The crystal structure of CaFe3+SiAlO6 and the crystal chemistry of Fe3+—Al3+ substitution in calcium Tschermak's pyroxene

The crystal structure of a synthetic CaFe³⁺Al-SiO₆ pyroxene (20 kb, 1,375° C) with unit cell dimensions a=9.7797(16), b=8.7819(14), c=5.3685(5) Å, =105.78(1), space group C2/c has been refined by the method of least squares to an R-factor of 0.025 based on 812 reflections measured on an automatic single crystal diffractometer. The octahedral M1 site is occupied by 0.82 Fe³⁺ and 0.18 Al³⁺. Within the tetrahedral T site, Si⁴⁺ (0.50), Al³⁺ (0.41) and Fe³⁺ (0.09) ions are completely disordered, although submicroscopic domains with short-range order are very likely. The octahedral site preference energy of the Fe³⁺ ions with respect to Al³⁺ ions in CaFe³⁺AlSiO₆ is about 10 kcal/mole, which is much higher than that found in Y₃Al _x Fe_5–2O₁₂ garnets. Topologically the structure of CaFe³⁺AlSiO₆ is intermediate between that of diopside and calcium Tschermak's pyroxene, CaAlAlSiO₆. For CaM³⁺ AlSiO₆ clinopyroxenes an increase in the size of the M1 octahedron is accompanied by an increase in the average M2-0, bridging T-0 and 03-03 distances and kinking of the tetrahedral chain.     

Magnetism in aluminous goethite

Magnetism in fine particles of goethite which contain 5, 11, 18 and 30 mole % aluminium has been investigated. Mössbauer spectroscopy was used, with the samples at 4.2 K and in applied magnetic fields of 0–8 T. Analysis was via a mean field model, in which the spectra from each sample were simultaneously least-squares fitted. The presence of aluminium increases the effective anisotropy, introduces a range of anisotropy fields, and induces ferrimagnetism. Quantitative measurements of these effects are presented. It is surmised that the anisotropy is principally determined by the local environment of ferric ions, and that the distribution of anisotropy fields is the result of a variety of atomic neighbour configurations. Ferrimagnetism is understood to arise from a strong preference, with 100% success at intermediate concentrations, for aluminium ions to crystallise on the same sublattice. Simulations of the net magnetic moment in small particle systems are presented, and it is shown that statistical fluctuations and the effects of surfaces are unimportant in the particles studied.     

Gibbs energy of aluminous minerals

The discrepancy between the tabulated Gibbs Energies of Formation for Al₂SiO₅ and corundum relative to muscovite and kaolinite is considered to lie principally with the latter two minerals. New values for heat of formation of gibbsite [Gbs] will affect the tabulated H _f ⁰ , G _f(298,1) ⁰ for the other aluminous minerals which are referred to gibbsite as calorimetric aluminum reference. Gibbs Energy Difference Functions, calculated from phase equilibria in the system CaO-Al₂O₃-SiO₂-(H₂O-CO₂), can be used to estimate consistent H _f ⁰ , G _f(298,1) ⁰ values for aluminous minerals. A self consistent data set is presented referred to G _f(298,1) ⁰ [Corundum]=–378.08 kcal mol^–1. Two independent values for G _f(298,1) ⁰ [Anorthite]=–961.52 and –960.29 kcal, from a recalculation of the H _f ⁰ [Anor] based upon the revised H _f(298,1) ⁰ [Gbs]=–309.325 kcal mol^–1 and from measurement of silica activity on the anorthite-saturated part of the CaO-Al₂O₃-SiO₂ liquidus, respectively, are considered to show the magnitude of the discrepancy and are used in the calculations.     

The effect of thermal history on the development of mineral assemblages during infiltration-driven contact metamorphism

 A one-dimensional model for coupled heat flow, fluid flow, and mineral reaction predicts the identity and spatial and temporal distributions of mineral assemblages produced during contact metamorphism of siliceous dolomite. Results are compared to mineral distributions predicted by transport models for fluid flow along steady-state temperature profiles and to mineral distributions observed in contact aureoles to assess whether a detailed analysis of heat flow is required to extract meaningful information of fluid flow history from field data. Results identify several mineral assemblages whose interpretation in terms of fluid flow is dependent on an understanding of their thermal history. Certain key mineral assemblages, however, are sensitive records of the geometry and amount of fluid flow but are insensitive to thermal history. The presence or absence of these mineral assemblages constrains the history of fluid flow during contact metamorphism of siliceous dolomite regardless of the details of heat flow. Occurrences of the key assemblages record both magmatic fluid flow in the direction of decreasing temperature (Alta, Elkhorn aureoles) and metamorphic fluid flow in the direction of increasing temperature (Beinn an Dubhaich, Kasuga-mura aureoles) during contact metamorphism. Time-integrated input fluid flux averages on the order of 100 mol/cm² with a range of ±1 order of magnitude. Received: 13 October 1995 / Accepted: 20 March 1996     

