灰岩和白云岩溶解速率控制机理的比较

碳酸盐岩溶解的速率控制过程包括:(1)岩石表面上的非均相化学反应;(2)离子从岩石表面通过扩散向溶液中的传输;(3)CO2向H 和HCO-3的转换.通常是这3个过程中的最慢过程决定着碳酸盐岩的溶解速率.然而,实验和理论分析发现,在条件相似的情况下,白云岩的初始溶解速率不仅只有灰岩的1/3～1/60,而且灰岩和白云岩的溶解呈现出不同的速率控制机理.如对灰岩而言,在实验中加入能催化CO2转换反应的生物碳酸酐酶(CA)后,其溶解速率增加出现在CO2分压>100Pa的区域,最高可达10倍;而对白云岩,其溶解速率增加出现在CO2分压<10000Pa的区域,且增加仅3倍左右.此外,虽然2类岩石的溶解也均受水动力条件(旋速或流速)的控制,且主要出现在CO2分压<1000Pa的区域,但灰岩的溶解对水动力条件的变化比白云岩溶解更敏感.这些发现在解释和揭示自然界白云岩和灰岩岩溶发育及其相关资源环境问题的差异方面具有重要意义.     

振动荷载及温度对白云岩和石灰岩溶蚀影响试验研究

在岩溶地区,列车运行期间的振动会对白云岩和石灰岩的水岩作用过程产生影响,再加上复杂的外界条件,这一影响会更加显著。以桂北岩溶石山地区白云岩和石灰岩为研究对象,利用自主设计的试验装置进行室内模拟试验,分别研究振动荷载及不同温度条件对白云岩和石灰岩的溶蚀的影响规律。研究发现：①常温常压条件下,在静止的酸性溶液中,白云岩的溶蚀速率小于石灰岩;②在H⁺充足的条件下,随着温度（0~45 ℃）的升高,石灰岩和白云岩的溶蚀速率都会增大,温度变化对于白云岩溶蚀影响更为显著;③白云岩和石灰岩的溶蚀受振动条件的强烈影响,二者的溶蚀量都随着振动次数的增加而增大,且在相同振动条件下石灰岩的溶蚀速率始终大于白云岩。该研究可为桂北岩溶石山地区的工程设计和建设提供理论依据。     

3种常见细菌胞外特征有机酸对方解石的溶蚀研究

为了更深入地了解微生物与大气可吸入矿物细颗粒的作用机理,实验以方解石(PM2.5)为研究对象,采用反相高效液相色谱(RP-HPLC)对硅酸盐细菌、金黄色葡萄球菌和大肠杆菌胞外有机酸主要组分进行定性定量分析,考察了细菌胞外有机酸对方解石的溶蚀效应。实验结果表明,3种常见细菌的胞外有机酸主要组分有草酸、乳酸、柠檬酸和琥珀酸;不同有机酸对方解石的溶蚀效应不同,但均能不同程度地促进Ca离子的释放;草酸、乳酸和琥珀酸对方解石发生作用后,表现为颗粒失重,而柠檬酸对方解石发生作用后,导致其发生重结晶行为,能改变方解石的表面形貌,使残余固体呈纤维状;在草酸和柠檬酸作用下,残余固体表现出了对有机酸基团(如C=O、C—C或C—H)的吸附,特别是柠檬酸在1 600~500 cm-1区域。     

Bank-derived carbonate sediment transport and dissolution in the Hawaiian Archipelago

This investigation used two approaches to examine the flux of bank-derived carbonate particles and determine the potential influence of benthic carbonate particle dissolution on the carbon chemistry of the waters around the Hawaiian Archipelago. First, the particle flux near several representative carbonate banks in the Hawaiian Archipelago was measured and compared with the flux at a distal site (ALOHA) approximately 100 km north of Oahu, Hawaii. The results of four sediment trap deployments on three carbonate banks in the Hawaiian Archipelago demonstrate that the flux of bank-derived carbonate particles are consistently one to two orders of magnitude higher than the fluxes at the distal station. Furthermore, the mineralogy of the carbonate flux near the banks, which includes very soluble bank-derived aragonite and magnesian calcite particles, is distinctly different from that of the distal fluxes. Second, the chemistry of the waters at each bank station along the archipelago was characterized and compared with the chemistry of the distal waters to determine if differences in the particle flux were reflected in the carbon chemistry. Higher alkalinity and carbonate ion concentrations were observed around all of the banks studied. The saturation state of these waters suggests that the dissolution of some magnesian calcite and aragonite phases could explain the higher alkalinity values. Calculations suggest that the dissolution of benthically-derived aragonite and magnesian calcite may be an important component of the North Pacific alkalinity budget and a potential sink for anthropogenic CO₂.     

