Olaf Medenbach

Refractive Index and Dispersion of Fluorides and Oxides

The refractive indices of 509 oxides and 55 fluorides were analyzed using two forms of a one-term Sellmeier equation: (1) 1/(n2−1)=−A/λ2+B, where A, the slope of the plot of (n2−1)−1 versus λ−2 in units of 10−16 m2, gives a measure of dispersion and B, the intercept of the plot at λ=∞, gives n∞=(1+1/B)1/2 and (2) n2−1=EdEo/(Eo2−(ℏω)2), where ℏω=the photon energy, Eo=the average single oscillator (Sellmeier) energy gap, and Ed=the average oscillator strength, which measures the strength of interband optical transitions. Form (1) was used to calculate n at λ=589.3 nm (nD) and n at λ=∞ (n∞), and the dispersion constant A. The total mean polarizabilility for each compound was calculated using the Lorenz–Lorentz equation: αe=3/4π [(Vm) (n∞2−1)/(n∞2+2)], where Vm is the molar volume in A3. Provided for each compound are: nD, n∞, Vm, 〈αe〉, 〈A〉, 〈B〉, 〈Ed〉, 〈Eo〉, the literature reference, the method of measurement of n and estimated errors in n. Results obtained by prism, infrared reflectivity, ellipsometry, and i...

Ellenbergerite, a new high-pressure Mg-Al-(Ti,Zr)-silicate with a novel structure based on face-sharing octahedra

Ellenbergerite occurs as purple millimetre-size grains associated with talc, kyanite, clinochlore, rutile, and zircon in composite inclusions within decimetre-large pyrope crystals (90–98 mole percent end-member) in the quartzite layer of the Dora Maira massif, Western Alps, from which coesite has been recently reported (Chopin 1984). It is hexagonal, a=12.255(8), c=4.932(4) Å, Z=1, space group P63. Mohs hardness 6.5; Dmes 3.15, Dcal 3.10; no cleavage. Uniaxial negative and vividly pleochroic,ω colourless,ε colourless to deep lilac with colour zoning. The intensely coloured variety hasω 1.6789(5),ε 1.670(1); microprobe analysis yields SiO2 39.1, P2O5 0.45, Al2O3 25.1, TiO2 4.0, MgO 22.2, FeO 0.20, sum 99.05 wt.% including H2O 8.0 (coulometrically). The formula calculated on a O28(OH)10 basis (implying 7.5 wt.% H2O) is Mg6.71 Fe0.03 Ti0.61 Al6.00 Si7.92 P0.08 O28(OH)10 The colour zoning is due to nearly complete Ti⇌Zr substitution. In addition ellenbergerite may contain more than 8 wt.% P2O5 with strictly correlated changes of Si, Mg, Al and Ti+Zr contents, over 80% of which represent the SiAl⇌PMg substitution.The structure has been determined from 1049 observed independent reflections and refined to R=0.034, Rw=0.031, including six of ten protons. It consists of single chains of face-sharing octahedra with one third vacancies extending along the six-fold screw axes, and of pairs of fully occupied face-sharing octahedra linked by edge-sharing to form octahedral double chains parallel to the twofold screw axes, all interconnected by SiO4 tetrahedra. It may be compared with the dumortierite polymorph with space group P63mc derived hypothetically by Moore and Araki (1978). The structural formula is (Mg,Ti,Zr,□)2 Mg6(Al,Mg)6 (Si,P)2 Si6 O28(OH)10 Face-sharing octahedra are an unusual feature in silicates which results in a dense structure and reflects, considering the common bulk composition, the uncommon high-pressure formation conditions (about 25–30 kbar, 700–800° C). Ti4+-Fe2+ charge transfer between face-sharing octahedra on the six-fold screw axes most likely accounts for the absorption scheme.