Electron optical investigation of sedimentary dolomites

Transmission electron microscope studies demonstrate a structural distinction between stoichiometric and ancient calcian dolomites of sedimentary origin. A modulated or tweed structure similar in appearance to spinodal microstructures characterizes ancient calcian dolomites, whereas stoichiometric dolomites are homogeneous. Recent calcium-rich dolomites lack a modulated structure, but have a heterogeneous microstructure with high densities of growth defects. Electron and optical diffraction and lattice imaging suggest a fluctuation in interplanar spacing in a coherent lattice. In addition, lattice imaging also reveals discontinuous faults throughout the modulated structure. Ordering and possibly incipient exsolution are suggested, c reflections in SAD patterns of calcian dolomites document a new superstructure in at least part of the lamellar structure and raise questions of different ordering states throughout the structure.The results provide new insight regarding the formation of sedimentary dolomites     

The stability of transition metal dolomites in carbonate systems: a discussion

The instability of transition metal dolomites [CaR²⁺(CO₃)₂ where R²⁺ is Fe, Co, Ni, Cu, or Zn] and the limited substitution of transition metal cations for Mg in the dolomite structure can be accounted for by the effect of octahedral distortion. For example, trigonal elongation of the Fe octahedron, due to the Jahn-Teller effect, observed in siderite and ankerite, results in elongation of the Ca octahedron which is sensitive to distortion because the radius of Ca²⁺ is close to the upper limit for octahedral coordination. Co, Ni, Cu, Zn octahedra are also thought to be deformed, relative to Mg octahedra, in carbonates.The free energy of formation (ΔG^o_f) of R²⁺CO₃ becomes more positive with increasing octahedral distortion. Estimated ΔG^o_f(dolomite) as well as stabilities and solubility limits of R²⁺ in natural and synthetic dolomites suggest a series in order of decreasing stability: Mg >Mn >Zn >Fe >Co >Ni >Cu.ΔG^o_f(est.) for the terminal Fe-dolomite solid solution [72 mol% CaFe(CO₃)₂] in the system CaCO₃-MgCO₃-FeCO₃ may represent an empirical threshold value for dolomite stability which lies between ΔG^o_f for Mn- and Zn-dolomites. While Zn-dolomite is probably not a stable phase, very extensive solid solution toward CaZn(CO₃)₂ is to be expected in the system CaCO₃-MgCO₃-ZnCO₃. The tendency for transition metal dolomites to contain excess CaCO₃ can also be accounted for in terms of octahedral distortion and AG^o_f.     

The reaction history of kyanite in high‐P aluminous granulites

Cathodoluminescence (CL) mapping of kyanite in high pressure, aluminous granulites from the central Grenville Province reveals internal structures that are linked to their metamorphic reaction history. In two samples, individual kyanite crystals are shown to be composite porphyroblasts comprising three distinct generations, defined by their CL intensity and Cr (±V, Ti, Fe and Ga) content, and each separated by resorbed interfaces. In contrast, a sub‐aluminous sample contains two types of kyanite, one as resorbed inclusions in garnet and another in the groundmass or replacing garnet. These textural variants of kyanite are interpreted within the framework of phase equilibria modelling. In P–T pseudosections, a first generation of kyanite, which is only present in the most aluminous samples, is potentially linked to staurolite breakdown, and its resorption is consistent with a subsequent increase in pressure. This kyanite represents the earliest remnant of prograde metamorphism identifiable in these rocks. The second generation, present in the porphyroblasts in the same samples and as inclusions in garnet in the sub‐aluminous sample, is interpreted to be the peritectic product of muscovite dehydration melting. Resorption of this kyanite is consistent with subsequent continuous dehydration melting of biotite, which is also inferred based on microstructural considerations. The final generation of kyanite, present as rims on the prograde kyanite porphyroblasts in aluminous samples and as part of the groundmass or replacing garnet in the sub‐aluminous rock, is interpreted to have grown during melt crystallization upon retrogression. The presence of retrograde kyanite implies that the melt crystallized over a wide range of temperatures, and provides an important constraint on the P–T conditions of the metamorphic peak and on the retrograde P–T path. CL mapping is crucial for identifying retrograde kyanite in aluminous samples, as it preferentially overgrows existing kyanite rather than replacing other prograde phases. The scarcity of kyanite in sub‐aluminous rocks allows retrograde kyanite to grow as discrete crystals that can be identified by optical microscopy. This work attests to the potential of unconventional tools such as CL imaging for deciphering the metamorphic history of rocks.     

