Showing papers in "Mineralogical Magazine in 1978"

Nomenclature of amphiboles

Abstract: 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 apfu (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 is herein slightly, but significantly changed from the original idealised 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 summarised.

Feldspars and fluids in cooling plutons

Abstract: Alkali feldspars in plutonic igneous rocks vary both in their exsolution textures and in the structural state of their components. The primary factor leading to this diversity is the availability of hydrothermal fluids during their cooling history. In many plutons feldspar variation is related to degree of fractionation as indicated by rock chemistry. The variation reflects build-up of water with magmatic evolution and implies that fluids did not circulate freely in the intrusives in the temperature range of unmixing and ordering. Experimental work bearing on the role of fluids in subsolidus changes in feldspars is reviewed, and points of overlap between crystallographic studies of feldspars and stable isotope studies are discussed.

Glauconite and celadonite: two separate mineral species

Abstract: Analyses of glauconites and celadonites from continental sedimentary rocks and sea-floor basalts using X-ray diffraction, electron probe microanalysis, infra-red spectroscopy, and X-ray fluorescence spectroscopy are reported. The minerals are shown to be distinct species; each an isomorphous replacement series, glauconite having an average half unit cell of K0·85(Fe3+, Al3+)1·34 (Mg2+, Fe2+)0·66(Si3·76Al0·24)O10(OH)2 whereas celadonite approaches the ideal half unit cell of K(Fe3+, Al3+)(Mg2+, Fe2+)Si4O10(OH)2. Considerable Fe3+Alvi interchangeability occurs in the octahedral layer in both minerals and considerable substitution of aluminium in the tetrahedral layer of glauconites results in the more disordered 1Md type of structure compared with the more highly ordered 1M structure of celadonites. Some mixed layer glauconite-smectites and celadonites were also examined and could be distinguished from true glauconites and celadonites by chemical analysis, XRD, and IR techniques. It is proposed that the terms ‘glauconite’ and ‘celadonite’ should be used only for those minerals containing less than 5% interlayering.

Major element chemistry of the geothermal sea-water at Reykjanes and Svartsengi, Iceland

Abstract: High-temperature geothermal fields in Iceland represent localized anomalies of hot, altered rock in the uppermost part of the crust, which coincide with points of maximum tectonic/magmatic activity. These points correspond to the intersection of oblique fault swarms to the plate boundaries. Geothermal activity under mid-ocean ridges follows probably similar tectonic/magmatic anomalies. Due to high permeability sea-water invades the bed-rock of the Reykjanes Peninsula, Iceland, and is overlain by a variably thick lens of dilute ground water of meteoric origin. The variable degree of salinity of geothermal waters in the Reykjanes Peninsula has resulted from different degree of mixing of fresh ground water with the underlying sea-water-ground-water in the downflow zones around the geothermal fields. At Reykjanes the geothermal water represents heated sea-water without any freshwater mixing. The difference in the composition of sea-water or sea-water/fresh water mixtures and the geothermal waters is due to basalt/water interaction at elevated temperatures. The major-element chemistry of the geothermal water represents an equilibrium composition at the relevant aquifer temperatures. The activities of silica, calcium, sulphate, and carbonate are thus limited by the solubilities of quartz, anhydrite, and calcite. Fluoride activity is thought to be controlled by an ionic exchange reaction where it substitutes for hydroxyl groups in phyllosilicates. The ratios of individual cations and hydrogen ion are governed by ionic exchange equilibria with hydrothermal minerals, probably smectite and chlorite. The equilibrium pH for the Reykjanes and Svartsengi geothermal waters is 5·5 and 5·1 respectively. Sea-water will become somewhat acid upon heating to more than about 300 °C and equilibration with basalt, the acidity increasing with temperature.