Laboratory study of self-potential signals during releasing of CO2 and N2 plumes

Carbon Capture Sequestration (CCS) projects require, for safety reasons, monitoring programmes focused on surveying gas leakage on the surface. Generally, these programmes include detection of chemical tracers that, once on the surface, could be associated with CO₂ degassing. We take a different approach by analysing feasibility of applying electrical surface techniques, specifically Self-Potential. A laboratory-scale model, using water-sand, was built for simulating a leakage scenario being monitored with non-polarisable electrodes. Electrical potentials were measured before, during and after gas injection (CO₂ and N₂) to determine if gas leakage is detectable. Variations of settings were done for assessing how the electrical potentials changed according to size of electrodes, distance from electrodes to the gas source, and type of gas. Results indicated that a degassing event is indeed detectable on electrodes located above injection source. Although the amount of gas could not be quantified from signals, injection timespan and increasing of injection rate were identified. Even though conditions of experiments were highly controlled contrasting to those usually found at field scale, we project that Self-Potential is a promising tool for detecting CO₂ leakage if electrodes are properly placed.     

CO2流体对火山碎屑岩改造作用的实验研究

以塔木察格盆地火山碎屑岩为研究对象,研究不同温度下(100、120、140、160、180℃)CO2流体对火山碎屑岩(流纹质凝灰岩、沉凝灰岩)成分的改造.研究发现:在CO2流体的作用下,火山碎屑岩中的长石、碳酸盐矿物发生溶蚀,且其溶蚀强度随温度的升高而增大,石英的溶蚀程度较弱;火山碎屑岩中的凝灰质成分易溶蚀,并且是CO2流体溶蚀火山碎屑岩的主要对象;通过扫描电镜观察发现沉凝灰岩在160℃下样品表面有绿泥石(?)和一水软铝石(?)生成.结合塔木察格盆地中的碳酸盐矿物(尤其是片钠铝石)的存在及盆地中次生溶孔大量发育的特征,认为盆地内有CO2流体活动且CO2流体对塔木察格盆地次生孔隙的形成有重要的贡献.     

四川盆地理论CO_2地质利用与封存潜力评估

结合CO_2地质利用与封存技术机理,在国际权威潜力评估公式的基础上,系统地提出了适合中国地质背景的次盆地尺度CO_2封存潜力评估方法及关键参数取值。同时,以四川盆地为例,依次开展了枯竭油田地质封存与CO_2强化石油开采、枯竭气田与CO_2强化采气、不可采煤层地质封存与CO_2驱替煤层气,以及咸水层地质封存技术的CO_2地质封存潜力。结果表明,四川盆地利用深部咸水层与枯竭天然气田CO_2地质封存潜力最大,期望值分别达154.20×10~8t和53.73×10~8t。其中,枯竭天然气田因成藏条件好、勘探程度高、基础建设完善,为四川盆地及其周边利用枯竭气田CO_2地质封存技术实现低碳减排提供了早期示范机会。CO_2地质利用与封存潜力评估方法,对进一步开展全国次盆地尺度理论封存潜力评估与工程规划具有重要意义。     

