X-ray diffraction, electron microprobe and STEM EDAX analyses of basalts from the South Pennines (Derbyshire) indicate that Fe-rich smectites are the dominant alteration products, occurring either as pseudomorphs or infilling veins and vesicles. Previously chlorite has been identified as the common alteration product but its occurrence is very infrequent.

The crystal structure of nechelyustovite, ideally Na4Ba2Mn1.5☐2.5Ti5Nb(Si2O7)4O4(OH)3F(H2O)6, a 5.447(1) Å, b 7.157(1) Å, c 47.259(9) Å, α 95.759(4)°, β 92.136(4)°, γ 89.978(4)°, V 1831.7(4) Å3, space group P, Z = 2, Dcalc. 3.041 g cm–3, from Lovozero alkaline massif, Kola Peninsula, Russia, has been solved and refined to R1 = 13.9% on the basis of 1745 unique reflections (Fo > 15σF). Electron microprobe analysis yielded the empirical formula (H20)6.01, Z = 2, calculated on the basis of 42 (O + F) a.p.f.u., H2O and OH are calculated from structure refinement (H2O = 6 p.f.u.; F + OH = 4 p.f.u.). The crystal structure of nechelyustovite is a combination of a TS (titanium silicate) block and an I (intermediate) block. The TS block consists of HOH sheets (H-heteropolyhedral, O-octahedral). The TS block exhibits linkage and stereochemistry typical for Group III (Ti = 3 a.p.f.u.) of Ti-disilicate minerals: two H sheets connect to the O sheet such that two (Si2O7) groups link to the trans edges of a Ti octahedron of the O sheet. There are two distinct TS blocks of the same topology, TS1 and TS2, that differ in the cations of the O sheet, [(Na1.5Mn1☐0.5)Ti] and [(Na2Mn0.5☐0.5)Ti] (4 a.p.f.u.) respectively. The TS1 and TS2 blocks have two different H sheets, H1,2 and H3,4, where (Si2O7) groups link to [5]- and [6]-coordinated (Ti,Nb) polyhedra respectively. There are three peripheral sites, AP(1—3), occupied mainly by Ba (less Sr and K) at 96, 86 and 26% and one peripheral site AP(4) occupied by Na at 50%. There are two I blocks: the I1 block is a layer of Ba atoms; the I2 block consists of H2O groups and AP(3) atoms. TS blocks alternate with I blocks or link through hydrogen bonds (as in epistolite). There is a sequence of four TS blocks and three I blocks per the c cell parameter: TS2 — I1 — TS1 — I2 — TS1 — I1 — TS2.

Duftite, PbCu(AsO4)(OH) is orthorhombic, space group P212121 with a = 7.768(1), b = 9. 211(1), c = 5.999(1) Å, Z = 4; the structure has been refined to R = 4.6% and Rw = 6.5% using 640 observed reflections [F> 2σ(F)]. The structure consists of chains of edge-sharing CuO6 ‘octahedra’, parallel to c; which are linked via AsO4 tetrahedra and Pb atoms in distorted square antiprismatic co-ordination to form a three dimensional network. The CuO6 ‘octahedra’ show Jahn-Teller distortion with the elongation running approximately along <627>. The hydrogen bonding network in the structure was characterized using bond valence calculations. ‘β-duftite’ is an intermediate in the duftite-conichalcite series, which has a modulated structure based on the intergrowth of the two structures in domains of approximately 50 Å. The origin of the modulation is thought to be associated with displacements in the oxygen lattice and is related to the orientation of the Jahn-Teller distortion of CuO6 ‘octahedra’. Approximately half of the strips show an elongation parallel to <627> while the other strips are elongated parallel to [010]. This ordering results in an increase in the b cell repeat compared to duftite and conichalcite.