Normal and anomalous tobermorites

Abstract: SUMMARY. Tobermorite minerals vary in some properlies, notably in whether or not unidimensional lattice shrinkage occurs by about 300 ~ to give a 9"3/~ form; specimens that do this are called normal, and ones that do not, anomalous. Data are compared for thirteen natural tobermorites and the extent to which normal or anomalous character is related to other properties is examined. The most definite correlations found are with chemical composition and morphology. The conditions of formation of normal and anomalous tobermorites are discussed in the light of synthetic evidence. TOBERMORITE minerals vary in some properties, most notably in whether loss of molecular water is accompanied by unidimensional lattice shrinkage in the c-direction. Such shrinkage has been observed with specimens from Crestmore, California (Taylor, I953; Farmer et al., 1966), Ballycraigy, Northern Ireland (McConnell, 1954), and Fuka, Japan (Mitsuda et al., I972), but not with those from Loch Eynort, Scotland (Gard and Taylor, 1957) or Heguri, Japan (Mitsuda, 1973). Tobermorites that show this lattice shrinkage have come to be called 'normal' and ones that do not, 'anomalous'. In this paper, data are compared for thirteen natural tobermorites, and the extent to which normal or anomalous character is related to other properties is discussed. Specimens. These are described as 14/~ or I I/~ tobermorite according to the approximate value of the layer thickness. All the specimens mentioned in this paper are I r A tobermorites unless otherwise stated. Of the specimens listed below, all were examined in the present work except numbers 4 and 12. Table I gives analyses for most of them. (l) Crestmore, California: the r4 A tobermorite originally described by M urdoch (196 I) as 'Mineral Z' and further studied by Farmer et al. {I966). (2) Bingham, Utah (USNM 127132): the 14 A tobermorite described by Stephens and Bray (I973).

The Origin of Rhythmic Layering

Abstract: Rhythmic layering in the Skaergaard intrusion shows variation in crystal size and in modal proportions of primocrysts with structural height. These changes are ascribed to variations in nucleation rates of primocrysts. The nucleation theory requires that crystallization occurs under supercooled conditions, and that the crystallization of one primocryst phase may change the composition of the magma in such a manner that the nucleation rates of other primocrysts are influenced. Both these constraints appear to have been fulfilled for the Skaergaard intrusion.

Cryptic variation in the Rhum layered intrusion

Abstract: Electron-microprobe analyses of cumulus olivine, chromite, pyroxene, and plagioclase from layered peridotites and allivalites of the Eastern and Western Layered Series of Rhum demonstrate the presence of cryptic variation Olivine varies from Fo88-78 within individual units, and there are corresponding changes in the Mg/(Mg+Fe2+) ratios in the pyroxenes and chromites Plagioclase changes are not so dramatic, but the An-content broadly follows the Mg/(Mg+Fe2+) ratio in the other minerals The most Fe-(and Na-) rich phases do not occur at the top of lithological units, but some way below The composition trend above them is reversed The data are interpreted as the result of periodic infilling of a magma chamber, the new magma mixing with the remains of the previous pulse Each pulse was followed by a period when fractional crystallization produced the layered rocks New data on Ni in the olivines suggests that the ratio of the volume of initial magma to volume of layered rocks was about four to one, the initial magma being allied to the high-calcium low-alkali tholeiitic basalts of Skye

Study of the Sector Structure in Adularia by Means of Optical Microscopy, Infra-Red Absorption, and Electron Microscopy

Abstract: Surface microtopography of habit faces, and internal textures of corresponding growth sectors of adularia from the Rhonegletscher, Switzerland, and the Seikoshi mine, Japan, were studied by means of optical polarizing microscopy, reflection interference contrast, electron microscopy, and infra-red absorption spectroscopy. Thin sections parallel to the growth plane were prepared from the specimen whose as-grown face was pasted on a glass slide. Most internal textures seen optically in the thin sections correlated well with the growth patterns for both specimens (fig. 1). Thin foils prepared by ion-thinning from the Rhonegletscher adularia show, under the electron microscope, coexisting heavily cross-hatched areas giving diffuse diffraction spots and homogeneous areas giving sharp diffraction spots. As a whole, adularia from the Seikoshi mine gives more homogeneous and much weaker cross-hatching than the Rhonegletscher specimens. Also, infra-red absorption spectra, which correlate very well with the Al/Si ordering and the optics, show that the {110} growth sector consists of triclinic disordered K-feldspar and the {001} and {01} sectors monoclinic high or low sanidine or orthoclase. These observations indicate that the characteristic internal textures seen on all optical micrographs are formed during the process of crystal growth, and are not due to monoclinic-triclinic transition. Although the transition process has taken place to a slight extent, the original growth textures remain well preserved throughout geological time. It is suggested from these observations that the degree of Al/Si ordering is mainly controlled by the different rates of growth and the different two-dimensional atomic arrangements exposed on the various growth surfaces. The higher the growth rate the more disordered are the phases that form. The symmetry of the two-dimensional atomic arrangement is kept during growth regardless of the degree of ordering; {001} and {01} growth sectors whose crystal surfaces are parallel to b-axis are monoclinic, and the {110} sector whose surface is inclined to the b-axis is triclinic.