Flue gas and pure CO2 sorption properties of coal: A comparative study

Presently many research projects focus on the reduction of anthropogenic CO₂ emissions. It is intended to apply underground storage techniques such as flue gas injection in unminable coal seams. In this context, an experimental study has been performed on the adsorption of pure CO₂ and preferential sorption behavior of flue gas. A coal sample from the Silesian Basin in Poland (0.68% V Rr), measured in the dry and wet state at 353 K has been chosen for this approach. The flue gas used was a custom class industrial flue gas with 10.9% of CO₂, 0.01% of CO, 9% of H₂, 3.01% of CH₄, 3.0% of O₂, 0.106% of SO₂ and nitrogen as balance.Adsorption isotherms of CO₂ and flue gas were measured upto a maximum of 11 MPa using a volumetric method. Total excess sorption capacities for CO₂ on dry and wet Silesia coal ranged between 1.9 and 1.3 mmol/g, respectively. Flue gas sorption capacities on dry and wet Silesia coal were much lower and ranged between 0.45 and 0.2 mmol/g, respectively, at pressures of 8 MPa. The low sorption capacity of wet coal has resulted from water occupying some of the more active adsorption sites and hence reducing the heterogeneity of adsorption sites relative to that of dry coal. Desorption tests with flue gas were conducted to study the degree of preferential sorption of the individual components. These experiments indicate that CO₂ is by far the prefered sorbing component under both wet and dry conditions. This is followed by CH₄. N₂ adsorbs very little on the coal in the presence of CO₂ and CH₄. It is also observed that the adsorption of CO₂ onto coal is not significantly hindered by the addition of other gases, other than dilution effect of the pressure.In addition to the sorption experiments, the density of the flue gas mixture has been determined up to 18 MPa at 318 K. A very good precision of these measurements were documented by volumetric methods.     

The rise of trees and their effects on Paleozoic atmospheric CO2 and O2

The rise of large vascular plants during the mid-Paleozoic brought about a major increase in the rates of weathering of silicate minerals that induced a drop in the level of atmospheric CO₂ and contributed, via the atmospheric greenhouse effect, to global cooling and the initiation of the most long lived and a really extensive glaciation of the past 550 million years. Sedimentary burial of the microbiologically resistant remains of the plants resulted during the Permo-Carboniferous in both further lowering of CO₂ and in elevation of atmospheric O₂. Evidence of changes in CO₂ and O₂ are provided by mathematical models, studies of paleosols, fossil plants, fossil insects, and the effects of modern plants on silicate weathering, and by laboratory studies of the effects of changes in O₂ on plants and insects. To cite this article: R.A. Berner, C. R. Geoscience 335 (2003).     

Experimental identification of CO2–water–rock interactions caused by sequestration of CO2 in Westphalian and Buntsandstein sandstones of the Campine Basin (NE-Belgium)

Geological sequestration of CO₂ is one of the options studied to reduce greenhouse gas emissions. Although the feasibility of this concept is proven, apart from literature data on modelling still little is known about the CO₂–water–rock interactions induced by CO₂-injection.To evaluate the effect of CO₂–water–rock interactions on three sandstone aquifers in NE-Belgium an experimental setup was built. Eighteen experiments were performed in which sandstones were exposed to supercritical CO₂. CO₂–water–rock interactions were deduced from the evolution of aqueous concentrations of 25 species and a thorough characterisation of the sandstones before and after treatment. The results show that dissolution of ankerite/dolomite and Al-silicates could enhance porosity/permeability. The observed precipitation of end-member carbonates could increase storage capacity if it exceeds carbonate dissolution. Precipitation of the latter and of K-rich clays as observed, however, can hamper the injection.     

Dissolution of Carbonate Sediments Under Rising <Emphasis Type="Italic">p</Emphasis>CO<Subscript>2</Subscript> and Ocean Acidification: Observations from Devil’s Hole,Bermuda

Rising atmospheric pCO₂ and ocean acidification originating from human activities could result in increased dissolution of metastable carbonate minerals in shallow-water marine sediments. In the present study, in situ dissolution of carbonate sedimentary particles in Devil’s Hole, Bermuda, was observed during summer when thermally driven density stratification restricted mixing between the bottom water and the surface mixed layer and microbial decomposition of organic matter in the subthermocline layer produced pCO₂ levels similar to or higher than those levels anticipated by the end of the 21st century. Trends in both seawater chemistry and the composition of sediments in Devil’s Hole indicate that Mg-calcite minerals are subject to selective dissolution under conditions of elevated pCO₂. The derived rates of dissolution based on observed changes in excess alkalinity and estimates of vertical eddy diffusion ranged from 0.2 mmol to 0.8 mmol CaCO₃ m⁻² h⁻¹. On a yearly basis, this range corresponds to 175–701 g CaCO₃ m⁻² year⁻¹; the latter rate is close to 50% of the estimate of the current average global coral reef calcification rate of about 1,500 g CaCO₃ m⁻² year⁻¹. Considering a reduction in marine calcification of 40% by the year 2100, or 90% by 2300, as a result of surface ocean acidification, the combination of high rates of carbonate dissolution and reduced rates of calcification implies that coral reefs and other carbonate sediment environments within the 21st and following centuries could be subject to a net loss in carbonate material as a result of increasing pCO₂ arising from burning of fossil fuels.     