东准噶尔贝勒库都克铝质A型花岗岩的厘定及意义

初步研究表明,长期以来被认为S型花岗岩的贝勒库都克黑云母花岗岩应为铝质A型花岗岩.该岩体以富硅(SiO2=75.25%～76.67%)和碱(Na2O+K2O=8.08%～8.97%),贫镁(MgO=0.02%～0.18%)和钙(CaO =0.39%～0.89%),氧化指数变化较大(W=0.02～0.15)以及高FeOT/MgO比值(12.71～84.51,平均34.55)为特征.其K2O＞Na2O,NK/A=0.86～0.95(平均0.92),A/CNK=0.97～1.02(＞0.95),属偏铝-过铝质钙碱-弱碱性岩石.在微量元素和稀土元素组成上,岩石富Ga、Zr和Hf等高场强元素,亏损Ba、Nb、Sr等元素.10 000 Ga/Al比值(2.97～4.20)均大于A型花岗岩的下限值(2.6),明显高于I型和S型花岗岩的平均值(分别为2.10和2.28).在Zr、Ce、Nb对10 000 Ga/Al以及FeOT/MgO对(Zr+Nb+Ce+Y)、SiO2等A型花岗岩多种判别图上,投影点均落在A型花岗岩区,而与高分异的I、S型花岗岩明显不同.这些特征表明,贝勒库都克黑云母花岗岩与国内外铝质A型花岗岩(如福建沿海、东西准噶尔和澳大利亚Lachlan褶皱带铝质A型花岗岩)十分相似.在Nb-Y-Ce、R1-Ga/Al和R1-R2构造环境判别图上,显示出造山后花岗岩的特征.贝勒库都克铝质A型花岗岩的厘定,不仅对探讨卡拉麦里地区地壳物质组成及构造演化有着重要的地质意义,还为我国新疆北部寻找与铝质A型花岗岩有关的锡矿资源开辟了方向.     

淮南煤田太原组铝质泥岩元素地球化学特征

华北聚煤盆地南缘淮南煤田晚石炭世-早二叠世太原组含煤岩系普遍沉积铝质泥岩,然而,对于多层铝质泥岩的形成条件及其地球化学特征尚未开展深入研究。在本次研究中,系统采集了淮南煤田张集煤矿补Y1钻孔岩心样品,采用XRF和ICP-OES、ICP-MS分别测试了主量元素和微量元素,对铝质泥岩地球化学特征及其地质成因进行分析。结果表明：不同层位的铝质泥岩来源于同一源区,铝土质泥岩可能受到了更强烈的红土化作用导致其明显偏离SiO₂/Al₂O₃和Fe₂O₃/Al₂O₃趋势线;Sr/Ba的结果表明铝质泥岩是在不稳定的海陆交互沉积环境下形成的,V/Cr和V/（V+Ni）的结果表明铝质泥岩是在贫氧到厌氧的沉积环境中形成的;综合主量元素和微量元素的结果,表明了不同层位铝质泥岩的母岩可能是附近古陆的中酸性火成岩,母岩风化产物经迁移至淮南地区沉积成岩。     

Crystal-chemistry of natural and heated aluminous orthopyroxenes

X-ray single crystal diffraction data of natural and heated Al-rich orthopyroxenes were used to study the cation ordering and the geometrical changes induced by Si+R ²⁺=Al^IV+R ³⁺ substitution. The calculated site populations and the observed bond distances in tetrahedral and octahedral sites suggest a total ordering of Al^IV in the TB tetrahedron and of R ³⁺ cations in the M1 octahedron, even in samples heated at 1000° C. The mismatch between the tetrahedral and octahedral layers along the c axis seems to play a crucial role in determining the limits of Si/Al^IV substitution.     