Amphibolitic rocks from the Precambrian of Grand Canyon" mineral chemistry and phase petrology

Abstract: SUMMARY. The amphibolites and mafic schists that occur in the Precambrian metamorphic rocks of the Grand Canyon are divided into five major groups: anthophyllite and cordierite-anthophyllite rocks, early amphibolites, Granite Park mafic complex, hornblendebeating dykes, and tremolite-bearing dykes. Many types of amphibole occur in these rocks. Microprobe analyses identify gedrite, anthophyllite, cummingtonite, and grunerite, as well as three groups of calcic amphiboles. These last range in composition from colourless tremolitic or actinolitic amphiboles, through pale-green hornblende, to strongly pleochroic green-brown hornblende, which contains a significant proportion of the tschermakite and pargasite endmembers. Phase relationships between the coexisting amphiboles and other minerals are presented for two regional metamorphic events. Assemblages containing chlorite, garnet, and hornblende were formed during the first event; from the absence of staurolite, but the presence of almandine garnet and oligocla.se-andesine, it is concluded that the metamorphic grade was between the upper greenschist and the lower amphibolite facies. The second period of metamorphism produced rocks of the staurolite and sillimanite zones, within which three main 'subfacies' can be distinguished on the basis of phase relationships in the mafic schists. 'OLDER Precambrian' metamorphic and igneous rocks, i4oo to 2o00 Ma in age, are exposed within the Upper and Lower Granite Gorges of the Grand Canyon, and at isolated localities between. The total length of outcrop along the Colorado River is over 144 kin, and there is further considerable exposure within side-canyons. In recent years these rocks have been intensively studied by the author and his associates, R. S. Babcock and E. H. Brown from Western Washington State University, Bellingham, and D. E. Livingston from the University of Arizona. The crystalline rocks of the region have been divided into widespread schists and associated lithologies (the Vishnu Group), a suite of intrusive granitic rocks (the Zoroaster Plutonic Complex), a layered mafic complex (the Granite Park Mafic Body), and two distinctive units of gneiss (the

Platinum-iron alloy sample containers for melting experiments on iron-bearing rocks, minerals, and related systems

Abstract: SUMMARY. A method of producing platinum-iron alloy sample containers has been developed that allows experiments to be carried out on iron- and volatile-bearing systems with minimal net exchange of iron between the sample and its container. Preliminary experiments carried out using these containers in the temperature range from IIoo ~ to I25o ~ and at pressures up to lo kb over periods of up to 75 hours produced an average iron loss of Io~ to I6% (as FeO) compared with more than 5o% FeO loss obtained using pure platinum containers. A more judicious choice ofaUoy composition would reduce this loss still further. MELTING experiments on igneous rocks and analogous synthetic systems can provide considerable information on the crystal-liquid equilibria involved in magmatic processes. Unfortunately systems containing iron in both of its oxidation states and also containing small but significant amounts of volatile components, such as CO2 and H20, have been particularly difficult to investigate at high temperatures, largely due to the lack of a suitable container material. The loss of iron to noble metal sample containers is a well-known but poorly quantified phenomenon. Attempts to minimize iron loss to platinum have involved an unsuccessful method of saturating the container with iron from a silicate melt, adding sufficient excess iron to the sample to compensate for the subsequent iron loss (Nicholls, t974) and reducing the experiment duration to a minimum. The latter methods risk failure to achieve equilibrium, especially with respect to the distribution of the minor and trace elements between crystals and liquids. It is possible to carry out melting experiments without iron loss in container materials such as graphite (Thompson, i974) , molybdenum (Biggar, ~97o), and iron metal (O'Hara and Humphries, 1977). These containers are suitable for volatile-free systems and the low oxygen fugacities appropriate

The High-Temperature Transformation of Andalusite (A12SiO5) into 3/2-Mullite (3Al2O32SiO2) and vitreous silica (SiO2)

Abstract: The high-temperature transformation of andalusite (Al2SiO5) into 3/2-mullite (3A2O32SiO2) plus vitreous silica (SiO2) has been studied within the temperature interval from 1300 to 1600 °C by means of X-ray powder and single-crystal techniques and by infrared spectroscopy. Results are interpreted in terms of a topotactic transformation in which (011) and (01) planes of andalusite transform into (20) and (201) planes of 3/2-mullite; in this way the a- and b-axes of the two phases are interchanged. From the structural and orientational relationship between the andalusite and 3/2-mullite latrices it is concluded that the aluminium-oxygen octahedral chains, running in both structures parallel to the crystallographic c-axis, are preserved during phase transformation.

