Abstract Modraite, ideally Ca19Fe2+Al4(Al7Fe2+)(SiO4)10(Si2O7)4O(OH)9, is a new member of the vesuvianite group. It was found at Modra, Harmónia skarn rock in the Malé Karpaty Mountains, Pezinok District, Slovakia. Modraite formed as a high-temperature and low-pressure metamorphic mineral on the contact zone between Devonian limestones and Carboniferous granitic rocks, in association with diopside-hedenbergite, rarely grossular, calcite, titanite, and clinozoisite-epidote. The new mineral forms dark brown microscopically transparent euhedral to subhedral prismatic crystals or radiating aggregates (up to 5 cm long) in diopside-hedenbergite groundmass. Modraite crystals are characterized by dominant {100} and minor {110} prism faces; the mineral is brittle, has an irregular (uneven) fracture without cleavage or parting. The Mohs’ hardness is 6−7. The measured and calculated densities are 3.35(2) and 3.42 g/cm3, respectively. The new mineral is microscopically transparent to translucent, optically uniaxial, negative, ω = 1.7221(2), ε = 1.7151(3). The mean chemical composition of modraite (wt%, electron microprobe, REE and Li by LA-ICP-MS, Fe2O3 and FeO by Mössbauer spectroscopy, H2O calculated from stoichiometry) is: SiO2 36.42, TiO2 1.79, Al2O3 16.37, V2O3 0.03, Cr2O3 0.03, La2O3 0.03, Ce2O3 0.08, Pr2O3 0.01, Nd2O3 0.04, Sm2O3 0.01, Gd2O3 0.01, Fe2O3 1.32, FeO 3.65, MnO 0.23, ZnO 0.05, MgO 0.87, CaO 34.92, BaO 0.26, Li2O 0.03, Na2O 0.09, H2O 1.80, F 1.49, Cl 0.16, O=F -0.63, O=Cl -0.04, total 99.02. The empirical formula based on 50 cations (excluding H) per formula unit is: X1–X4(Ca18.60Na0.09Ba0.05Ce0.02La0.01Nd0.01)Σ18.78Y1(Fe2+0.81Fe3+0.13Al0.09)Σ1.03Y2Al4Y3(Al5.50Fe2+0.71Ti4+0.67Mg0.65Fe3+0.36Mn2+0.10Li+0.06Zn2+0.02 V3+0.01Cr3+0.01)Σ8.09T1(□4) T2□ (Z1–Z2Si1.01O4)10 (Z3Si2O7)4O10–O11(OH5.97F2.34O1.55Cl0.14)Σ10.00. Mössbauer spectroscopy indicates a dominant proportion of Fe2+ (76 %) over Fe3+ (24 %), with the dominancy of Fe2+ over Fe3+ at both the Y1 and Y3 sites. The crystal structure was refined to R1 = 0.0215 for 3970 unique reflections. The mineral is tetragonal, space group P4/nnc, single-crystal X-ray diffraction data of a modraite are: a = 15.559(2) Å, c = 11.804(2) Å, V = 2857.4(9) Å3, Z = 2. The seven strongest lines of the powder X-ray diffraction pattern are [d(Å)−I(%)−hkl]: 3.235−19−402, 2.949−29−004, 2.749−100−432, 2.593−59−522, 2.458−37−620, 1.625−16−526, 1.621−30−922. Modraite is the Y1Fe2+-dominant member, ideally with 7 Al3+ and 1 Fe2+ cations at the Y3 site, and a strong O10-H10…O10 hydrogen bond. Modraite is named after the type locality, Modra, western Slovakia.

