Uptake and incorporation of Al, Cr, V, Zn and Mo in hematite: Competition, synergies and influence on structural properties

This study investigated multiple (di-, tri- and tetra-)incorporation of selected minor and trace elements (Al³⁺, Cr³⁺, V^3-5+, Zn²⁺, Mo⁶⁺ and As⁵⁺) into hematite. The purpose was to improve understanding of how hematite may control trace element mobility in the environment, and how physical and chemical properties of hematite are impacted by multi-element incorporation at x/Fe molar ratios of up to 10%. Simultaneous structural incorporation of Al±Cr±V±Zn into hematite was achieved, with both synergistic and antagonistic effects occurring between certain element combinations. Cr+Al had synergistic effects on their co-incorporation, while V negatively affected Al incorporation, and both V and Zn negatively affected Cr incorporation. In contrast, Mo was minimally associated with hematite, and As prevented hematite formation completely. X-ray diffraction indicated contraction and expansion of the hematite unit-cell upon substitution was related to the ionic radius of the substituting element in single-element samples, while V predominantly controlled the direction of deviation in multi-element samples. X-ray absorption near-edge structure spectroscopy indicated V was present as a mixture of V³⁺-V⁵⁺, with a higher average V oxidation state associated with multi-element samples. Results provide new insights into trace element geochemistry within hematite, and highlight the importance of multi-element studies to better understand natural and anthropogenic systems.

The effects of climate change on the Pleistocene rock art of Sulawesi

The equatorial tropics house some of the earliest rock art yet known, and it is weathering at an alarming rate. Here we present evidence for haloclasty (salt crystallisation) from Pleistocene-aged rock art panels at 11 sites in the Maros-Pangkep limestone karsts of southern Sulawesi. We show how quickly rock art panels have degraded in recent decades, contending that climate-catalysed salt efflorescence is responsible for increasing exfoliation of the limestone cave surfaces that house the ~ 45 to 20-thousand-year-old paintings. These artworks are located in the world's most atmospherically dynamic region, the Australasian monsoon domain. The rising frequency and severity of El Niño-induced droughts from anthropogenic climate change (that is, higher ambient temperatures and more consecutive dry days), combined with seasonal moisture injected via monsoonal rains retained as standing water in the rice fields and aquaculture ponds of the region, increasingly provide ideal conditions for evaporation and haloclasty, accelerating rock art deterioration.

Synthesis and characterisation of new Bi(iii)-containing apatite-type oxide ion conductors: the influence of lone pairs

New Bi-containing apatite-type germanates, characterised by X-ray and neutron diffraction, impedance spectroscopy and bond valence analysis, display high oxide ion conductivity.

P–V–Tequation of state of synthetic mirabilite (Na2SO4·10D2O) determined by powder neutron diffraction

Neutron powder diffraction data have been collected from Na2SO4·10D2O (the deuterated analogue of mirabilite), a highly hydrated sulfate salt that is thought to be a candidate rock-forming mineral in some icy satellites of the outer solar system. These measurements, made using the OSIRIS instrument on the ISIS neutron spallation source, covered the range 0.1 <P< 545 MPa and 150 <T< 270 K. The refined unit-cell volumes as a function of pressure and temperature are parameterized in the form of a Birch–Murnaghan third-order equation of state, and the anisotropic linear incompressibilities are represented in terms of the elastic strain tensor. At 270 K, the bulk modulusK0,270= 19.6 (1) GPa, its first pressure derivative ∂K/∂P= 5.8 (5) and its temperature dependence ∂K/∂T = −0.0175 (6) GPa K−1. The stiffest direction at 270 K, with a linear bulk modulus of ∼82 GPa, is coincident with the twofold axis of this monoclinic crystal. Of the remaining two principal directions, the most compressible (K≃ 44 GPa) is roughly aligned with thecaxis, and the intermediate value (K≃ 59 GPa) is therefore approximately collinear witha*. With the aid of additional published data, a number of other important thermodynamic quantities have been derived, including the Grüneisen and Anderson–Grüneisen parameters, and the volume and enthalpy of melting along the high-pressure melting curve. Additional data obtained during this work, concerning the elastic properties of deuterated ice IV, are also presented.

Crystal structures and thermal expansion of α-MgSO4and β-MgSO4from 4.2 to 300 K by neutron powder diffraction

Detailed neutron powder diffraction measurements have been carried out on two polymorphs of anhydrous magnesium sulfate, α-MgSO4and β-MgSO4. α-MgSO4is orthorhombic, space groupCmcm(Z= 4); at 4.2 K the unit-cell dimensions area= 5.16863 (3),b= 7.86781 (5),c= 6.46674 (5) Å,V= 262.975 (2) Å3[ρcalc= 3040.16 (2) kg m−3], and at 300 K,a= 5.17471 (3),b= 7.87563 (5),c= 6.49517 (5) Å,V= 264.705 (2) Å3[ρcalc= 3020.29 (2) kg m−3]. The axial and volumetric thermal expansion coefficients are positive at all temperatures and exhibit no unusual behaviour. Structures were refined at 4.2 and 300 K toRP< 3%; less precise structural parameters were determined during warming from 4.2 to 300 K. β-MgSO4has a more complex structure, crystallizing in space groupPbnm(Z= 4); the unit-cell dimensions at 4.2 K area = 4.73431 (8),b= 8.58170 (12),c= 6.67266 (11) Å,V= 271.100 (5) Å3[ρcalc= 2949.04 (5) kg m−3], and at 300 K,a= 4.74598 (7),b= 8.58310 (10),c= 6.70933 (10) Å,V= 273.306 (4) Å3[ρcalc= 2925.42 (4) kg m−3]. The thermal expansivities of theaandcaxes, and the volumetric thermal expansion coefficient, are positive at all temperatures and normally behaved. However, the thermal expansion of thebaxis is both very small and negative below ∼125 K. Structural and thermal motion parameters for β-MgSO4as a function of temperature are also reported.