Forêtite, ideally Cu2Al2(AsO4)(OH,O,H2O)6, is a new secondary arsenate mineral from the Cap Garonne mine, Var, France (IMA2011-100). It has also been identified at the Salsigne gold mine, Languedoc-Roussillon, France. Although it was identified as a potentially new mineral in 1993, a formal description has only been possible as a result of a new find in a chamber called Annex S located near the entrance of the Cap Garonne mine. Forêtite occurs as pale sky blue to aqua aggregates, up to ∼0.1 mm across, made up of minute plates no more than 20 μm in length. It has a very pale blue streak. Individual crystals have a vitreous lustre and are transparent, whereas clusters appear translucent. The calculated density is 3.286 g cm–3. The crystals are brittle, with an irregular fracture and have a hardness of ∼3–4 on Moh's scale. Forêtite is found in direct association with bariopharmacoalumite, cyanotrichite, parnauite, chalcophyllite and mansfieldite in an Al-rich assemblage which is presumed to have formed under acidic conditions. It is biaxial; the average refractive index measured in white light on aggregates of forêtite crystals is 1.620(5). The empirical formula (based on 10 oxygen atoms per formula unit) is Cu1.94(Al1.96Fe0.04)Σ2.00(As0.84S0.09Si0.04)Σ0.97O10H5.19. Raman spectroscopy confirms the presence of OH and H2O in the structure. Forêtite is triclinic, space group P, with a = 6.969(9), b = 7.676(9), c = 8.591(11) Å, α = 82.01(9), β = 71.68(8), γ = 102.68(8)°, V = 415(1) Å3 and Z = 2. The five strongest lines in the X-ray powder diffraction pattern [d in A ˚ , (I), (hkl)] are as follows: 7.307, (100), (010, 00); 3.141, (24), (200, 00); 2.818, (24), (20, 20); 4.519, (23), (111); 2.343, (22), (11). The mineral is named in honour of Dr Jean-Paul Forêt, who co-founded the project that turned the Cap Garonne mine into a protected site and museum.

Mendeleevite-(Nd), (Cs,□)6(□,Cs)6(□,K)6(REE,Ca)30(Si70O175)(OH,H2O,F)35 is a new mineral from the Darai-Pioz alkaline massif, Tajikistan. Mendeleevite-(Nd) was found in a pectolite aggregate in silexites (quartz-rich rocks) which consist of fine to medium pectolite grains, quartz, aegirine and fluorite, with minor khvorovite, mendeleevite-(Ce), sokolovaite, hyalotekite, orlovite, kirchhoffite, pekovite, neptunite, zeravshanite, senkevichite, nordite-(Nd), alamosite, pyrochlore-group minerals and baratovite. Mendeleevite-(Nd) forms colourless cubic crystals 10–40 μm in size; it has a vitreous lustre and a Mohs hardness of 5–5.5; Dmeas. = 3.20(2) g/cm3, Dcalc. = 3.155 g/cm3. Mendeleevite-(Nd) is optically isotropic, with the refractive index n = 1.582(2). Mendeleevite-(Nd) is cubic, space group Pm3̄, a = 21.9106(4) Å; Z = 2. The six strongest reflections in the powder X-ray diffraction pattern are [d (Å), I (%), (h k l)] are: 11.01, 100, (0 0 2); 15.63, 55, (0 1 1); 3.47, 42, (2 0 6); 3.099, 42, (3 4 5); 2.192, 42, (0 0 10); 1.819, 41, (3 6 10). Chemical analysis by electron microprobe gave SiO2 42.30, Ce2O3 10.12, La2O3 3.60, Nd2O3 16.19, Pr2O3 2.79, Sm2O3 4.19, Gd2O3 1.69, Eu2O3 0.47, SrO 2.99, CaO 2.20, Cs2O 8.50, K2O 0.85, H2O 3.85, F 1.25, –O = F2 –0.53, sum 100.46 wt.%, with H2O calculated by analogy with mendeleevite-(Ce). The empirical formula based on 210 (O + F) apfu, with F + OH + H2O = 35 pfu, is Cs6(□4.20K1.80)∑6{[(Nd9.57Ce6.13Sm2.39La2.20Pr1.68Gd0.93Eu0.27)∑23.17(Ca3.90Sr2.87)∑6.77]∑29.94□0.06}∑30(Si70.03O175)(OH14.47F6.54)∑21.01 (H2O)14, Z = 2. The simplified and ideal formulae are (Cs,□)6 (□,Cs)6(□,K)6 (REE,Ca)30 (Si70O175)(OH, H2O,F)35 and Cs6(REE23Ca7)(Si70O175)(OH,F)19(H2O)16, respectively. The compatibility index (from measured density) = – 0.039 (excellent). Mendeleevite-(Nd) is a Nd analogue of mendeleevite-(Ce), (Cs,□)6(□,Cs)6(□,K)6(REE,Ca,□)30(Si70O175)(H2O,OH,F,□)35. Both minerals are named after Dmitri Mendeleev (1834–1907), the great Russian chemist, author of the periodic table of chemical elements, who has had a significant impact on the development of natural sciences and industry, both in Russia and around the world.