University, ABSTRACT Tektites, a rare type of impact glass, are known to occur only in five distinct strewn fields on Earth. We report on the discovery of a new tektite strewn field in northeastern Brazil. This recent finding resulted in the collection of similar to 500 specimens found within a strewn field at least 90 km long. Mostly black in color, the masses, hereby named "geraisites" after the Brazilian state of Minas Gerais, range from <1 g up to 85.4 g and exhibit various shapes, rarely showing lechatelierite inclusions. In terms of chemical composition, they fall in the dacite and rhyolite fields of the total alkali versus silica diagram, with similar SiO2 and slightly higher Na2O + K2O content in comparison with other known tektites. Some variations in trace elements from sample to sample, such as for Cr and Ni, are observed. Water contents of the glass are extremely low, ranging between 70 and 110 ppm. The best estimate for their age of formation measured by the 40Ar/39Ar method is currently ca. 6.3 Ma, although the possible presence of inherited 40Ar* will require more analyses to fully establish their age. Strontium, Nd, and Hf isotopic compositions as well as Nd and Hf model ages provide insights into the age and composition of the source rocks, having a continental crustal composition, most likely Mesoarchean felsic rocks.

The crystal structures of the low‐temperature (LT) and high‐temperature (HT) modifications of silver pyrophosphate, Ag 4 P 2 O 7 , were determined from single‐crystal X‐ray data. Thermal analysis, vibrational and 31 P‐MAS‐NMR spectroscopy studies, and density functional theory calculations complement the results. The crystal structure of HT‐Ag 4 P 2 O 7 ( T = 487 °C, , Z = 2, a = 5.5734(5) Å, c = 13.7613(18) Å) is very similar to that of the aristotype, γ ‐K 4 P 2 O 7 , whereas the crystal structure of LT‐Ag 4 P 2 O 7 ( T = 25 °C, P 3 1 21/ P 3 2 21, Z = 6, a = 5.5128(1) Å, c = 40.8723(11) Å) differs significantly from LT‐K 4 P 2 O 7 . The experimental results and those of DFT structure optimization yield bent pyrophosphate groups (bridging angle ∠(P−O b −P) ≈ 129°) for both LT‐ and HT‐Ag 4 P 2 O 7 , while their conformations change from eclipsed (HT) to staggered (LT). For one half of the Ag + cations in the HT modification, the unusual ninefold coordination in a truncated hexagonal pyramid changes to a distorted tetrahedral coordination in the LT form. Structural/dynamic instability expresses itself by formation of multinary compounds in the systems A 4 P 2 O 7 /Ag 4 P 2 O 7 ( A = Li, Na). Thus, the crystal structures of Li 3 AgP 2 O 7 and LiAg 3 P 2 O 7 show no similarity at all to that of Ag 4 P 2 O 7 , while the closely related crystal structure of Na 2 Ag 2 P 2 O 7 shows distinct ordering of Na + /Ag + despite similar ionic radii.

Needle to lath-shaped single crystals of KNaCaSi4O10 were obtained from the crystallization of a glass with a molar ratio of K2O:Na2O:CaO:SiO2 = 2:2:1:10. The crystals within the samples were intergrown. They form an interleaved microstructure, thereby providing mechanical support to one another. Structural investigations of this new synthetic compound in the quaternary system K2O-Na2O-CaO-SiO2 were performed using single-crystal X-ray diffraction experiments. The basic crystallographic data are as follows: triclinic symmetry, space group , a = 7.0784(5)& Aring;, b = 8.0744(6) & Aring;, c = 10.1449(9) & Aring;, alpha = 103.029(7)degrees, beta = 99.873(7)degrees, gamma = 116.103(8)degrees, V = 482.39(7) & Aring;(3), and Z = 2. The determination of the structure was performed using direct methods. Subsequent least-squares refinements of the coordinates and anisotropic displacement parameters for all 17 symmetrically independent atoms in the asymmetric unit converged at R(|F|) = 0.0351 for 1721 reflections and 154 parameters. According to Liebau's classification of oxosilicates, the present compound belongs to the group of tubular chain silicates. The fundamental chains of the tubes run parallel to the a-axis. More specifically, the tubes can be described as loop-branched dreier double chains:{lB, 2(1) (infinity)}[3Si(8)O(20)]. . An individual tube contains four-, six-, and eight-membered rings of silicate tetrahedra. The sodium and calcium atoms are distributed in an ordered manner on two different cation sites. Both positions show a similar 5 + 1 coordination environment. In fact, the inner [NaO5]- and [CaO5]-polyhedra can be approximated as distorted quadratic pyramids. These pyramids share common edges and form a second set of double chains, again running parallel to [100]. Linkage between adjacent silicate tubes is further provided by the potassium ions located in the centers of the aforementioned eight-membered rings. Each K cation has ten nearest oxygen ligands. Structural investigations were completed using Raman and infrared (IR) spectroscopy. The spectroscopic data were interpreted, and the bands were assigned to specific vibrational species with the help of density functional calculations at the HSEsol level of theory. The network of interlaced lath-shaped crystals formed during the devitrification process could lead to crack deflection and toughening. This makes KNaCaSi4O10 a potential new candidate for a crystalline phase in silicate-based glass ceramics.

