interstitial complexes are made for the borate, sulfate and uranyl-oxide-hydroxy-hydrate minerals. There is fairly close agreement between the predicted ranges of interstitial complex and those observed in Nature.

Mushroom elbaite from the Kat Chay mine, Momeik, near Mogok, Myanmar: I. Crystal chemistry by SREF, EMPA, MAS NMR and Mössbauer spectroscopy

Abstract Tourmaline from the Kat Chay mine, Momeik, near Mogok, Shan state, Myanmar, shows a variety of habits that resemble mushrooms, and it is commonly referred to as ‘mushroom tourmaline’. The structure of nine single crystals of elbaite, ranging in colour from pink to white to black and purple, extracted from two samples of mushroom tourmaline from Mogok, have been refined (SREF) to R indices of ~2.5% using graphite-monochromated Mo-Kα X-radiation. 11B and 27Al Magic Angle Spinning Nuclear Magnetic Resonance spectroscopy shows the presence of [4]B and the absence of [4]Al in samples with transition-metal content low enough to prevent paramagnetic quenching of the signal. Site populations were assigned from refined site-scattering values and unit formulae derived from electron-microprobe analyses of the crystals used for X-ray data collection. 57Fe Mössbauer spectroscopy shows that both Fe2+ and Fe3+ are present, and the site populations derived by structure refinement show that there is no Fe at the Z site; hence all Fe2+ and Fe3+ occurs at the Y site. The 57Fe Mössbauer spectra also show peaks due to intervalence charge-transfer involving Fe2+ and Fe3+ at adjacent Y sites. Calculation of the probability of the total amount of Fe occurring as Fe2+–Fe3+ pairs for a random short-range distribution is in close accord with the observed amount of Fe involved in Fe2+-Fe3+, indicating that there is no short-range order involving Fe2+ and Fe3+ in these tourmalines.

Some systematics of the garnet structure

Equations relating the positional parameters of the anion in the oxide garnets to the mean constituent ionic radii of the cations occupying the {X}, [Y], and (Z) sites have been derived from published garnet structures using multiple regression analysis: x = 0.0278(22)r {X} +0.0123(28)r [Y] −0.0482(16)r(Z) +0.0141 y = −0.0237(25)r {X} +0.0200(32)r [Y] + 0.0321(18)r(Z) +0.0523 z = −0.0102(20)r {X} +0.0305(25)r [Y] −0.0217(14)r(Z) +0.6519 Variations of mean bond lengths with constituent ionic radius are examined for the garnet structures. Deviations of mean bond length from the sum of the constituent ionic radii may be correlated with the ionic radius of the cations at the other sites in the structure.