Two feldspars of composition Ca0.3Sr0.7Al2Si2O8 (An30SrF70) and Ca0.6Sr0.4Al2Si2O8 (An60SrF40) were synthesized from gel and annealed at temperatures between 1150 and 1480ºC for durations between 0.025 (1.5 min) and 1073 h. The An60SrF40 feldspar is triclinic for all annealing temperatures, whereas the An30SrF70 possesses, at room temperature, a monoclinic symmetry, C2/m in the shortest annealing runs and I2/c after slightly longer heating, and becomes triclinic after prolonged annealing. A TTT plot for the triclinic–monclinic transition shows an activation energy of 101±12 kcal, less than, but still close to, that observed for Al-Si order in anorthite. TEM investigation shows antiphase domains and Carlsbad twins the size of which increases with annealing. IR spectroscopy was performed on a series of samples of An30SrF70, annealed at T = 1350ºC. The linewidth in hard mode is related to the spontaneous strain of the monoclinic–triclinic transition via . The evolution of the order parameter with time can be modelled as lnt, as predicted in order-disorder processes at intermediate degrees of order. It is shown that even after the longest annealing runs in An30SrF70, equilibrium was not obtained whereas, under the same conditions, equilibrium Al-Si order can be obtained in anorthite and Sr-feldspar.

The uptake of inorganic Hg2+ and organometallic CH3Hg+ from aqueous solutions by 11 different natural zeolites has been investigated using a batch distribution coefficient (Kd) method and supported by a preliminary voltammetric study. The effect of mercury concentration on the Kd response is shown over an environmentally appropriate concentration range of 0.1–5 ppm inorganic and organometallic Hg using a batch factor of 100 ml g-1 and 20 h equilibration. Analcime and a Na-chabazite displayed the greatest methylmercury uptakes (Kd values at 1.5 ppm of 4023 and 3456, respectively), with mordenite as the smallest at 578. All uptake responses were greater for methylmercury than for the inorganic mercuric nitrate solutions, suggesting a distinctive sensitivity of zeolites to reaction with different types of solute species. It is likely that this sensitivity is attributable to the precise nature of the resultant Hg-zeolite bonds. Additionally, both the Si-Al ratio and the Na content of the initial natural zeolite samples are shown to influence the Kd responses, with positive correlations between Kd and Na content for all zeolites excluding mordenite.

Subsilicic sodium gedrite has been found in a hydrothermal vein together with sekaninaite (Fe-cordierite), andalusite, plagioclase, topaz, sillimanite, quartz, biotite, magnetite, ilmenite, hercynite, wolframite, pyrrhotite, chalcopyrite and pyrite. This vein is part of a hydrothermal vein system consisting predominantly of quartz-feldspar veins, some of which contain andatusite, sekaninaite, biotite, muscovite, fluorite and accessory oxides and sulphides, which was formed during the later stages of crystallization of a high-level anorogenic Svecofennian granite. Petrographic observations suggest the following crystallization sequence for the quartz-feldspar veins: plagioclase-quartz-andalusite-sekaninaite-microcline-biotite-albite-oxides and sulphides-muscovite-fluorite, and for the subsilicic sodium gedrite-bearing vein: andalusite-sekaninaite-subsilicic sodium gedrite-biotite-quartz-albite-sillimanite-topaz-oxides and sulphides. Electron microprobe analysis revealed that all subsilicic sodium gedrite is relatively homogeneous with only the following compositional variation: Na(A) 0.57–0.81, Aliv 2.31–2.57, XMg 0.15–0.21. The temperature (T) for the formation of the hydrothermal vein system is estimated at 550–600°C and the pressure (P) is estimated to be less than 3 kbar.

Fly ash from secondary Pb metallurgy was submitted to the pH-static leaching procedure according to the PrEN 14997 European leaching standard. The 48 h pH-static leaching experiments were performed on (1) fresh untreated fly ash and (2) previously washed fly ash with a cumulative wash step of 60 l kg-1. Greater release ofmetallic contaminants (Pb, Cd, Zn) was observed in the acidic pH range for both ashes. Washing significantly reduced the release of Cd and Zn, but greater concentrations of Pb were observed in leachates from washed fly ash due to the more important leaching of anglesite (PbSO4). The PHREEQC-2 speciation-solubility calculations showed that anglesite, phosgenite (PbCl2·PbCO3) and laurionite (Pb(OH)Cl) are the most important solubility-controlling phases for Pb, which is the most important contaminant.