The debate in the literature whether the triclinic room-temperature crystal structure of iron(II) pyrophosphate (Fe2P2O7) is centrosymmetric or not has been clearly resolved on the basis of new single-crystal X-ray intensity measurements. This study additionally revealed that Fe2P2O7 undergoes three reversible phase transitions between -140 and 190°C, with the modifications denoted with decreasing temperature as β, α3, α2 and α1. The room-temperature form, α2-Fe2P2O7, indeed crystallizes in a centrosymmetric but incommensurately modulated structure, a fact that has not been recognized for more than 40 years. For better comparison with the C-centred monoclinic thortveitite-type aristotype (space group type C2/m), the structure of the hettotype α2-Fe2P2O7 is described in the superspace group C1(αβγ)0 with a = 6.6393 (6), b = 8.4748 (6), c = 4.4839 (3) Å, α = 90.036 (5), β = 103.962 (7), γ = 92.929 (6)° and a modulation wavevector q = 0.4489 (3)a* + 0.2517 (3)b* + 0.3646 (3)c*. The α2 modification undergoes two phase transitions towards periodic structures. On heating, a triclinic structure described in C1 with very similar lattice parameters is realized above 85°C for the corresponding α3 modification. It can be considered as the non-modulated basic structure of the α2 modification. At about 185°C, α3-Fe2P2O7 transforms to the thortveitite-type β modification, which remains stable up to at least 1000°C. On cooling the α2 modification, a triclinic structure of the low-temperature α1 modification forms below -140°C, which can be considered as a twofold superstructure of the α3 modification with q = ½a* + ½b* + ½c*. The result of these phase transitions from the thortveitite-type β-modification via the triclinic α3 phase and the incommensurately modulated triclinic α2 modification to α1-Fe2P2O7 is the complete ordering of the pyrophosphate anion in the low-temperature phase with a P-O-P bridging angle of 151.91 (8)°. This ordering is accompanied by the lowering of the coordination number of one half of the Fe2+ ions from 6 to 5.

For decades, unraveling the heat treatment of sapphire has been a challenging issue. The present study offers new aspects that support the detection of heat treatment of sapphire. Natural geuda sapphire exhibits orange to red luminescence under longwave ultraviolet (LWUV; 365 nm) light, while heated geuda sapphire shows blue luminescence under shortwave ultraviolet (SWUV; 225 nm) light. The presence of melt inclusions in dissolved silks serves as an indicator of sapphire heat treatment. Fourier-transform infrared (FTIR) spectroscopy alone is insufficient for distinguishing unheated from heated sapphire. By combining orange to red luminescence with blue luminescence and melt inclusions, we provide a practical method for accurately differentiating natural gem-quality sapphire and heated gem-quality sapphire.