The importance of iron speciation (Fe+2/Fe+3) in determining mineral assemblages: an example from the high‐grade aluminous metapelites of southeastern Madagascar

Metapelitic granulites from the Anosyen domain of southeastern Madagascar are exposed in three intercalated formations: the Amparihy, Bakika and Ihosy formations. Although mineralogically distinct from each other, the rocks from these formations show very similar bulk‐rock compositions when measured on a Fe_T basis. The preserved mineral assemblages thus do not reflect differences in the ratios of the main rock‐forming oxides (i.e. Al₂O₃:Fe_T:MgO), but instead reflect variations in the pre‐metamorphic oxidation state of the protolith rocks. These differences in oxidation state are manifested via differences in iron speciation – either Fe⁺² or Fe⁺³. The relatively reduced rocks of the Amparihy Formation preserve the assemblage bi–sp–sill–g–cd, which contrasts markedly with the mostly garnet and spinel‐absent bi–cd–sill–mt assemblages preserved in the strongly oxidized rocks of the Ihosy Formation. Compositionally intermediate rocks of the Bakika Formation are garnet bearing, but sillimanite‐absent, and contain the assemblage sp–g–cd–mag. Modelling of these rocks in the Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–O system suggests that they evolved along a heating and cooling P–T path with only limited decompression accompanying cooling on the retrograde path. Peak temperatures and pressures of ～880–920 °C and 6–6.5 kbar are inferred for the majority of the Anosyen domain, with slightly lower peak temperatures (～840 °C) estimated in the extreme northwest of the area. The high‐temperature and relatively low‐pressure nature of metamorphism suggests high geothermal gradients existed during orogenesis, which in southern Madagascar is related to the amalgamation of Gondwana (580–520 Ma). Although metamorphic temperatures may have been augmented via thermal advection from the emplacement of the syn‐ to post‐tectonic Ambalavao suite, the high geothermal gradients nevertheless suggest thin and consequently hot lithosphere existed prior to orogenesis.     

The geometry and petrogenesis of hydrothermal dolomites at Navan, Ireland

The dolomites at Navan, Ireland, formed in Courceyan peritidal and shallow-shelf limestones. The dolomite body has a plume-like geometry, cross-cutting both lithological boundaries and diagenetic barriers generated by sea-floor cementation and emergence. The dolomitizing fluids rose parallel to major faults to diffuse laterally through the succession, controlled by variations in permeability that reflect both facies variation on deposition and pre-dolomitization diagenesis. Cathodoluminescent zones reveal three principal stages of dolomite emplacement, separated by dissolution surfaces, with each stage reflecting several changes in the character of dolomitizing solutions. The predominance of dull zones indicates burial rather than surface conditions. The dolomites formed some time after burial in response to an areally limited hydrothermally-driven flow. Isotope values (σ¹⁸O of — 6σ6 to — 10.4%_δ and σ¹³C of — 0σ2 to +2σ5%_δ PDB), and fluid inclusion data, suggest that these fluids had compositions similar to those of Carboniferous seawater. However, these became hotter with time, with temperature increasing from 60 to 160δC. The Navan dolomites are closely associated with Europe's largest zinc-lead deposit. The distribution of the ores follows the same trend as that of the dolomites and paragenetic relationships indicate that dolomitization and mineralization were temporally and genetically related.     

Kinetics of Fe2+-Mg distribution in aluminous orthopyroxenes

Kinetic studies of isothermal heating experiments (600–800° C) on aluminous pyroxenes (Mg_0.942Fe _0.880 ²⁺ Fe _0.068 ³⁺ Mn_0.016Ca_0.010Al_0.084) (Si_1.848Al_0.152) permit the determination of rate constant of isothermal disordering as 2.5457 E13(±1.4 E13) min^?1. The activation energy is determined as 278 (±23) kJ/mol. Data on two other aluminous pyroxenes at 700° C indicate that the rate constant decreases significantly with increasing amount of trivalent cations. There is a similar but reverse correlation between the concentration of trivalent cations and the Fe²⁺-Mg equilibrium distribution between sites. The site distribution coefficient increases with increasing concentration of trivalent cations at constant temperature.     