Ellipsometric measurements in the spectral range 250–850 nm have been made on polished single crystals of natural cuprite from Namibia and of artificial cuprite grown by a total-oxidation method from pure copper. Derived values of the refractive indices, absorption coefficients, normal reflectances, and the complex dielectric constants for the two materials are all closely similar, suggesting that the stoichiometry and purity are essentially the same. Values of the absorption coefficient at the red end of the spectrum are inconsistent with the observed transparency of the crystals, and are compatible with a 10 nm surface oxidation film of cupric oxide. This study is part of a wider survey of the effects of doping and stoichiometry on the optical parameters of cuprite.

Chemical and single crystal X-ray studies have shown the existence of a 9-layer polytype of taaffeite. The parameters of the hexagonal unit-cell are a 5·675 Å, c 41·096 Å (arh 14·085 Å, α 23·24°). The oxide composition of the mineral is Al2O3 71·44, MgO 15·76, FeO 6·78, BeO 5·50%. It is pale olive green, with refractive indices ω = 1·739, ε = 1·735, and a specific gravity of 3·68.

It is now more than sixty years since Tschermak first published his classical paper on the relationship between the pyroxenes and amphiboles, but, in spite of our greatly increased knowledge of these important rock-forming minerals, many of the problems concerning their constitution and genesis remain unsolved. In particular, the nature of the conditions which determine whether a pyroxene or an amphibole will separate from a crystallizing magma are imperfectly understood, although the point is one of very considerable petrological significance. Much fuller information is necessary, particularly with regard to the relationship between the composition of the ferromagnesian minerals and the rocks in which they occur. The present paper merely represents a preliminary attempt to correlate the available chemical evidence and to determine what other possible factors might influence the character of the ferromagnesian crystallization.

The pyroxmangite and rhodonite, here examined by X-ray methods, are from the Lewisian schists at Glen Beag, in the Glenelg district, Inverness-shire. The material was exhibited by Prof. C. E. Tilley at the meeting of the Society on November 5, 1936, and it has since been described by him. The specimen of ‘iron-rhodonite’ was from the iron slag described by Whiteley and Hallimond, and is no. 7a in their table of analyses.

The unit cell of the ‘iron-rhodonite’ was determined by means of the Weissenberg goniometer and the data obtained are compared in table I with Gossner and Brückl's determination for rhodonite. The unit cell of pyroxmangite was determined by taking a Weissenberg photograph about the c-axis and oscillation photographs about the a- and b-axes.

Sr- and Zr-bearing perrierite-(Ce) occurring in aegirinized syenite pegmatites of the Burpala massif, Russia, is compositionally intermediate between perrierite-(Ce) and hezuolinite and occupies a compositional gap in minerals of the chevkinite group. Its crystal structure has been determined using a single-crystal diffractometer fitted with a CCD detector and MoKα X-ray radiation. The mineral is monoclinic; a = 13.815(1), b = 5.668(1), c = 11.842(1) Å , β = 113.843(3)º, V = 848.18(4) Å3, space group C2/m, Z = 2. The crystal structure was refined with the occupancies [(Ce1.2La1.0Nd0.15) (Sr1.0Ca0.5Na0.15)]4(Zr0.5Fe0.3Mn0.2)(Ti1.3Fe0.7)2Ti2(Si2O7)2O8 on the basis of chemical composition although the allocation of cations to particular sites was performed on the basis of the number of refined electrons in each unique site. The dominance of Zr in the B site links the Burpala perrierite-(Ce) to more Sr-Zr-rich members of the chevkinite group, such as hezuolinite and rengeite. As in all of the perrierite members, there is a distortion of the D site octahedra, which is interpreted as due to the packing of the REE ions.