Almandine-spessartine garnet in a Moldanubian peraluminous pegmatoid (Bohemian Massif, Austria) shows asymmetric morphology, compositional zoning, and microstructural zoning, indicating directed crystal growth. Sector-specific variations in inclusion abundance and microstructures in {112} and {110} garnet sectors indicate facet-specific crystallization processes, associated with individual garnet surface configurations and a compositional boundary layer (CBL) present in the melt adjacent to growing garnet (Kohn et al., 2024). Rutile inclusions show distinct changes in abundance, aspect ratio, shape preferred orientations (SPOs) and crystallographic orientation relationships (CORs) between garnet growth zones. We quantified the SPOs of > 2400 rutile needles in two crystallographically equivalent {112}Grt rim sectors, and recorded the COR, location, habit and SPO of > 350 rutile inclusions in a transect across core and rim zones within one {112} Grt sector.Rutile inclusions are elongated parallel to the four ⟨111⟩ Grt directions, the three ⟨100⟩ Grt directions and one ⟨112⟩ Grt direction. The most frequent SPO for a given {112} Grt sector is the one closest to the garnet growth direction (i.e. the facet normal), whereas the SPO lying in the facet is exceedingly rare. Sectioning effects cannot explain these frequency variations. Based on the facet-specific SPO and COR statistics, we infer rutile inclusions formed by nucleation at the advancing garnet surface and subsequent co-growth with the host.Based on directly correlated SORs and CORs between elongate rutile inclusions and garnet host, specific CORs were pooled into three COR groups: 103R/111G (“one Rutile direction one Garnet direction”), 001R/111G and 001R/100G. Within one {112} Grt sector, core domains exhibit lower aspect ratios and higher abundance of rutile inclusions, with COR group 103R/111G being predominant. Contrastingly, the rim domain exhibits highly elongate rutile needles with lower abundance. The dominant COR group changes to 001R/111G and COR group 001R/100G appears. We suggest the decrease in inclusion abundance signals a decrease in the ratio of rutile nucleation rate to rutile growth rate, while the increase in aspect ratio signals an increase in the growth rate of rutile compared to the garnet growth rate (normal to the facet). The needle-bearing rim supposedly crystallized from a melt with higher Na, Si and OH– content compared to the core (Kohn et al. 2024). A corresponding increase in diffusion rates of components in the melt is hypothesized to have decreased supersaturation with respect to rutile in a CBL, decreasing rutile nucleation rates and affecting relative growth rates. The preference for particular SPO-COR combinations should be influenced by the garnet surface configuration upon heterogeneous nucleation of rutile. Radial and lateral variations of rutile CORs and SPOs are thus attributed to changes in the nature of the garnet/melt interface, and/or the garnet growth mechanism.Based on comparison with previous studies, changes in rutile COR group frequencies associated with increasing Si- (and likely OH–) content of the melt are a systematic feature of magmatic fractional crystallization in peraluminous pegmatitic systems containing rutile-bearing garnet.Funded by Austrian Science Fund (FWF): I4285-N37 and Slovenian Research Agency (ARRS): N1-0115Kohn et al. (2024), Lithos, DOI: 10.1016/j.lithos.2023.1074

The debate in the literature whether the triclinic room-temperature crystal structure of iron(II) pyrophosphate (Fe 2 P 2 O 7 ) is centrosymmetric or not has been clearly resolved on the basis of new single-crystal X-ray intensity measurements. This study additionally revealed that Fe 2 P 2 O 7 undergoes three reversible phase transitions between −140 and 190°C, with the modifications denoted with decreasing temperature as β, α 3 , α 2 and α 1 . The room-temperature form, α 2 -Fe 2 P 2 O 7 , indeed crystallizes in a centrosymmetric but incommensurately modulated structure, a fact that has not been recognized for more than 40 years. For better comparison with the C -centred monoclinic thortveitite-type aristotype (space group type C 2/ m ), the structure of the hettotype α 2 -Fe 2 P 2 O 7 is described in the superspace group C 1 (αβγ)0 with a = 6.6393 (6), b = 8.4748 (6), c = 4.4839 (3) Å, α = 90.036 (5), β = 103.962 (7), γ = 92.929 (6)° and a modulation wavevector q = 0.4489 (3) a * + 0.2517 (3) b * + 0.3646 (3) c *. The α 2 modification undergoes two phase transitions towards periodic structures. On heating, a triclinic structure described in C 1 with very similar lattice parameters is realized above 85°C for the corresponding α 3 modification. It can be considered as the non-modulated basic structure of the α 2 modification. At about 185°C, α 3 -Fe 2 P 2 O 7 transforms to the thortveitite-type β modification, which remains stable up to at least 1000°C. On cooling the α 2 modification, a triclinic structure of the low-temperature α 1 modification forms below −140°C, which can be considered as a twofold superstructure of the α 3 modification with q = ½ a * + ½ b * + ½ c *. The result of these phase transitions from the thortveitite-type β-modification via the triclinic α 3 phase and the incommensurately modulated triclinic α 2 modification to α 1 -Fe 2 P 2 O 7 is the complete ordering of the pyrophosphate anion in the low-temperature phase with a P—O—P bridging angle of 151.91 (8)°. This ordering is accompanied by the lowering of the coordination number of one half of the Fe 2+ ions from 6 to 5.