The effect of Fe-Mg substitution on phase relations in marly rocks of the Western Hohe Tauern (Austria)

Based on mineral assemblages and compositions in metamorphic marly rocks of the Western Hohe Tauern, the effect of Fe-Mg substitution on previously deduced phase relations in a simplified marly rock system (Hoschek 1980) is estimated. Fe-Mg partitioning in coexisting minerals is influenced, amongst others, by the F-OH substitution. Calculations with extrapolated mean K _D ^Fe-Mg values for F-free minerals and with the assumption of ideal Fe-Mg solid solution show similar effects of the Fe-Mg and the F-OH substitution on phase relations. The consideration of the Fe²⁺ and F distribution leads to a better compatibility between experimentally determined mineral stabilities and observed mineral assemblages in the marly rocks of the Western Hohe Tauern.     

The distribution of microfossil assemblages in Proterozoic rocks

A biostratigraphic model of the temporal distribution of distinctive Proterozoic microfossil assemblages is suggested, based on studies of upper Precambrian chert-embedded and compression-preserved organic-walled microfossils from the reference sections of Eurasia, North America and Australia. Microfossils from 2.0 to 0.542 Ga can be divided into seven successive informal global units which can be compared to standard units of the International and Russian time scales. Each unit is characterized by a particular association of taxa, typified by the fossil assemblage that gives it its name. These form broad biostratigraphic units comparable to assemblage zones of Phanerozoic successions; in general (but with minor differences) they correspond to chronostratigraphic units accepted by the Internal Commission on Stratigraphy. The units are: (1) Labradorian, the upper part of the Paleoproterozoic (Orosirian and Statherian), 2.0–1.65 Ga; (2) Anabarian, lower Mesoproterozoic (Calymmian–Ectasian)/Lower Riphean–lower Middle Riphean, 1.65–1.2 Ga; (3) Turukhanian, upper Mesoproterozoic (Stenian)/upper Middle Riphean, 1.2–1.03 Ga; (4) Uchuromayan, lower Neoproterozoic (late Stenian–Tonian)/lower Upper Riphean, 1.03–0.85 Ga; (5) Yuzhnouralian, upper Neoproterozoic (Cryogenian)/upper Upper Riphean, 0.85–0.63 Ga; (6) Amadeusian, lower Ediacaran/lower Vendian, 0.63–0.55 Ga; (7) Belomorian, upper Ediacaran/upper Vendian, 0.55–0.542 Ga.     

Metamorphism of siliceous dolomites at Naxos,Greece

Siliceous dolomitic marbles at Naxos, Greece, are crystallized in a range of metamorphic temperatures, estimated from 350 to 700 °C and a total pressure of about 6 kb. A number of low variant assemblages of this rock-system have been studied. The sequence of these metamorphic assemblages is a function of increasing metamorphism, and this is in agreement with other fielddata and with experimental data for the chemical system CaO-MgO-SiO₂-H₂O-CO₂ (-HF). The composition of the associated fluid inclusions may be considered reasonably correct in the light of experimentally obtained X _CO2 values for the mineral equilibria, extrapolated for a total pressure of about 6 kb at the interpolated temperatures at the sample locations.     

The influence of disordered,non-equilibrium dolomites on the Mg-solubility in calcite in the system CaCO3-MgCO3

The influence of different degrees of disorder of dolomites on the solubility of MgCO₃ in calcite has been studied under isothermal and isobaric conditions. At 900° C, 4kb and 1000° C, 5 and 7kb, varied smoothly as function of the particular structural and cationic disorder of coexisting dolomite. Higher degrees of disorder of dolomite, estimated by the d _00.6/d _11.0 values and the peak height ratio I _01.5/I_00.6, lead to greater solubility of MgCO₃ in calcite. The run time for all experiments was 96 h, much longer than in previous work. The influence of disorder of dolomite on appears to be larger than that of temperature, as shown by the large range of (0.12–0.30) in calcite at 900° C 4 kb, found in this study. The state of order of dolomite seems to control the solubility limits in this system, and may explain discrepancies found in previous experimental work.     