Whole-rock major- and trace-element data are presented on a sample collection from the >3 Ga Amikoq Layered Complex (ALC), and hosting amphibolites within the Mesoarchean Akia terrane, SW Greenland. The lithologies range from leuconorite to melanorite/feldspathic orthopyroxenite, orthopyroxenite to harzburgite through to dunite, and tholeiitic basaltic–picritic mafic host rocks. The Amikoq Layered Complex samples are primitive (Mg#: 65–89) with elevated Ni and Cr contents. However, the absence of troctolitic lithologies and the presence of two orthopyroxene compositional trends, suggests that the successions might not be comagmatic. On the basis of trace-element cumulate models, relatively low Ni contents and minor negative Sr-Eu anomalies in some high-Ti ultramafic rocks, it is not possible to exclude a petrogenesis related to a melt similar to that of the mafic host-rocks. Ultramafic samples with U-shaped trace-element distribution patterns are petrogenetically related to the noritic sequences, either through cumulus mineral accumulation or melt-rock reactions. Assimilation-fractional-crystallisation modelling of melanorites nevertheless require the parental melt to have been contaminated/mixed with a component of island-arc-like tholeiite affinity. A boninite-like parental melt might have been derived from the subcontinental lithospheric mantle of the Akia terrane, or alternatively via assimilation of an ultramafic parental melt with island-arc-like tholeiite. Given the complex geological evolution and high-grade metamorphic overprint of the Amikoq Layered Complex, we are unable to differentiate between the two models.

Thérèsemagnanite was originally described from the Cap Garonne mine, Var, France. Its ideal formula was reported as (Co,Zn,Ni)6(SO4)(OH,Cl)10·8H2O; without crystal structure data, only the powder X-ray diffraction pattern was given. Revision of the holotype material revealed that thérèsemagnanite is identical to ‘cobaltogordaite’ (IMA2014-043), recently described from the Blue Lizard mine, Utah, USA. Thérèsemagnanite is thus redefined in accordance with the new data obtained for the neotype specimen from Blue Lizard (formerly the holotype specimen of ‘cobaltogordaite’) and ‘cobaltogordaite’ has been discredited by the International Mineralogical Association Commission on New Mineral Nomenclature and Classification (IMA CNMNC). Thérèsemagnanite has the ideal, end-member formula NaCo4(SO4)(OH)6Cl·6H2O. The empirical formulae of the holotype (Cap Garonne) and the neotype (Blue Lizard), both based on microprobe analyses and calculated on the basis of 17 O + Cl atoms per formula unit (with fixed 6 OH groups and 6 H2O molecules; H content is calculated by stoichiometry) are (Na0.64K0.09)Σ0.73(Co2.35Zn1.22Ni0.50)Σ4.07S1.02O3.98(OH)6Cl1.02·6H2O and Na1.01(Co1.90Zn1.37Ni0.48Cu0.15Mn0.05)Σ3.95S1.03O4.09(OH)6Cl0.91·6H2O, respectively. Thérèsemagnanite is trigonal, P$\overline 3 $, a = 8.349(3), c = 13.031(2) Å, V = 786.6(4) Å3 and Z = 2 (neotype). The strongest powder X-ray diffraction lines are [dobs in Å (hkl) (Irel)]: 13.10 (001)(100), 6.53 (002)(8), 4.173 (110)(4), 3.517 (112)(5), 2.975 (104, 10$\overline 4 $)(4), 2.676 (211)(5) and 2.520 (12$\bar 2$)(5) (neotype). Thérèsemagnanite is a cobalt analogue of gordaite, NaZn4(SO4)(OH)6Cl·6H2O. These minerals represent the gordaite group, accepted by the IMA CNMNC.

Bulachite specimens from Cap Garonne, France, comprise two intimately mixed hydrated aluminium arsenate minerals with the same Al:As ratio of 2:1 and with different water contents. The crystal structures of both minerals have been solved using data from low-dose electron diffraction tomography combined with synchrotron powder X-ray diffraction. One of the minerals has the same powder X-ray diffraction pattern (PXRD) as for published bulachite. It has orthorhombic symmetry, space group Pnma with unit-cell parameters a = 15.3994(3), b = 17.6598(3), c = 7.8083(1) Å and Z = 4, with the formula [Al6(AsO4)3(OH)9(H2O)4]⋅2H2O. The second mineral is a higher hydrate with composition [Al6(AsO4)3(OH)9(H2O)4]⋅8H2O. It has the same Pnma space group and unit-cell parameters a = 19.855(4), b = 17.6933(11) and c = 7.7799(5) Å i.e. almost the same b and c parameters but a much larger a parameter. The structures are based on polyhedral layers, parallel to (100), of composition [Al6(AsO4)3(OH)9(H2O)4] and with H-bonded H2O between the layers. The layers contain [001] spiral chains of edge-shared octahedra, decorated with corner connected AsO4 tetrahedra that are the same as in the mineral liskeardite. The spiral chains are joined together by octahedral edge-sharing to form layers parallel to (100). Synchrotron PXRD patterns collected at different temperatures during heating of the specimen show that the higher-hydrate mineral starts transforming to bulachite when heated to 50°C, and the transformation is complete between 75 and 100°C.