Almandine-spessartine garnet from the Moldanubian Zone in the Bohemian Massif (Austria) records microstructural and compositional features of crystal growth imprinted during the transition from pegmatoid magmatic to subsolidus state. Directed garnet growth results in asymmetric compositional, morphological and microstructural zoning. In addition, the garnet core shows sector zoning reflected by the predominance of phosphate inclusions in {110} sectors, and rutile inclusions in {112} sectors of garnet. These sector-specific abundances of inclusions are also expressed in the electron probe microanalytical data of the non-equivalent garnet sectors. The contribution of the submicron and nano-inclusions of phosphates and rutile is of crucial importance for thermobarometric methods based on P, Na and Ti in garnet. The sector-specific abundance of inclusions is referred to the surface configuration of individual garnet facets. Contrastingly, the microstructural growth banding observed in the {112} garnet sectors supposedly results from compositional fluctuations of a boundary layer at the garnet-melt interface. These fluctuations control the details of the inclusion microstructures, which differ even in crystallographically equivalent sectors.

Synthesis experiments were conducted in the quaternary system K2O-Na2O-CaO-SiO2, resulting in the formation of a previously unknown compound with the composition K0.72Na1.71Ca5.79Si6O19. Single crystals of sufficient size and quality were recovered from a starting mixture with a K2O:Na2O:CaO:SiO2 molar ratio of 1.5:0.5:2:3. The mixture was confined in a closed platinum tube and slowly cooled from 1150°C at a rate of 0.1°C min-1 to 700°C before being finally quenched in air. The structure has tetragonal symmetry and belongs to space group P4122 (No. 91), with a = 7.3659 (2), c = 32.2318 (18) Å, V = 1748.78 (12) Å3, and Z = 4. The silicate anion consists of highly puckered, unbranched six-membered oligomers with the composition [Si6O19] and point group symmetry 2 (C2). Although several thousands of natural and synthetic oxosilicates have been structurally characterized, this compound is the first representative of a catena-hexasilicate anion, to the best of our knowledge. Structural investigations were completed using Raman spectroscopy. The spectroscopic data was interpreted and the bands were assigned to certain vibrational species with the support of density functional theory at the HSEsol level of theory. To determine the stability properties of the novel oligosilicate compared to those of the chemically and structurally similar cyclosilicate combeite, we calculated the electronegativity of the respective structures using the electronegativity equalization method. The results showed that the molecular electronegativity of the cyclosilicate was significantly higher than that of the oligostructure due to the different connectivities of the oxygen atoms within the molecular units.

