A Dihydroxo-Bridged Ferric Dimer

Recent investigations have produced a large number of dimeric complexes containing the oxo-bridged structural unit Fe_2O^(4+). Here we report the isolation and characterization of [Fe (pic)_2OH]_2 which apparently is the first example of a crystalline iron(III) dimer which utilizes dihydroxo bridging in its coordination structure

Camaronesite, [Fe^(3+)(H_2O)_2(PO_3OH)]_2(SO_4)•1-2H_2O, a new phosphate-sulfate from the Camarones Valley, Chile, structurally related to taranakite

Camaronesite (IMA 2012-094), [Fe^(3+)(H_2O)_2(PO_3OH)]_2(SO_4)•1-2H_2O, is a new mineral from near the village of Cuya in the Camarones Valley, Arica Province, Chile. The mineral is a low-temperature, secondary mineral occurring in a sulfate assemblage with anhydrite, botryogen, chalcanthite, copiapite, halotrichite, hexahydrite, hydroniumjarosite, pyrite, römerite, rozenite and szomolnokite. Lavender-coloured crystals up to several mm across form dense intergrowths. More rarely crystals occur as drusy aggregates of tablets up to 0.5 mm in diameter and 0.02 mm thick. Tablets are flattened on {001} and exhibit the forms {001}, {104}, {015} and {018}. The mineral is transparent with white streak and vitreous lustre. The Mohs hardness is 2½, the tenacity is brittle and the fracture is irregular, conchoidal and stepped. Camaronesite has one perfect cleavage on {001}. The measured and calculated densities are 2.43(1) and 2.383 g/cm^3, respectively. The mineral is optically uniaxial (+) with ω = 1.612(1) and ε = 1.621(1) (white light). The pleochroism is O (pale lavender) > E (colourless). Electron-microprobe analyses provided Fe_2O_331.84, P_2O_529.22, SO_315.74, H_2O 23.94 (based on O analyses), total 100.74 wt.%. The empirical formula (based on 2 P a.p.f.u.) is: Fe_(1.94)(PO_3OH)_2(S_(0.96)O_4)(H_2O)_4•1.46H_2O. The mineral is slowly soluble in concentrated HCl and extremely slowly soluble in concentrated H_2SO_4. Camaronesite is trigonal, R32, with cell parameters:a = 9.0833(5), c = 42.944(3) Å, V = 3068.5(3) Å3 and Z = 9. The eight strongest lines in the X-ray powder diffraction pattern are [d_(obs) Å (I)(hkl)]: 7.74(45)(101), 7.415(100)(012), 4.545(72)(110), 4.426(26)(018), 3.862(32)(021,202,116), 3.298(93)(027,119), 3.179(25)(208) and 2.818(25)(1•1•12,125). In the structure of camaronesite (R_1 = 2.28% for 1138 F_o > 4σF), three types of Fe octahedra are linked by corner sharing with (PO_3OH) tetrahedra to form polyhedral layers perpendicular to c with composition [Fe^(3+)(H_2O)_2(PO_3OH)]. Two such layers are joined through SO_4 tetrahedra (in two half-occupied orientations) to form thick slabs of composition [Fe^(3+)(H_2O)_2(PO_3OH)]_2(SO_4). Between the slabs are partially occupied H_2O groups. The only linkages between the slabs are hydrogen bonds. The most distinctive component in the structure consists of two Fe octahedra linked to one another by three PO_4 tetrahedra yielding an [Fe_2(PO_4)_3] unit. This unit is also the key component in the sodium super-ionic conductor (NASICON) structure and has been referred to as the lantern unit. The polyhedral layers in the structure of camaronesite are similar to those in the structure of taranakite. The Raman spectrum exhibits peaks consistent with sulfate, phosphate, water and OH groups

Vesuvianite From Pajsberg, Sweden, and the Role of Be In the Vesuvianite Structure