Manganvesuvianite and tweddillite, two new Mn3+-silicate minerals from the Kalahari manganese fields, South Africa

Abstract The new minerals manganvesuviante and tweddillite, both formed by hydrothermal alteration of primary manganese ores, are described from the Kalahari manganese fields (Republic of South Africa). In addition, single-crystal X-ray structure refinements of both new minerals are presented. Manganvesuvianite is a tetragonal vesuvianite mineral with the simplified formula Ca19Mn3+(Al,Mn3+,Fe3+)10(Mg,Mn2+)2Si18O69(OH)9, characterized by Mn3+ occupying the five-coordinated position (square pyramid). The crystals have simple prismatic forms: {100}, {110} terminated by {101} and exhibit deep maroon red colour. With polarized light the crystals are strongly pleochroic, yellowish parallel to E and dark red to lilac parallel to O. Tweddillite is an epidote-groupmineral (space group space group P21/m, a = 8.932(5), b = 5.698(4), c = 10.310(5) Å, β = 114.56(4), V = 477.3(8) Å3) with the simplified formula CaSr(Mn3+,Fe3+)2Al[Si3O12](OH), closely related to strontiopiemontite. The difference between strontiopiemontite and tweddillite is the concentration of octahedral Mn3+. Strontiopiemontite has Mn3+ mainly on the M3 site whereas tweddillite has Mn3+ with minor Fe3+ on M3 and M1. Tweddillite forms aggregates of very thin dark red {001} blades characterized by striking pleochroism. The crystals appear dark red parallel to b and orange-yellow parallel to a. Perpendicular to (001) the blades appear magenta to red.

Ti-poor hoegbomite in kornerupine-cordierite-sillimanite rocks from Ellammankovilpatti, Tamil Nadu, India

Hoegbomite occurs sparingly in minute (mostly 0.1 mm) grains with fine-grained hercynite, magnetite, and rutile in two coarse-grained kornerupine-cordierite-sillimanite rocks from Ellammankovilpatti, Tamil Nadu, India. The hoegbomite is Ti-poor (2.5 wt% TiO2), Fe-rich (25–26% Fe as FeO), and contains 6.2–6.8% MgO, 59.8–60.1% Al2O3, 1.0–1.3% ZnO, 0.3–0.7% Cr2O3 and 0.02% Li2O. Minor amounts (estimated not to exceed 0.2 wt% oxide) of V, Co, Ni, Ga, and Sn were detected on the electron microprobe, but Be, Nb, and Zr were not detected with the ion microprobe mass analyser. Assuming the crystal structure refined by Gatehouse and Grey (1982) to be applicable to the Ellammankovilpatti hoegbomite, the analyses were recalculated on a basis of 22 cations, 30 oxygens, and two hydroxyls, resulting in 49 to 53% of the iron being ferric. Identification of hoegbomite was confirmed by X-ray powder diffraction. Associated cordierite (Fe/(Fe+Mg)=0.14) and kornerupine (Fe/(Fe+Mg)= 0.27) contain 0.02 weight % Li2O and 0.05–0.07% BeO, while only the kornerupine contains B2O3 — 1.57% (ion microprobe analyses). Hoegbomite and the other oxides may have crystallized at temperatures between 680 and 720° C (P≈6.5 kbar) following attainment of peak conditions by the reaction: kornerupine+sillimanite±rutile+ZnO+H2O+O2 =cordierite+chlorite+hercynite+hoegbomite +magnetite+B2O3.The conditions for hoegbomite formation at Ellammankovilpatti appear to be characteristic of many hoegbomite parageneses. Critical for hoegbomite are silica undersaturation and relatively high oxygen and water activities at fairly high temperatures, conditions which are most commonly attained in later phases of a metamorphic cycle in upper amphibolite- and granulite-facies terrains.

Eifelite, KNa3Mg4Si12O30, a new mineral of the osumilite group with octahedral sodium

Eifelite of variable composition is uniaxial positive withn0 near 1.543 andne near 1.544, a between 10.14 and 10.15 Å, andc about 14.22 Å, space groupP 6/m 2/c 2/c. There is a complete series of solid solution between the eifelite end member KNa3Mg4Si12O30 and roedderite, KNaMg5Si12O30, following the 2 Na⇌Mg substitution. Both eifelite and roedderite have milarite-type structures, but Na is always in six-coordinated sites: In roedderite Na occupies solely a newly defined B′[6]-position which is slightly displaced alongc from the ideal B[9]-position lying on the (001/2)-mirror plane in K2Mg5Si12O30. In eifelite Na is located both inB′[6] and in theA[6]-positions, where it partially replaces Mg. Eifelite has the highest cation occupancy of all osumilite group minerals known thus far.Both eifelite and roedderite occur in vesicles of contact metamorphosed basement xenoliths ejected with the leucite tephrite lava of the Quaternary Bellerberg volcano in the Eifel, West Germany. They are considered to be precipitates from highly alkaline, MgSi-rich, but Al-deficient gas phases that originated through interaction of gaseous igneous differentiates with the xenoliths.