塔里木盆地寒武-奥陶系白云岩结构构造类型及其形成机理

塔里木盆地寒武-奥陶系白云岩度大、分布广,是重要的油气储集层.根据岩石学研究并结合地球化学特征将其分为4类,其特点如下:(1)泥晶-粉晶白云岩,白云石晶体小、晶形差,有序度在4类中最低(0.73);Na、K、Sr含量在4类中最高;δ13C值及Z值较高,分别为-0.94‰、122.3;Fe、Mn含量较低;阴极发光为亮桔红色;常呈纹层状,具暴露及成化标志.这些特征表明其结晶较快,形成环境蒸发强烈、盐度较高、氧化性较强,是在萨布哈环境中由浓缩海水发生白云石化所形成.(2)藻纹层白云岩,白云石晶体小,晶形较好,有序度较高(0.85);藻纹层基本保留了其原始微细结构;Na、K、Sr含量较低:δC值在4类中最高,平均为-0.50‰;含有较多黄铁矿.这些特征说明其可能形成于成岩作用的稍早期阶段且其成因与藻类活动密切相关,成岩环境盐度较低,可能是在藻类作用下,由富藻层浓集的镁与残留海水在浅埋藏环境中共同作用所形成.(3)雾心亮边白云岩,白云石普遍具雾心亮边结构,雾心含少量包裹体及灰质残余,亮边则少见或无;阴极发光下,雾心发亮桔黄色光,亮边发光暗或不发光;局部见此类白云石;呈星散状分布于亮晶颗粒灰岩或泥晶灰岩中;白云石晶体较大,晶形较好.此类白云岩的δ13C及Z值在4类中最低,K、Na、Sr、Fe含量相对较低,Mn含量最高;有序度较高,均值为0.83.这些特征表明此类白云岩中白云石的雾心可能是在早期浅埋藏环境中,由富镁孔隙水交代周围灰质所形成;亮边则形成于埋藏较深的环境,其所需镁可能来源于粘土矿物的转化.(4)重结晶白云岩:白云石晶体粗大且污浊,接触紧密;局部见白云石晶体具环带结构,局部见颗粒幻影结构;阴极发光较暗;Fe含量在4类中最高,Mn含量较高;有序度最高,平均为0.95;包裹体均一温度平均为110.2℃.这些特征表明其可能是在还原性较强的深埋藏环境中,由早期形成的白云岩经较强的重结晶作用所形成.     

新疆东准噶尔苏吉泉铝质A型花岗岩的确立及其初步研究

新疆东准噶尔卡拉麦里地区是一个重要的锡成矿带,分布有多种类型花岗岩,其中黑云母花岗岩长期以来被认为是S型花岗岩。本文研究表明,苏吉泉黑云母花岗岩富集Rb、K等大离子亲石元素及Zr、Hf等高场强元素,其FeO/MgO和10000Ga/Al值大,明显不同于典型的I型和S型花岗岩,属于典型的铝质A型花岗岩。锆石的LA-ICPMSU-Pb定年结果显示其形成时代为304±2Ma,比该区钙碱性花岗岩侵位晚,而与碱性花岗岩形成时代相近。这些黑云母花岗岩具有高εNd(t)的同位素特征,但它们不是直接来源于亏损地幔,而更可能是源自地幔且被深埋的年轻地壳物质部分熔融和结晶分异作用的产物。花岗岩微量元素构造判别图显示它们是一种后碰撞花岗岩,标志晚石炭世卡拉麦里地区造山作用的结束和板内构造演化的开始。     

The stabilities of gibbsite,boehmite, aluminous goethites and aluminous hematites in bauxites,ferricretes and laterites as a function of water activity,temperature and particle size

Stability relationships between Al-goethite, Al-hematite, boehmite and gibbsite are presented in terms of water activity [H₂O], temperature (T), grain size and bulk-composition in the system Fe₂O₃-Al₂O₃-H₂O at a total pressure of 1 bar.Al-goethite and Al-hematite are treated as ideal solid solutions, the former of the end-members FeOOH (goethite) and AlOOH (diaspore) and the latter of the end-members Fe₂O₃ (hematite) and Al₂O₃ (corundum). Using log K_sp provided by the literature for the various phases involved, the common associations observed in laterites, bauxites and ferricretes do not have stability fields over geologically reasonable intervals of [H₂O] and T. Consequently a new internally consistent set of log K_sp values is proposed and used, and allows such associations to have actual stability fields in the appropriate diagrams. The new log K_sp values used in the calculations are such that the solubilities of the end members are greater than those commonly listed. This is in agreement with natural observations which show that such minerals, are generally poorly crystallized and of very small size. The assumption of an ideal solid solution in aluminous goethite and aluminous hematite combined with the new log K_sp values leads to prediction of composition limits for these two minerals which agree well with observed values. The fact that an ideal solid solution must extend continuously from one end-member to the other is masked by the appearance of other stability fields (e.g. gibbsite or boehmite) which cross and overlap a part of the solid solution stability fields of AlOOH-FeOOH and Fe₂O₃-Al₂O₃.