Two contrasting reaction coronae were developed around rare earth element (REE) accessory phosphates in Variscan metagranitic rocks, which have been overprinted by Alpine blastomylonitisation from the Fabova Hol'a Massif, in the Veporic Unit, Western Carpathians, Central Slovakia. The Th–U–Pb total EPMA age determination of primary magmatic monazite-(Ce) from the metagranite indicates a Carboniferous (Mississippian, Tournaisian) age of 355 ± 1.9 Ma. Monazite-(Ce) breakdown resulted in impressive, though common, fluorapatite ± Th-silicate + allanite-(Ce) + clinozoisite coronae. The alteration of xenotime-(Y) produced a novel type of secondary coronal micro-texture consisting of a massive fluorapatite mantle zone and tiny satellite crystals of hellandite-(Y) [(Ca,REE)4Y2Al□2(B4Si4O22)(OH)2] and hingganite-(Y) [Y2□Be2Si2O8(OH)2] of ~1–5 μm, and rarely ≤10 μm in size. The localised occurrence of Y–B–Be silicates, which are associated closely with other secondary minerals, suggests the involvement of B and Be during the metasomatic alteration transformation of xenotime-(Y). General reactions for monazite-(Ce) and xenotime-(Y) decomposition, including the fluids involved, can be written as follows: Mnz + (Ca, Fe, Si, Al and F)-rich fluid → FAp + Ht + Aln + Czo; Xtm + (Ca, Fe, Si, Al, F, B and Be)-rich fluid → FAp + Hld + Hin + Czo.

The granitic rocks underwent Early Cretaceous burial metamorphism under greenschist- to lower amphibolite-facies P–T conditions. Subsequently, Alpine post-collisional uplift and exhumation of the Veporic Unit, starting from the Late Cretaceous epoch, was accompanied by a retrograde tectono-metamorphic overprint; the activity of external fluids, caused the formation of secondary coronae minerals around monazite-(Ce) and xenotime-(Y). A portion of B (± Be) should have been liberated from the metagranite feldspars, micas, or xenotime-(Y) enriched in (Nb,Ta)BO4 (schiavinatoite or béhierite) components. However, the principal source of B and Be in fluids necessary for the production of hellandite and hingganite, was probably of external origin from adjacent magmatic, metamorphic, or sedimentary rocks (Permian granites, rhyolites and sedimentary rocks, and Palaeozoic metapelites).

Geological disposal facilities (GDF) are intended to isolate and contain radioactive waste within multiple protective barriers, deep underground, to ensure that no harmful quantities of radioactivity reach the surface environment. The last line of defense in a multi-barrier GDF is the geosphere, where iron is present in the host rock mineralogy as either Fe(II) or Fe(III), and in groundwater as Fe(II) under reducing conditions. The mobility of risk-driving radionuclides, including uranium and technetium, in the environment is affected significantly by their valence state. Due to its low redox potential, Fe(II) can mediate reduction of these radionuclides from their oxidized, highly mobile, soluble state to their reduced, insoluble state, preventing them from reaching the biosphere. Here a study of five types of potential host rocks, two granitoids, an andesite, a mudstone and a clay-rich carbonate, is reported. The bulk rocks and their minerals were analysed for iron content, Fe(II/III) ratio, and for the speciation and fine-grained nature of alteration product minerals that might have important controls on groundwater interaction. Total iron content varies between 0.9% in clays to 5.6% in the andesite. X-ray absorption spectroscopy reveals that Fe in the granitoids and andesite is predominantly Fe(II), and in mudstones, argillaceous limestone and terrestrial sandstone is predominantly Fe(III). The redox reactivity of the potential host rocks both in the presence and absence of Fe(II)-containing 'model' groundwater was investigated using an azo dye as a probe molecule. Reduction rates as determined by reactivity with the azo dye were correlated with the ability of the rocks to uptake Fe(II) from groundwater rather than with initial Fe(II) content. Potential GDF host rocks must be characterized in terms of mineralogy, texture, grain size and bulk geochemistry to assess how they might interact with groundwater. This study highlights the importance of redox reactivity, not just total iron and Fe(II)/(III) ratio, when considering the host rock performance as a barrier material to limit transport of radionuclides from the GDF.