Zn-rich högbomite formed from gahnite in the metabauxites of the Menderes Massif, SW Turkey

Gahnite, ZnAl2O4, present as an accessory mineral in regionally metamorphosed low-grade diasporites, has reacted in adjacent higher-grade, corundum-bearing metabauxite equivalents (emeries) to form Zn-rich högbomite, (Zn,Fe2+,Mg,Ni)t-2x (Ti,Sn)xAl2O4, of the 4H polytype. Commonly, the initial högbomite crystals grew epitactically along the octahedral faces of gahnite, which was subsequently dissolved, so that högbomite now forms spectacularly intergrown sets of eight crystals in perfect crystallographic orientation to each other. This indicates a metamorphic reaction, probably involving a fluid, transporting mainly the elements Zn and Al. Reactant Ti minerals in the diasporites were rutile and titanian hematite (10–15 mol% FeTiO3). In the emeries högbomite coexists with still more Ti-rich hematites containing between 26 and 37 mol% FeTiO3. The overall reaction relations involving partial reduction may be subdivided into the intial univariant reaction, gahnite+diaspore+Ti-hematite+rutile=högbomite+H2O+O2. This was followed, in the absence of gahnite, by compositional readjustments of högbomite and Ti-hematite and the appearance of magnetite. Core to rim zoning profiles indicate that, with continued growth, the högbomite crystals became poorer in Zn and Ti, but richer in Fe2+, while the Ti-contents of coexisting hematite increased. Högbomite formation at the expense of gahnite started at temperatures as low as about 400° C for an estimated pressure of 5–6 kbar.

Kulkeite, a new metamorphic phyllosilicate mineral: Ordered 1∶1 chlorite/talc mixed-layer

Kulkeite occurs as platy, colorless, porphyroblastic, single crystals up to 2 mm in size in a low-grade dolomite rock associated with a Triassic meta-evaporite series at Derrag, Tell Atlas, Algeria, It is associated with sodian aluminian talc, unusual chlorite polytypes, and both K and Na phlogopite. Kulkeite is optically biaxial, negative, nx=1.552, ny=1.5605, nz=1.5610, 2Vz=24° (obs.). Based on microprobe analysis the empirical formula is (Na0.38K0.01Ca0.01)(Mg8.02Al0.99)[Al1.43Si6.57O20](OH)10 with some variation in Na, Si, and tetrahedral Al. The crystals are monoclinic with a=5.319(1), b=9.195(2), c=23.897(10) Å, β=97° 1(3)′; Z=2; the calculated density is 2.70 g cm−3. The four strongest lines in the X-ray powder pattern are (d, I, hkl): 7.90, 100, 003; 1.533, 100, 060; 7.42, 80, 002; 3.38, 80, 007; the 001 reflection with 23.7 Å has intensity 10.Transmission electron microscopy confirms the nature of a regular 1∶1 mixed-layer, which consists of 14 Å chlorite (clinochlore) sheets alternating with sheets of one-layer (9.5 Å) talc characterized by the lattice substitution NaAl→Si just as in the talc occurring as a discrete mineral co-existing with kulkeite. Kulkeite is intergrown with lamellae of clinochlore that represent two-layer and five-layer (70 Å) polytypes with optical birefringence exceeding the normal value for clinochlore by a factor of 3.The origin of kulkeite is due to low-grade metamorphism with temperatures probably not exceeding 400° C. As the clinochlore lamellae and sodian aluminian talc are found in mutual contact, kulkeite seems to represent a metastable mineral at least during the latest phase of metamorphism. However, at an earlier stage, prior to clinochlore formation, kulkeite might have been stable, and the incorporation of Na and Al into its talc component could indeed be the decisive factor for the formation of the mixed-layer.