Vesuvianite from Pajsberg, Sweden contains about one atom of Mn, based on 50 cations per formula unit, and small amounts of Be, B, and As. Optical absorption analysis suggests that the Mn is predominantly or entirely trivalent. Crystal-structure analysis indicates that Mn is housed at the general octahedral site Y3, which exhibits only minor distortion from ideal octahedral symmetry. Arsenic is housed at Y2 and Z2, and the formula derived from electron microprobe and LA-ICP-MS analyses suggests minor substitution of Al for Si, also at Z2. Beryllium and B are at T1, between the edge-sharing trimers Y3Y2Y3, as is the case for B in the boron-dominant vesuvianite species wiluite. The total content at T1 is interpreted as 0.82Be, 0.34B, and 0.037Fe^(3+)

Joteite, Ca_2CuAl[AsO_4][AsO_3(OH)]_2(OH)_2•5H_2O, a new arsenate with a sheet structure and unconnected acid arsenate groups

Joteite (IMA2012-091), Ca_2CuAl[AsO_4][AsO_3(OH)]_2(OH)_2•5H_2O, is a new mineral from the Jote mine, Tierra Amarilla, Copiapó Province, Atacama, Chile. The mineral is a late-stage, lowtemperature, secondary mineral occurring with conichalcite, mansfieldite, pharmacoalumite, pharmacosiderite and scorodite in narrow seams and vughs in the oxidized upper portion of a hydrothermal sulfide vein hosted by volcanoclastic rocks. Crystals occur as sky-blue to greenish-blue thin blades, flattened and twinned on {001}, up to ~300 µm in length, and exhibiting the forms {001}, {010}, {110}, {210} and {111}. The blades are commonly intergrown in wheat-sheaf-like bundles, less commonly in sprays, and sometimes aggregated as dense crusts and cavity linings. The mineral is transparent and has a very pale blue streak and vitreous lustre. The Mohs hardness is estimated at 2 to 3, the tenacity is brittle, and the fracture is curved. It has one perfect cleavage on {001}. The calculated density based on the empirical formula is 3.056 g/cm^3. It is optically biaxial (–) with α = 1.634(1), β = 1.644(1), γ = 1.651(1) (white light), 2V_(meas) = 78(2)° and 2V_(calc) = 79.4°. The mineral exhibits weak dispersion, r Y (pale greenish blue) > X (colourless). The normalized electron-microprobe analyses (average of 5) provided: CaO 15.70, CuO 11.22, Al_2O_38.32, As_2O_546.62, H_2O 18.14 (structure), total 100 wt.%. The empirical formula (based on 19 O a.p.f.u.) is: Ca_(1.98)Cu_(1.00)Al_(1.15)As_(2.87)H_(14.24)O_(19). The mineral is slowly soluble in cold, concentrated HCl. Joteite is triclinic, P1, with the cell parameters: a = 6.0530(2), b = 10.2329(3), c = 12.9112(4) Å, a = 87.572(2), b = 78.480(2), g = 78.697(2)°, V = 768.40(4) Å^3 and Z = 2. The eight strongest lines in the X-ray powder diffraction pattern are [d_(obs) Å (I)(hkl)]: 12.76(100)(001), 5.009(23)(020), 4.206(26)(120,003,121), 3.92(24)(022,022,102), 3.40(25)(113), 3.233(19)(031,023,123,023), 2.97(132,201) and 2.91(15)(122,113). In the structure of joteite (R_1 = 7.72% for 6003 F_o > 4σF), AsO_4 and AsO_3 (OH) tetrahedra, AlO_6 octahedra and Cu^(2+)O_5 square pyramids share corners to form sheets parallel to {001}. In addition, 7- and 8-coordinate Ca polyhedra link to the periphery of the sheets yielding thick slabs. Between the slabs are unconnected AsO_3(OH) tetrahedra, which link the slabs only via hydrogen bonding. The Raman spectrum shows features consistent with OH and/or H_2O in multiple structural environments. The region between the slabs may host excess Al in place of some As