Abstract Rutile is an important accessory mineral in metamorphic rocks and is used as a geothermobarometer or geochronometer. This study aims to bridge the gap between diffusion studies in simplified and complex natural systems by investigating the incorporation and mobility of Al in natural rutile and its high-pressure polymorph TiO 2 (II). Experiments were performed at 0.1 MPa to 7 GPa, 1223–1373 K, at buffered μ(Al 2 O 3 ) and with f O 2 constrained to ≤CCO, which is the equilibrium between graphite and a CO-CO 2 gas phase. Based on electron probe microanalysis, secondary ion mass spectrometry and Fourier transform infrared analyses, we suggest a complex combination of mechanisms to explain the incorporation of Al and H in natural rutile and TiO 2 (II). This includes: (1) the incorporation of Al 3+ on octahedral Ti-sites charge balanced by the formation of oxygen vacancies; and (2) the incorporation of oxygen in interstitial positions charge balanced by hydrogen interstitials. Determined Al-diffusivities in natural TiO 2 are approximately eight to nine orders of magnitude faster compared to previously published data. A possible explanation includes a significantly enhanced rate of ionic diffusion through the combined effect of hydrolytic weakening, enhanced Al-diffusion through extended defects and to a minor extent oxygen fugacity variations. Consequently, results of this study question that the inferred high closure temperatures for the Al-in-rutile geothermobarometer can be applied to all natural systems.

AbstractThe Drewer quarry located in the Rhenish Massif is a well-studied outcrop that comprises Upper Devonian (Famennian) to Lower Carboniferous (Viséan) strata. Within the Drewer deposits two black shale intervals have been described that are linked to two global oceanic anoxic events, the Hangenberg Event and the Lower Alum Shale Event. The black shales associated with the Middle Tournaisian Lower Alum Shale Event contain abundant phosphatic concretions, which were investigated using thin section petrography, powder X-ray diffraction, Fourier-transform infrared spectrometry and scanning electron microscopy. The concretions formed during several growth phases under anoxic and at least episodically sulphidic conditions within the sediment and served as a substrate for subsurface microbial mats that formed phosphatic microstromatolites. The microstromatolites occur either as partially branched columns of up to 600 µm in length attached to the phosphatic concretions or as smaller, bulbous aggregates surrounding the concretions. Element mapping identified the presence of pyrite and other metal sulphides within the phosphatic microstromatolites. The carbon and oxygen stable isotopic composition of phosphate-associated carbonate within the phosphatic microstromatolites suggests that the mat-forming microorganisms were probably anaerobic, chemotrophic microbial communities dwelling in the anoxic environment during the Lower Alum Shale Event. Such interpretation agrees with the deeper-water depositional setting of the Lower Alum Black Shale and its high content of organic matter, suggesting that chemotrophic microbial mats are potent agents of phosphogenesis in general, and of the formation of phosphatic stromatolites in particular.

Synthetic and naturally occurring forms of tricopper orthotellurate, CuII3TeVIO6 (the mineral mcalpineite) have been investigated by 3D electron diffraction (3D ED), X-ray powder diffraction (XRPD), Raman and infrared (IR) spectroscopic measurements. As a result of the diffraction analyses, CuII3TeVIO6 is shown to occur in two polytypes. The higher-symmetric CuII3TeVIO6-1C polytype is cubic, space group Ia3, with a = 9.537 (1) Å and V = 867.4 (3) Å3 as reported in previous studies. The 1C polytype is a well characterized structure consisting of alternating layers of CuIIO6 octahedra and both CuIIO6 and TeVIO6 octahedra in a patchwork arrangement. The structure of the lower-symmetric orthorhombic CuII3TeVIO6-2O polytype was determined for the first time in this study by 3D ED and verified by Rietveld refinement. The 2O polytype crystallizes in space group Pcca, with a = 9.745 (3) Å, b = 9.749 (2) Å, c = 9.771 (2) Å and V = 928.3 (4) Å3. High-precision XRPD data were also collected on CuII3TeVIO6-2O to verify the lower-symmetric structure by performing a Rietveld refinement. The resultant structure is identical to that determined by 3D ED, with unit-cell parameters a = 9.56157 (19) Å, b = 9.55853 (11) Å, c = 9.62891 (15) Å and V = 880.03 (2) Å3. The lower symmetry of the 2O polytype is a consequence of a different cation ordering arrangement, which involves the movement of every second CuIIO6 and TeVIO6 octahedral layer by (1/4, 1/4, 0), leading to an offset of TeVIO6 and CuIIO6 octahedra in every second layer giving an ABAB* stacking arrangement. Syntheses of CuII3TeVIO6 showed that low-temperature (473 K) hydrothermal conditions generally produce the 2O polytype. XRPD measurements in combination with Raman spectroscopic analysis showed that most natural mcalpineite is the orthorhombic 2O polytype. Both XRPD and Raman spectroscopy measurements may be used to differentiate between the two polytypes of CuII3TeVIO6. In Raman spectroscopy, CuII3TeVIO6-1C has a single strong band around 730 cm-1, whereas CuII3TeVIO6-2O shows a broad double maximum with bands centred around 692 and 742 cm-1.