Textural evidence for liquid immiscibility in tholeiites

Abstract: The residual liquids of many tholeiitic basalts and andesites, on cooling, split into iron-rich and silica-rich fractions, which may quench to brown glassy globules and clear glass respectively. More commonly, however, cooling is sufficiently slow for the iron-rich liquid to crystallize to globular single crystals of pyroxene. Depending on the cooling rate, these crystallized globules range in shape from spheres to elongated globules bounded by crystal faces. The fine grain size of the mesostasis of most tholeiites is partly due to these small crystallized globules. The silica-rich fraction, on the other hand, is more commonly quenched to a glass, and when preserved as globules in the crystallized iron-rich fraction, it may be bounded by negative crystal faces of the surrounding pyroxene. Globules of the iron-rich liquid commonly nucleate on the surface of the plagioclase crystals where they can become trapped, later crystallizing to spherical pyroxene grains that mostly contain a minor opaque phase. In contrast, iron-rich globules that form next to pyroxene grains commonly become attached to these crystals, giving them lobate boundaries. The immiscible silica-rich liquid becomes trapped between these lobes and, with sufficiently slow cooling, results in finger-like quartzo-feldspathic inclusions extending in from the margins of the pyroxene grains. These textures can provide evidence of immiscibility in a wide range of volcanic and hypabyssal rocks, even when no glass is present.

Hercynite as a Breakdown Product of Staurolite from Within the Aureole of the Ardara Pluton, Co. Donegal, Eire

Abstract: Zincian hercynite is present in pelites within the aureole of the Ardara Pluton. The hercynite is always surrounded by cordierite, garnet, or feldspar and is never in contact with quartz. From textural and chemical evidence it is apparent that the hercynite is produced by the reaction of zincian staurolite---, zincian hercynite + cordierite + sillimanite. A temperature of about 725 ~ at pressure of about 2.6 kb and anfo~ below that defined by the Q.F.M. buffer is postulated for this reaction.

The Geochemistry of Loveringite, a Uranium-Rare-Earth-Bearing Accessory Phase from the Jimberlana Intrusion of Western Australia

Abstract: The distribution and geochemistry of loveringite, an accessory Ti, Fe_+ Cr oxide containing U and rare-earth elements (Ln) from the Jimberlana Intrusion, have been studied. Loveringite is most abundant in bronzite cumulates; it is found in trace amounts in early plagioclase-augite-hypersthene cumulates, but is not found in the olivine cumulates or in the late-stage differentiates. Loveringites from the bronzite cumulates have a high Cr content compared with those from the plagioclase-augite-hypersthene cumulates, suggesting that the mineral is stabilized by the presence of Cr in the intercumulus liquid. The Ln pattern shows a strong depletion trend from La to Eu, a sharp reversal between Eu and Tb and a second depletion pattern from Tb to Lu. This pattern suggests that the Ln are substituting into two sites, one much larger than the other.

Deweylites, Mixtures of Poorly Crystalline Hydrous Serpentine and Talc-Like Minerals

Abstract: An X-ray and chemical examination of deweylites reveals that they are intimate mixtures of very poorly ordered trioctahedral 2:1 and 1:1 layer silicates. The 2:1 mineral exhibits no swelling in the presence of ethylene glycol or water and is best described as an extremely fine-grained and highly disordered form of talc, for which the term kerolite is often used. Stevensite is not a component of deweylites. The 1:1 component most closely resembles a disordered chrysotile. Evaluation of the chemical data, including values for H2O +, shows that both end-members are hydrated to various degrees. This hydration is most probably attributable to the presence of water molecules and hydroxyls associated with the large surface areas and unbalanced surface bonds. Formulae for the end-members are approximately R 3Si4O10(OH)2·0·3 0·7 H2O and R 3Si2O3(OH)4·0·3–0·7 H2O where R is principally Mg, but samples with more or less H2O are entirely possible due to variations in crystallinity. No specific formula can be given to deweylites as they are mixtures with variable proportions of the components. The name is useful, however, as a field or ‘box’ term, similar to the use of garnierite and limonite.