Rendering an Account: An Open-State Archive in Postgraduate Supervision

The paper begins with a brief account of the transformation of research degree studies under the pressures of global capitalism and neo-liberal governmentality. A parallel transformation is occurring in the conduct of research through the use of information and communication technologies. Yet the potential of ICTs to shape practices of surveillance or to produce new student-supervisor relations and enhance the processes of developing the dissertation has received almost no critical attention. As doctoral supervisor and student, we then describe the features and uses of a web-based open state archive of the student's work-in-progress, developed by the student and accessible to his supervisor. Our intention was to encourage more open conversations between data and theorising, student and supervisor, and ultimately between the student and professional community. However, we recognise that relations of accountability, as these have developed within a contemporary "audit revolution" (Power, 1994, 1997) in universities, create particular "lines of visibility" (Munro, 1996). Thus while the open-state archive may help to redefine in less managerial terms notions of quality, transparency, flexibility and accountability, it might also make possible greater supervisory surveillance. How should we think about the panoptical potential of this archive? We argue that the diverse kinds of interactional patterns and pedagogical intervention it encourages help to create shifting subjectivities. Moreover, the archive itself is multiple, in bringing together an array of diverse materials that can be read in various ways, by following multiple paths. It therefore constitutes a collage, which we identify as a mode of cognition and of accounting distinct from but related to argument and narrative. As a more "open" text (Iser, 1978) it has an indeterminacy which may render it less open to abuse for the technologies of managerial accountability

Silica coatings on the 1974 Kilauea flow: new SEM and SIMS results and implications for Mars

Despite the predominately mafic character of martian surface rocks, silica-rich materials have long been predicted to occur on Mars; recently, those predictions have been validated. CRISM spectra from numerous regions of Mars have revealed H_2O and OH-bearing phases most consistent with amorphous silica. Additionally, the detection of high-silica materials at Home Plate by MER Spirit implied aqueous alteration and leaching in a volcanic environment [3]. In order to fully understand the environments in which silica-rich materials are formed on Mars, it is useful to study silica in analogous terrestrial settings. We focus on silica and Fe-Ti oxide coatings in the Ka’u Desert on the island of Hawaii, an analog to Mars characterized by low levels of rainfall and strong acid-sulfate alteration processes [4]. Many formation mechanisms for these coatings have been proposed, including dissolution of wind-blown tephra [5], leaching of volcanic glass [6], and vapor deposition [7]. We focus on a suite of samples from the 1974 Kilauea pahoehoe flow, collected in 2003. The chemistry and morphology of these coatings were previously presented [8]. Here we present new morphological, spectral and isotopic analyses of the coating suite. The goal of the study is to characterize the coatings and their formation mechanism and describe the implications for silica mobility on Mars

Discovery of bridgmanite, the most abundant mineral in Earth, in a shocked meteorite

Meteorites exposed to high pressures and temperatures during impact-induced shock often contain minerals whose occurrence and stability normally confine them to the deeper portions of Earth's mantle. One exception has been MgSiO_3 in the perovskite structure, which is the most abundant solid phase in Earth. Here we report the discovery of this important phase as a mineral in the Tenham L6 chondrite and approved by the International Mineralogical Association (specimen IMA 2014-017). MgSiO_3-perovskite is now called bridgmanite. The associated phase assemblage constrains peak shock conditions to ~24 gigapascals and 2300 kelvin. The discovery concludes a half century of efforts to find, identify, and characterize a natural specimen of this important mineral

Trapping an Iron(VI) Water-Splitting Intermediate in Nonaqueous Media

We report in situ spectroscopic measurements in nonaqueous media designed to trap an exceptionally strong oxidant generated electrochemically from an iron-containing nickel layered double hydroxide ([NiFe]-LDH) material. Anodic polarization of this material in acetonitrile produces metal-oxo vibrational spectroscopic signatures along with an extremely narrow near-infrared luminescence peak that strongly indicate that the reactive intermediate is cis-dioxo-iron(VI). Chemical trapping experiments reveal that addition of H_2O to the polarized electrochemical cell produces hydrogen peroxide; and, most importantly, addition of HO– generates oxygen. Repolarization of the electrode restores the iron(VI) spectroscopic features, confirming that the high-valent oxo complex is active in the electrocatalytic water oxidation cycle