Hydrometallurgical processing and recovery of molybdenum trioxide from spent catalyst

Hydrometallurgical processing of spent hydrodesulphurisation (HDS) catalyst for the recovery of molybdenum using sodium carbonate and hydrogen peroxide mixtures was investigated. The results indicated that the recovery of molybdenum was largely dependent on the concentrations of Na₂CO₃ and H₂O₂ in the reaction medium, which controls the acidity of the leach liquor and carry over of impurities such as Al, Ni, P, Si and V. Leaching process was exothermic and leaching efficiency of molybdenum decreased with increasing solid to liquid ratio. Large scale leaching of spent catalyst, under optimum conditions: 20% pulp density, 85 g/L Na₂CO₃, 10 vol.% H₂O₂ and 1 h reaction, resulted a leaching efficiency of 84% Mo. The obtained leach liquor contained (g/L): Mo — 22.0, Ni — 0.015 and Al — 0.82, P — 1.1, Si — 0.094 and minor quantities of V — 8 mg/L, As and Co — < 1 mg/L. Recovery of Mo from leach solution as MoO₃ of 97.30% purity was achieved by ammonium molybdate precipitation method.     

高温下TiO2的相转变动力学及后期颗粒生长研究

利用原位高温XRD方法测量了锐钛矿相TiO2在不同温度下(850、863、875、888、900℃)时间与转化率之间的关系,证实锐钛矿-金红石相转变为一级反应。计算的相转变活化能为432.788±25.657 kJ/mol,指前因子为4.847 454×1015 s-1。利用ESEM观察了样品在相转变完成后,经历不同温度(888、900、930、960、1 000℃)相同烧结时间(10 h),和相同温度(1 000℃)不同烧结时间(0、5、10、15、20 h)下颗粒晶型变化和颗粒长大情况。在1 000℃以下时,温度对颗粒粒度的影响并不明显,并且不出现完好的晶型。1 000℃时,随着时间的延长,颗粒粒度不断增大,并在10 h以后出现完好晶型。根据lnDt(Dt为t时间下颗粒的平均粒度)和lnt的关系图,认为颗粒生长应经过两个不同时期。     

The control of Phanerozoic atmosphere and seawater composition by basalt–seawater exchange reactions

We present results from a long term geochemical cycling model, with a focus on the sensitivity of atmospheric carbon dioxide, oxygen, and the major element composition of seawater to seafloor spreading rates. This model incorporates rock weathering, basalt–seawater exchange reactions, and the formation and destruction of chemical sediments and organic matter. Hydrothermal reactions between seafloor and seawater involving calcium, magnesium, sodium, potassium, sulfate and carbon are the high temperature counterparts to low temperature redox, weathering, precipitation and diagenetic reactions. A major source of uncertainty is the extent to which these exchange fluxes are controlled by seafloor spreading rate. In addition, the return fluxes of these components to the atmospheric and primary silicate reservoirs reflect not only the overall rates of subduction and metamorphism, but the distribution of the overlying sedimentary burden and authigenic minerals formed during basalt alteration as well. In particular, we show how the stoichiometry of exchange fluxes (Mg/Ca and SO₄/Ca) may buffer atmospheric CO₂ and O₂ concentrations.     

Climatic and lithologic controls on the temporal and spatial variability of CO2 consumption via chemical weathering: An example from the Australian Victorian Alps

A detailed geochemical study on river waters of the Australian Victorian Alps was carried out to determine: (i) the relative significance of silicate, carbonate, evaporite and sulfide weathering in controlling the major ion composition and; (ii) the factors regulating seasonal and spatial variations of CO₂ consumption via silicate weathering in the catchments. Major ion chemistry implies that solutes are largely derived from evaporation of precipitation and chemical weathering of carbonate and silicate lithologies. The input of solutes from rock weathering was determined by calculating the contribution of halite dissolution and atmospheric inputs using local rain and snow samples. Despite the lack of carbonate outcrops in the study area and waters being undersaturated with respect to calcite, the dissolution of vein calcite accounts for up to 67% of the total dissolved cations, generating up to 90% of dissolved Ca and 97% of Mg. Dissolved sulfate has δ³⁴S values of 16 to 20‰_CDT, indicating that it is derived predominantly from atmospheric deposition and minor gypsum weathering and not from bacterial reduction of FeS₂. This militates against sulphuric acid weathering in Victorian rivers. Ratios of Si vs. the atmospheric corrected Na and K concentrations range from ~ 1.1 to ~ 4.3, suggesting incongruent weathering from plagioclase to smectite, kaolinite and gibbsite.Estimated long-term average CO₂ fluxes from silicate weathering range from ~ 0.012 × 10⁶ to 0.039 × 10⁶ mol/km²/yr with the highest values in rivers draining the basement outcrops rather than sedimentary rocks. This is about one order of magnitude below the global average which is due to low relief, and the arid climate in that region. Time series measurements show that exposure to lithology, high physical erosion and long water–rock contact times dominate CO₂ consumption fluxes via silicate weathering, while variations in water temperature are not overriding parameters controlling chemical weathering. Because the atmospheric corrected concentrations of Na, K and Mg act non-conservative in Victorian rivers the parameterizations of weathering processes, and net CO₂ consumption rates in particular, based on major ion abundances, should be treated with skepticism.     