Metasomatism in the Wairere Serpentinite, King Country, New Zealand

Abstract: Metasomatic zones formed at the borders of various mafic inclusions in the Wairere Serpentinite, central North Island, New Zealand, show eucritic and hornblende gabbros grading through distinct zones of altered gabbro (hydrogarnet-free) to coarse and finegrained rodingites (hydrogarnet-bearing). The hydrated, calcium-rich, aluminium-silicate mineral assemblages of each reaction zone change between the zones, with one mineral phase being altered to another at each replacement front. Initial alteration of the gabbros albitized, prehnitized, and epidotized plagioclase and chloritized hornblende and clinopyroxene. Continuing metasomatism garnetized prehnite and generated secondary amphibole and pyroxene while the original host harzburgite was highly serpentinized and, adjacent to the inclusions, the serpentinite was chloritized so as to form a further distinct zone between peridotite and gabbro. These mineral replacement reactions are reflected in the bulk chemistry of the reaction zones, which show that increasing alteration of the gabbro is accompanied by a marked increase in Ca and H2O, a possible small gain in Mg, and depletion in Si, Al, and alkalis. These trends are balanced in the host peridotite by Si enrichment and Ca loss upon serpentinization. Clinopyroxenes and hornblendes of the zones show an increase in Ca and Mg and a decrease in Si with increasing metasomatic alteration. Garnets also decrease in Si but chlorites show an increase in both Si and Mg. Additional minerals analysed from both primary and altered rocks include olivine, orthopyroxene, biotite, plagioclase, pectolite, rosenhahnite, sphene, and chrome spinel. The zones are regarded as the result of reaction between the inclusions and Ca- and Al-rich solutions derived, in turn, by action of alkaline, CO2-poor solutions, which percolated through fractures in the original host peridotite and, in the process of serpentinization, released Mg and enhanced Ca and Al levels. At the contact of the mafic inclusions, chemical gradients were generated down which various chemical species diffused to different levels forming the zonation. The reactions probably took place in the vicinity of 260 to 350 °C, 2 to 3 kb, and at constant volume. Cataclasis and brecciation of the outer zones indicate more than one metasomatic episode.

Microprobe Analyses of Cadmium-Rich Tetrahedrites from Tyndrum, Perthshire, Scotland

Abstract: removal of some of the sulphur from the site of alteration. Nevertheless, the general pattern of elemental gain or loss during alteration is probably portrayed reasonably well by the equations. A feature common to all equations is that the alteration requires an over-all addition of iron. The metals released (nickel and manganese), however, appear to be accommodated in the alteration assemblage. Nickel released by the alteration of pentlandite is probably consumed by the formation of nickelian mackinawite from alabandine. Manganese released from the alteration of alabandine is probably consumed by the precipitation of the carbonate phase. The volumetric proportions of the alteration products empirically suggest that the expelled nickel and manganese are mainly redistributed locally without net removal from the system. It seems likely therefore that alteration is induced by high iron activity but postulation of a mechanism for concentrating iron under these conditions remains a problem. The unusual mode of occurrence of the pyrite and pyrrhotine in this deposit could perhaps be significant. There is good evidence to suggest that the pyrrhotine monosulphide was derived from pre-existing pyrite by contact metamorphism. Much of the pyrrhotine so formed is thought to have been subsequently re-pyritized. Where areas ofpyrrhotine are pervasively replaced by pyrite, the

Pumpellyite-bearing basic igneous rocks from the Lower Ordovician of North Pembrokeshire, Wales

Abstract: The first occurrence of pumpellyite in the Lower Ordovician volcanic rocks of North Pembrokeshire is reported and its form and chemistry are described. The significance of pumpellyite in relation to the grade of metamorphism attained during the Caledonian orogenic episode is discussed.