Olmiite, CaMn[SiO3(OH)](OH), the Mn-dominant analogue of poldervaartite, a new mineral species from Kalahari manganese fields (Republic of South Africa)

Abstract Olmiite, ideally CaMn[SiO3(OH)](OH), is a newly identified mineral from the N’Chwaning II mine of the Kalahari manganese fields (Republic of South Africa), which occurs as a product of hydrothermal alteration associated with poldervaartite, celestine, sturmanite, bultfonteinite and hematite. The mineral occurs as wheat-sheaf aggregates consisting of pale to intense reddish pink minute crystals. Olmiite is transparent with vitreous lustre, and exhibits deep-red fluorescence under short-wave UV-light. The mineral is brittle, with irregular fracture. Streak is white and Mohs hardness is 5-5½ No cleavage was observed. The measured density (pycnometer method) is 3.05(3) g/cm3. The calculated density is 3.102 g/cm3 or 3.109 g/cm3 using the unit-cell volume from single-crystal or powder data, respectively. Olmiite is biaxial positive, with refractive indices α = 1.663(1), β = 1.672(1), γ = 1.694(1) (589 nm), 2Vmeas= 71.8(1)°, 2Vcalc = 66(8)°. The optical orientation is X - a, Y - c, Z = b and dispersion: r > v, distinct. Pleochroism is not observed. Chemical analysis by electron microprobe yielded the chemical formula (Ca2-xMnxFey)[SiO3(OH)](OH), with 0.84 ≤ x ≤ 0.86, and y ≤ 0.01. Olmiite is orthorhombic, space group Pbca, with a - 9.249(3), b = 9.076(9), c = 10.342(9) Å, V = 868(1) Å3 and Z = 8. The strongest five powder-diffraction lines [d in Å, (I/Io), hkl] are: 4.14, (45), 021; 3.19, (100), 122; 2.807, (35), 311; 2.545 (35), 312; 2.361, (40), 223. Single-crystal structure refinement (R1 = 2.74% for 1012 observed reflections) showed that the atomic arrangement of olmiite is similar to that of poldervaartite, with all Mn ordered on the Ml site. Significant variations in bond distances and angles are related to the pronounced difference in the Mn content. Olmiite, therefore, is the Mn-dominant analogue of poldervaartite. The name poldervaartite should be reserved for samples having Ca dominant at the M2 site.

Boromullite, Al9BSi2O19, a new mineral from granulite-facies metapelites, Mount Stafford, central Australia: a natural analogue of a synthetic "boron-mullite"

Boromullite is a new mineral corresponding to a 1:1 polysome composed of Al5BO9 and Al2SiO5 modules. Electron-microprobe analysis of the holotype prism is SiO2 19.01(1.12), TiO2 0.01(0.02), B2O3 6.52(0.75), Al2O3 74.10(0.95), MgO 0.07(0.03), CaO 0.00(0.02), MnO 0.01(0.04), FeO 0.40(0.08), Sum 100.12 wt.%, which gives Mg0.01Fe0.03Al8.88Si1.93B1.14O18.94 (normalised to 12 cations), ideally Al9BSi2O19. Overall, in the type specimen, it ranges in composition from Mg0.01Fe0.03Al8.72Si2.44B0.80O19.20 to Mg0.01Fe0.03Al9.22Si1.38B1.35O18.67. Single-crystal X-ray diffraction gives orthorhombic symmetry, Cmc21, a 5.7168(19) Å, b 15.023(5) Å, c 7.675(3) Å, V 659.2(7) Å3, calculated density 3.081 g/cm3, Z = 2. The refined structure model indicates two superimposed modules present in equal proportions in the holotype prism. Module 1 has the topology and stoichiometry of sillimanite and carries all the Si, whereas module 2 is a type of mullite defect structure in which Si is replaced by B in triangular coordination and by Al in tetrahedral coordination, i.e., Al5BO9. The strongest lines in the powder pattern [d in Å, (Imeas.), (hkl)] are 5.37(50) (021), 3.38(100) (022, 041), 2.67 (60) (042), 2.51(60) (221, 023), 2.19(80) (222), 2.11(50) (043), 1.512(80) (263). Boromullite is colourless and transparent, biaxial (+), nx 1.627(1), ny 1.634(1), nz 1.649(1) (589 nm). 2Vz (meas) = 57(2)◦, 2Vz (calc) = 69(12)◦. In the type specimen boromullite tends to form prisms or bundles of prisms up to 0.4 mm long, typically as fringes or overgrowths on aggregates of sillimanite or as narrow overgrowths around embayed werdingite prisms. In other samples boromullite and sillimanite are intergrown on a fine scale (from < 1 μm to > 10 μm). Sekaninaite-cordierite, potassium feldspar, biotite, werdingite and its Fe-dominant analogue, hercynite, and ilmenite are other commonly associated minerals, whereas ominelite-grandidierite, plagioclase, andalusite, and tourmaline are much subordinate. The most widespread accessories are monazite-(Ce), an apatite-group mineral and zircon. Boromullite formed during anatexis of B-rich pelitic rocks under granulite facies conditions (810 ◦C ≈ T ≥ 775−785 ◦C, P = 3.3–4 kbar), possibly due to a shift in bulk composition to lower SiO2 and B2O3 contents associated with melt extraction. The assemblage boromullite + cordierite + sillimanite lies at lower SiO2 and B2O3 contents than the assemblage werdingite + cordierite + sillimanite and thus a decrease in SiO2 and B2O3 leads to the replacement of werdingite by boromullite, consistent with textural relations. Key-words: boron, new mineral, Australia, electron microprobe, crystal structure, granulite facies, anatexis, boromullite.