Chemical structures of coal lithotypes before and after CO2 adsorption as investigated by advanced solid-state C nuclear magnetic resonance spectroscopy

Four lithotypes (vitrain, bright clarain, clarain, and fusain) of a high volatile bituminous Springfield Coal from the Illinois Basin were characterized using advanced solid-state ¹³C nuclear magnetic resonance (NMR) spectroscopy. The NMR techniques included quantitative direct polarization/magic angle spinning (DP/MAS), cross polarization/total sideband suppression (CP/TOSS), dipolar dephasing, CH_n selection, and recoupled C-H long-range dipolar dephasing techniques. The lithotypes that experienced high-pressure CO₂ adsorption isotherm analysis were also analyzed to determine possible changes in coal structure as a result of CO₂ saturation at high pressure and subsequent evacuation. The main carbon functionalities present in original vitrain, bright clarain, clarain and fusain were aromatic carbons (65.9%-86.1%), nonpolar alkyl groups (9.0%-28.9%), and aromatic C-O carbons (4.1%-9.5%). Among these lithotypes, aromaticity increased in the order of clarain, bright clarain, vitrain, and fusain, whereas the fraction of alkyl carbons decreased in the same order. Fusain was distinct from other three lithotypes in respect to its highest aromatic composition (86.1%) and remarkably small fraction of alkyl carbons (11.0%). The aromatic cluster size in fusain was larger than that in bright clarain. The lithotypes studied responded differently to high pressure CO₂ saturation. After exposure to high pressure CO₂, vitrain and fusain showed a decrease in aromaticity but an increase in the fraction of alkyl carbons, whereas bright clarain and clarain displayed an increase in aromaticity but a decrease in the fraction of alkyl carbons. Aromatic fused-rings were larger for bright clarain but smaller for fusain in the post-CO₂ adsorption samples compared to the original lithotypes. These observations suggested chemical CO₂-coal interactions at high pressure and the selectivity of lithotypes in response to CO₂ adsorption.     

苏北盆地黄桥地区富CO_2流体对二叠系龙潭组砂岩储层的改造与意义

富CO_2流体-砂岩相互作用是砂岩储层次生孔隙的重要形成机制。苏北黄桥地区作为中国重要的CO_2气产区,富CO_2流体对上二叠统龙潭组砂岩储层的改造问题备受关注。为揭示富CO_2流体的作用特征及其对储层的影响,对黄桥地区典型钻井开展了系统的岩心描述和岩矿鉴定,并进行了微区原位观测和相关地球化学分析。结果表明,在靠近CO_2流体活动强烈的断裂带部位(特别是断层上盘),砂岩中碳酸盐胶结物基本溶蚀殆尽,仅存少量交代成因菱铁矿,同时钾长石类碎屑溶蚀非常强烈,并伴随高岭石等矿物沉淀,以及石英次生加大,还发育片钠铝石等指示高浓度CO_2作用的特征矿物,形成与CO_2流体作用相关的特征矿物组合(片钠铝石+高岭石+次生石英+菱铁矿);而在远离断裂的部位,受CO_2流体影响较弱,溶蚀作用也较弱,有较多的次生方解石沉淀,形成了以方解石+菱铁矿为主的自生矿物组合。前者次生孔隙发育,后者则更加致密。据此提出了深源断裂主控下与富CO_2流体作用相关的储层发育模式,为油气勘探和开发提供了新的思路。     