An entry from the Inorganic Crystal Structure Database, the world’s repository for inorganic crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the joint CCDC and FIZ Karlsruhe Access Structures service and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

MII3(TeIVO3)2(OH)2 (M = Mg, Mn, Co, Ni) compounds crystallize isotypically in the hexagonal space group P63mc (No. 186) with unit-cell parameters of a ≈ 13 Å, c ≈ 5 Å. In the crystal structure, a framework with composition M3(TeO3)2(OH)1.50.5+ defines large hexagonal channels extending along [001] where the remaining OH− anions are located. Crystal-growth studies under mild hydrothermal conditions with subsequent structure analyses on basis of X-ray diffraction methods revealed that parts of other anions present in solution such as CO32−, SO42−, SeO42−, NO3−, Cl− or Br− could partly replace the OH− anions in the channels. The incorporation of such anions into the M3(TeO3)2(OH)2 structure was confirmed by energy-dispersive X-ray spectrometry (EDS) measurements and Raman spectroscopy of selected single-crystals.

Additional information on methods, Figures S1–S3, and Tables S1–S4.

Abstract Marine hydrocarbon seeps are sites of chemosynthetic microbial activity and authigenic carbonate formation. Seep limestones are typified by a range of geochemical signatures of microbial hydrocarbon oxidation, but only few seep deposits reveal mesofabrics that can be regarded as evidence of microbial activity. A Cretaceous methane‐seep limestone from Waipiro Bay, New Zealand, exhibits a fabric composed of cryptocrystalline carbonate fluorapatite between carbonate coated grains and spheroidal calcite. To understand the unusual Waipiro deposit, a paragenetic sequence has been derived for coated grains, spheroidal calcite and carbonate fluorapatite using thin section petrography, scanning electron microscopy, Fourier‐transform infrared spectroscopy and stable isotope geochemistry. The formation of 13C‐depleted coated grains (δ13C values as low as −15.8‰ Vienna‐Pee Dee Belemnite) and spheroidal calcite (δ13C values as low as −21.3‰) was favoured by hydrocarbon oxidation. Fibrous banded and botryoidal cement, a typical early diagenetic phase of hydrocarbon‐seep deposits, features δ13C values as low as −22.9‰. Coated grains and spheroidal calcite grew by displacive growth in a gel‐like medium, probably a microbial mat. Phosphorus is a mobile element and marine pore waters are typically undersaturated with respect to carbonate fluorapatite. Specific conditions are consequently required to retain sufficient concentrations to precipitate carbonate fluorapatite. Possible sources of phosphorus for the formation of the 13C‐depleted Waipiro carbonate fluorapatite (mean δ13C value of −15.4‰) include (a) the oxidation of sedimentary organic matter by organoclastic sulphate reduction, (b) the degradation of the microbial mat itself and (c) the active release of polyphosphate by sulphide‐oxidizing bacteria. This study suggests that the formation of the Cretaceous Waipiro seep limestone involved an interplay of biogeochemical processes including sulphate‐driven anaerobic oxidation of methane, organoclastic sulphate reduction and possibly nitrate‐dependent sulphide oxidation. It further demonstrates that coated grains resembling oncoids are not restricted to shallow water, photosynthesis‐dependent ecosystems.