Stornesite-(Y), (Y, Ca)□2Na6(Ca,Na)8(Mg,Fe)43(PO4)36, the first terrestrial Mg-dominant member of the fillowite group, from granulite-facies paragneiss in the Larsemann Hills, Prydz Bay, East Antarctica

Abstract Stornesite-(Y), end-member formula Y⃞2Na6(Ca5Na3)Mg43(PO4)36, is a new Y-dominant analog of the meteoritic mineral chladniite. A representative electron microprobe analysis is SiO2 = 0.02, P2O5 = 48.11, SO3 = 0.05, MgO = 23.16, MnO = 0.24, FeO = 15.55, Na2O = 5.04, CaO = 5.66, SrO = 0.02, Y2O3 = 1.43, Yb2O3 = 0.24, UO2 = 0.01, Sum = 99.53 wt%, which gives Y0.68Yb0.06Na8.69Ca5.40Sr0.01Mg30.71Fe11.56 Mn0.18Si0.02S0.04P36.22O144. Overall, Y + REE range from 0.542 to 0.985 atoms per formula, and atomic Mg/(Mg + Fe) ratio from 0.684 to 0.749. Single-crystal X-ray diffraction gives trigonal symmetry, R3̅, a = 14.9628(27) Å, c = 42.756(11) Å, V = 8290(4) Å3, calculated density = 3.196 g/cm3, Z = 3. The mineral is isostructural with synthetic chladniite, but the (0, 0, 0) site is dominantly occupied by Y instead of Ca. Bond lengths are considerably shorter than for Ca sites; Y and Yb are fully ordered at this site, which is our rationale for recognizing stornesite-(Y) as a distinct species. The strongest lines in the powder pattern [d in Å, (I), (hkl)] are 3.67 (40) (0 3 6, 3 0 6), 3.52 (40) (0 0 12, 3 1 2, 1 3 2̅), 2.94 (60) (0 1 14, 3 2 2̅, 2 3 2), 2.73 (100) (2 0 14, 0 3 12, 3 0 12), 1.84 (40) (1 5 14, 5 11̅4̅ , 0 6 12, 6 0 12). The mineral is optically uniaxial +, nω = 1.6215(10) and nε = 1.6250(10) at 589 nm. Its color is pale yellow in standard thin sections. Stornesite-(Y) is found as inclusions in fluorapatite nodules in two paragneiss specimens from Johnston Fjord, Stornes Peninsula (whence the name) and in a third from Brattnevet, Larsemann Hills. Associated minerals are wagnerite, xenotime-(Y), monazite-(Ce), P-bearing K-feldspar, biotite, sillimanite, quartz, and pyrite; it is commonly altered to rusty material and secondary phosphates. Grains are anhedral, subhedral, or locally euhedral with hexagonal or rhombic outlines; maximum dimensions are 1 × 0.25 mm. It is inferred to have formed at 800.860 °C, 6.7 kbar by reaction of biotite with an anatectic melt locally enriched in P by interaction with fluorapatite.