新疆阿尔泰塔拉特铁铅锌矿床流体包裹体研究及矿床成因

塔拉特铁铅锌矿位于新疆阿尔泰造山带南缘的阿巴宫多金属成矿带,矿体赋存于克兰盆地下泥盆统康布铁堡组中,为一套海相中酸性火山岩-火山碎屑岩、陆源碎屑沉积岩-碳酸盐岩建造,脉状矿体受阿巴宫大断裂次级断裂控制.根据矿物组合和脉体穿插关系,塔拉特铁铅锌矿可分为4个成矿阶段:矽卡岩,氧化物,硫化物和碳酸盐阶段,后3个阶段均有石英共生.其中,硫化物(方铅矿-闪锌矿±磁黄铁矿±黄铜矿)阶段是铅锌成矿的主要阶段.不同阶段石英中广泛发育流体包裹体,可分为水溶液包裹体(W型)、纯CO2包裹体(PC型)、CO2-NaCl-H2O包裹体(C型)及含子矿物多相包裹体(S型)4类.冷热台显微测温和激光拉曼分析表明,氧化物阶段石英含有4种类型的包裹体,以W型为主,C型和S型包裹体次之,包裹体均一温度介于271～ 426℃,W型和C型盐度范围0.5％～22.4％ NaCleqv,S型包裹体盐度30.5％ ～40.6％ NaCleqv;硫化物阶段的石英流体包裹体为W型、C型和PC型,均一温度为204 ～ 269℃,盐度介于0.2％～15.6％ NaCleqv之间;碳酸盐阶段的矿物只含W型包裹体,均一温度集中在175～211℃之间,盐度为1.1％ ～9.9％ NaCleqv.利用C型包裹体对硫化物阶段成矿压力估算,得到107 ～ 171MPa,对应深度为4～6km.塔拉特铁铅锌矿初始成矿流体具有高温、高盐度、富CO2的特征,但碳酸盐阶段低盐度、贫CO2,流体不混溶和混合作用导致了成矿物质的沉淀.塔拉特铁铅锌矿的地质和成矿流体特征显示其为碰撞造山体制形成的矽卡岩型成矿系统.     

Time-lapse crosswell seismic and VSP monitoring of injected CO<Subscript>2</Subscript> in a brine aquifer

Seismic surveys successfully imaged a small scale CO₂ injection (1,600 ton) conducted in a brine aquifer of the Frio Formation near Houston, Texas. These time-lapse borehole seismic surveys, crosswell and vertical seismic profile (VSP), were acquired to monitor the CO₂ distribution using two boreholes (the new injection well and a pre-existing well used for monitoring) which are 30 m apart at a depth of 1,500 m. The crosswell survey provided a high-resolution image of the CO₂ distribution between the wells via tomographic imaging of the P-wave velocity decrease (up to 500 m/s). The simultaneously acquired S-wave tomography showed little change in S-wave velocity, as expected for fluid substitution. A rock physics model was used to estimate CO₂ saturations of 10–20% from the P-wave velocity change. The VSP survey resolved a large (∼70%) change in reflection amplitude for the Frio horizon. This CO₂ induced reflection amplitude change allowed estimation of the CO₂ extent beyond the monitor well and on three azimuths. The VSP result is compared with numerical modeling of CO₂ saturations and is seismically modeled using the velocity change estimated in the crosswell survey.     

A computer simulation study of OH defects in Mg2SiO4 and Mg2GeO4 spinels

Classical atomistic simulation techniques have been used to investigate the energies of hydrogen defects in Mg₂SiO₄ and Mg₂GeO₄ spinels. Ringwoodite (γ-Mg₂SiO₄) is considered to be the most abundant mineral in the lower part of the transition zone and can incorporate large amounts of water in the form of hydroxyls, whereas the germanate spinel (γ-Mg₂GeO₄) corresponds to a low-pressure structural analogue for ringwoodite. The calculated defect energies indicate that the most favourable mechanisms for hydrogen incorporation are coupled either with the reduction of ferric iron or with the creation of tetrahedral vacancies. Hydrogen will go preferentially into tetrahedral vacancies, eventually leading to the formation of the hydrogarnet defect, before associating with other negatively charged point defects. The presence of isolated hydroxyls is not expected. The same trend is observed for germanate, and thus γ-Mg₂GeO₄ could be used as a low-pressure analogue for ringwoodite in studies of water-related defects and their effect on physical properties.