Alexander M. Glenn

Samples from the sphalerite-dominated zone of a seafloor massive sulfide chimney, the Satanic Mills Chimney of the PACMANUS hydrothermal field, have been investigated to determine the internal macrostructure and microstructure of this zone, the phases present, and the distribution of metals. A combination of electron probe microanalysis, electron backscattered diffraction, and x-ray diffraction has been used. At the macroscale, this zone of the chimney wall is heavily porous and is comprised primarily of sphalerite, enclosing minor chalcopyrite, pyrite, and wurtzite. A Pb–As sulfosalt layer of possible microbial origins is present at the outer edge of the sphalerite matrix, next to a pore. The sphalerite has grown in globules on the order of 300μm in diameter. At the microscale, the sphalerite features a colloform texture and a duplex-type grain structure consisting of either fine-grain regions in the center surrounded by coarse-grained regions or radiating coarse grains only. Pb- and As-rich bands have been detected in the colloform sphalerite, and growth twins have been observed in both the sphalerite and chalcopyrite crystals. A qualitative description of the growth of a typical globule is given, including nucleation, crystal growth, and solute redistribution.

Hyperspectral soft X-ray emission (SXE) and cathodoluminescence (CL) spectrometry have been used to investigate a carbonaceous-rich geological deposit to understand the crystallinity and morphology of the carbon and the associated quartz. Panchromatic CL maps show both the growth of the quartz and the evidence of recrystallization. A fitted CL map reveals the distribution of Ti⁴⁺ within the grains and shows subtle growth zoning, together with radiation halos from ²³⁸U decay. The sensitivity of the SXE spectrometer to carbon, together with the anisotropic X-ray emission from highly orientated pyrolytic graphite, has enabled the C Kα peak shape to be used to measure the crystal orientation of individual graphite regions. Mapping has revealed that most grains are predominantly of a single orientation, and a number of graphite grains have been investigated to demonstrate the application of this new SXE technique. A peak fitting approach to analyzing the SXE spectra was developed to project the C Kα 2p_z and 2p_(x+y) orbital components of the graphite. The shape of these two end-member components is comparable to those produced by electron density of states calculations. The angular sensitivity of the SXE spectrometer has been shown to be comparable to that of electron backscatter diffraction.

Active sea floor hydrothermal systems are modern analogues of ancient volcanogenic massive sulfide deposits. Hydrothermal sulfide chimneys are one of the important components in these systems and are formed by rapid mixing between seawater and metal-rich hydrothermal fluids venting onto the sea floor. Previous models of chimney growth have been built up mainly based on studies including optical petrographic observations and scanning electron microscopy (SEM). The present study, conducted on a sample from the PACMANUS hydrothermal field (Manus basin, Papua New Guinea), reports for the first time the electron backscattered diffraction (EBSD) observations on a modern sea floor chimney, coupled with synchrotron X-ray fluorescence (SXRF) and scanning electron microscopy-backscattered electron (SEM-BSE) imagery to reveal fine-scale and primary microstructures of sphalerite, allowing the reconstruction of crystal growth history. The results show that sphalerite clusters are formed via the coalescence of multiple smaller sphalerite globules. Electron backscattered diffraction images also highlight that each globule includes an inner zone with fine-grained particles (μm) and an outer zone with elongate blade-shaped crystals (length up to 40 μm), in some cases showing branching dendritic habit. Both zones are dominated by sphalerite with minor other sulfides, such as chalcopyrite, pyrite, and wurtzite. The individual globules are interpreted as forming under conditions of supersaturation within high-temperature gradients, for example, during the mixing between high-temperature (e.g., 300°C) hydrothermal fluids and ambient cold seawater. The occurrence of these contrasting inner and outer zones reflects fluctuation between two regimes: bursts of rapid nucleation from supersaturated fluids that occurred almost instantaneously during the initial mixing and the skeletal crystal growth of particular crystal faces from limited nucleation sites where diffusion-limited boundary layers developed around the growing crystals. Those growing globules then coalesced into columnar aggregates. These observations could potentially have important implications for identifying fossil chimneys in ancient ore deposits. Moreover, this study emphasizes the benefits of advanced techniques, such as EBSD and SXRF, in order to characterize sulfides in various hydrothermal chimneys to reveal crystal growth and fluid mixing history and gain further insight into chimney growth processes.

Schmidite, Zn(Fe³⁺_0.5Mn²⁺_0.5)₂ZnFe³⁺(PO₄)₃(OH)₃(H₂O)₈ and wildenauerite, Zn(Fe³⁺_0.5Mn²⁺_0.5)₂Mn²⁺Fe³⁺(PO₄)₃(OH)₃(H₂O)₈ are two new oxidised schoonerite-group minerals from the Hagendorf-Süd pegmatite, Hagendorf, Oberpfalz, Bavaria, Germany. Schmidite occurs as radiating sprays of orange–brown to copper-red laths on and near to altered phosphophyllite in a corroded triphylite nodule, whereas wildenauerite forms dense compacts of red laths, terminating Zn-bearing rockbridgeite. The minerals are biaxial (+) with α = 1.642(2), β = 1.680(1), γ = 1.735(2) and 2V_meas = 81.4(8)° for schmidite, and with α = 1.659(3), β = 1.687(3), γ = 1.742(3) and 2V_meas = 73(1)° for wildenauerite. Electron microprobe analyses, with H₂O from thermal analysis and FeO/Fe₂O₃ from Mössbauer spectroscopy, gave FeO 0.4, MgO 0.3, Fe₂O₃ 23.5, MnO 9.0, ZnO 15.5, P₂O₅ 27.6, H₂O 23.3, total 99.6 wt.% for schmidite, and FeO 0.7, MgO 0.3, Fe₂O₃ 25.2, MnO 10.7, ZnO 11.5, P₂O₅ 27.2, H₂O 24.5, total 100.1 wt.% for wildenauerite. The empirical formulae, scaled to 3 P and with OH^– adjusted for charge balance are Zn_1.47Mn²⁺_0.98Mg_0.05Fe²⁺_0.04Fe³⁺_2.27(PO₄)₃(OH)_2.89(H₂O)_8.54 for schmidite and Zn_1.11Mn²⁺_1.18Mg_0.05Fe²⁺_0.08Fe³⁺_2.47(PO₄)₃(OH)_3.25(H₂O)_9.03 for wildenauerite. The two minerals have orthorhombic symmetry, space group Pmab and Z = 4. The unit-cell parameters from refinement of powder X-ray diffraction data are a = 11.059(1), b = 25.452(1) and c = 6.427(1) Å for schmidite, and a = 11.082(1), b = 25.498(2) and c = 6.436(1) Å for wildenauerite. The crystal structures of schmidite and wildenauerite differ from that of schoonerite in having minor partitioning of Zn from the ^[5]Zn site to an adjacent vacant tetrahedral site ^[4]Zn, separated by ~1.0 Å from ^[5]Zn. The two minerals are distinguished by the cation occupancies in the octahedral M1 to M3 sites. Schmidite has M1 = M2 = (Fe³⁺_0.5Mn²⁺_0.5) and M3 = Zn and wildenauerite has M1 = M2 = (Fe³⁺_0.5Mn²⁺_0.5) and M3 = Mn²⁺.