A Method for Preparing Doubly Polished Thin Sections Suitable for Microthermometric Examination of Fluid Inclusions

Abstract: MICROSCOPIC studies of fluid inclusions in minerals are best made on thin sections polished on both sides. Previous workers have used sections about I mm in thickness for routine studies (Roedder, 1976; Brumby and Shepherd, I976 ), prepared using conventional polishing laps. However, as pointed out by Schwartzkopff et al. (I974) serious temperature gradients arise when sections of this thickness are examined using the heating and freezing stage, leading to anomalously high results for inclusion filling temperatures. For this reason, as well as for ease of microscopic observation, it is much better to use a thinner section. The optimum thickness clearly depends to some extent upon the size range and density of occurrence of the fluid inclusions: for inclusions in the common size range 5-3o #m, for example, a section 6o/~m in thickness is appropriate. Sections thinner than this are very fragile when removed from their supporting glass slides and consequently difficult to transfer to and from the microscope stage. The method outlined here has been in routine use for some time to produce high quality, doublypolished sections 6o /~m in thickness, and has proved extremely satisfactory. Twenty-four finished sections can be prepared in three days. The polishing is performed using 'Vibromet' vibrating polishers, which utilize aluminium oxide slurries, a procedure that avoids the premature heating of the specimen that can be occasioned by the use of lead polishing laps, for example, and which can cause modification or even bursting of the fluid inclusions.

The Crystal Structure of Stilpnomelane. Part III: Chemistry and Physical Properties

Abstract: Full chemical analysis, cell dimensions, γ refractive index, density, and infra-red absorption spectrum have been determined for fourteen stilpnomelanes. Coupled with data from previously described material and electron probe analyses of three further samples, the new information shows that γ = 1·616+0·0047 Fe3+ −0·0037 Mg; substitution of Mn for Fe appears to have little effect on γ. For ferrostilpnomelane, D = 2·85−0·011 Mg, for ferristilpnomelane, D = 2·89−0·011 Mg. Cell dimensions clearly distinguish ferro-, ferri-, and man-ganstilpnomelanes; for ferri-stilpnomelane a = 22·03−0·00446 Mg−0·00786 Fe3+ (A). A convenient approximation to the structural formula using a ⅛ subcell, is K0·6(Mg, Fe2+Fe3+)6Si8Al(O,OH)27 2–4 H2O.

Tin-Rich Garnet, Pyroxene, and Spinel from a Slag

Abstract: Garnet containing up to 26% SnO2, and pyroxene and spinel containing i2~ and I0% SnOz respectively, occur in a tin-refining slag in association with cassiterite. These tin contents probably represent minimum values tbr the limit of ionic replacement of Sn 4 + in the structure of these minerals. The absence of tin from coexisting melilite suggests that Sn 4 ~ is onIy able to substitute for ions in octahedral co-ordination.

The Identification of Sulphide Minerals by Infra-Red Spectroscopy

Abstract: THE value of infra-red spectra in identifying sulphide minerals has been assessed by surveying the spectra of some forty specimens. Spectra of finely ground samples dispersed in polyethylene discs were obtained in the region 420-9o cm-1 with a resolution of 3-4 cm-1, using a Beckman-RIIC Fourier-Transform Interferometer. Except for minerals whose metallic conductivity obliterated vibrational features, the spectra permitted rapid recognition of sulphide minerals either alone or in mixtures. Spectra of the following twenty-five pure, or nearly pure specimens are presented: cinnabar, galena, pyrrhotine, alabandine, sphalerite, wurtzite, realgar, orpiment, stibnite, bismuthinite, arsenopyrite, tetrahedrite, pyrargyrite, proustite, enargite,

Welshite, Ca2Mg4Fe3+Sb5+O2[Si4Be2O18], a new member of the aenigmatite group

Abstract: Welshite, triclinic, Ca2Mg4Fe3+Sb5+O2 [Si4Be2O18], a = 10·28 A, b = 10·69 A, c = 8·83 A, α = 106·1°, β = 96·3°, γ = 124·8°, P¯1 with pseudo-monoclinic cell a m = 9·68(1) A, b m = 14·77(2) A, c m = 5·14(1) A, βm = 101·5(2)°, c-centred, occurs as lustrous subadaman-tine, thick prismatic reddish-black crystals up to 3 mm in greatest dimension associated with romeite, adelite, swedenborgite, berzeliite, manganoan phlogopite, richterite in crystalline dolomite from Langban, Sweden. Forms include b {010}, a{100}, k{130}, m{110}, j{021}, p{111}, q{121}, v{131}, r{¯111}, s{¯121}, t{¯131}, and v{¯141} (monoclinic setting). Streak pale-brown, hardness 6, specific gravity 3·77, fracture conchoidal, no cleavage, α = 1·81(1), γ = 1·83(1), 2E(obs.) ≈45°, no noticcable pleochroism. Welshite is a new member of the aenigmatite-rhonite group and obtains from the substitution Sb5+→ Ti4+ and Be2+ → Al3+ in rhonite.