Scanning electron microscopic (SEM) examination of the quartz isolated from chert of Transvaal and Swaziland, of 2000 and 3400 million year (MY) ages, respectively, in southern Africa revealed marked differences in quartz morphology. Well-defined individual euhedral quartz crystals, with polyhedral triple-point faces, were clearly evident on freshly fractured Transvaal chert surfaces as well as with the quartz isolates from the chert. The morphology and coarseness suggest crystal growth with little, if any, metamorphism; however, the δ18O values of 23.8–24.1‰ suggest crystallization temperatures of perhaps 40–45°C. In contrast, fracture surfaces of the older, strongly metamorphosed Swaziland cherts revealed a high degree of grain intergrowth which inhibited fracture between quartz grains. Quartz isolates from these showed strongly interlocked quartz crystal clusters and elongated chips of quartz with poorly defined irregular faces. Intercalation of mineral veins in the cherts on a mm scale and the intergrown character of the quartz grain boundaries provide evidence that the latter cherts have been strongly metamorphosed and recrystallized, in keeping with 14.6–15.1‰ δ18O values, corresponding to 80°C fractionation with water. The SEM micrographs of the fine quartz (1–10 µm) isolated from the Otavi dolomite formation of the 700-MY Damara System and from the 2000-MY Transvaal Dolomite Series revealed well-defined subhedral and euhedral quartz crystals of small size which, together with the 26.9–27.8‰ and 23.8‰ δ18O values, respectively, indicate that these dolomites have been affected little, if any, by post-depositional metamorphism; their crystallization temperatures fall in the range of 25–30 and 40–45°C, respectively.

Although numerous, small, manganese oxide deposits associated with dolomite in the Eastern Transvaal escarpment, Republic of South Africa, have been known for many years, their mineralogical make-up is somewhat controversial. Chemical, mineralogical, and morphological properties of the weathering products of dolomite and the coexisting manganese oxide material in the Graskop area were therefore determined. Mn and Fe occur only in minor accessory minerals in the original rock; however, in the weathering residue, these elements are concentrated and occur as separate mineral phases, chiefly birnessite, nsutite, and goethite. Thin veins of pure muscovite and quartz traverse the residua. Rare, pure calcite and maghemite nodules were noted throughout the residual manganese material. The properties of this weathering sequence suggest that the manganese wad deposits were formed in situ as a result of the congruent dissolution of dolomite, leaving a porous, sponge-like structure, highly enriched in Mn and Fe oxides.

Deposits of sepiolite, trioctahedral smectite (mixed-layer kerolite/stevensite), calcite, and dolomite, found in the Amargosa Flat and Ash Meadows areas of the Amargosa Desert were formed by precipitation from nonsaline solutions. This mode of origin is indicated by crystal growth patterns, by the low Al content for the deposits, and by the absence of volcanoclastic textures. Evidence for low salinity is found in the isotopic compositions for the minerals, in the lack of abundant soluble salts in the deposits, and in the crystal habits of the dolomite. In addition, calculations show that modern spring water in the area can precipitate sepiolite, dolomite, and calcite following only minor evaporative concentration and equilibration with atmospheric CO2. However, precipitation of mixed-layer kerolite/stevensite may require a more saline environment. Mineral precipitation probably occurred during a pluvial period in shallow lakes or swamps fed by spring water from Paleozoic carbonate aquifers.

Trioctahedral smectite is a constituent of Mg-rich carbonate crusts and moonmilks (pasty deposits) in caves of the Guadalupe Mountains of southeastern New Mexico. Energy dispersive X-ray microanalysis of individual crystallites and their aggregates along with the X-ray diffraction analysis indicates that the smectite is probably stevensite. Saponite is likely present in some samples also. The smectite is intimately associated with dolomite crusts and huntite moonmilks in Carlsbad Cavern, Lechu-guilla Cave, and other dolostone caves. Clay particles appear as fibers and films, with aggregates comprising decimicron-sized filamentous masses that envelop crystals of dolomite, huntite, and magnesite. The occurrence of smectite is related to the genesis of the Mg-rich carbonate minerals. In water films, progressive evaporation and carbon dioxide loss results in the sequential precipitation of Mg-rich calcite, aragonite, dolomite, huntite, and magnesite. This sequence of carbonate precipitation removes Ca and greatly increases the Mg/Ca ratio in the solutions. Silica is commonly available probably because of high pH conditions, and consequently, smectite forms in the Mg-rich alkaline environment. Along with the Mg-rich carbonate minerals, opal, quartz, and uranyl vanadates may precipitate with the smectite.

Chlorite and corrensite are common clay minerals in lacustrine mudrocks from the Devonian Orcadian Basin, Scotland. The relationship of their occurrence to vitrinite reflectance data demonstrate that they are authigenic minerals, formed during burial diagenesis/metamorphism at temperatures of ≥120°C. Whole rock mineralogical and chemical analyses show that chlorite authigenesis occurred by reactions between the detrital dioctahedral clay mineral assemblage and dolomite that was formed under early evaporitic conditions in the lacustrine environment.

XRD and electron microprobe analyses indicate that phases intermediate between corrensite and chlorite are probably mixed-layer chlorite/corrensite with a tendency towards segregation of layer types. Chemically, the conversion of corrensite to chlorite involves an increase in Al for Si substitution in tetrahedral sites, but there is no change in the Fe/Mg ratio of octahedral cations. There is also no relationship of mixed-layer proportions to paleotemperature; only a general paleotemperature interval of approximately 120° to 260°C in which a range of phases between corrensite and chlorite occurs. Chlorite polytypes are exclusively IIb, indicating the formation of this polytype at diagenetic temperatures.

The occurrence of corrensite and Mg-rich chlorite in evaporite and carbonate successions is probably a reliable indicator of diagenetic alteration at temperatures of ≥ 100°C. Burial diagenetic reactions between dioctahedral clay minerals and Mg-rich carbonates may possibly explain many occurrences of corrensite and Mg-rich chlorite in such rocks.

The Lower Pliocene lacustrine sediments of the Sakarya and Porsuk Formations in the Sivrihisar and Yunusemre-Biçer regions consist of claystone, argillaceous carbonate, carbonate, and evaporites. No detailed studies of paleoclimatic conditions have been performed previously. The present study aimed to determine the depositional environment and paleoclimatic conditions for the formation of these economically important sepiolite/palygorskite/carbonate/evaporite deposits based on detailed mineralogical, geochemical, and isotopic studies. Samples from various lacustrine sediments were examined using polarized-light microscopy, X-ray diffraction, scanning electron microscopy, and chemical and isotopic analysis methods. Dolomites are predominantly of micrite, which is partly recrystallized to dolomicrosparite/dolosparite close to desiccation fractures. The presence of ostracods and dacycladecean algae in the carbonates reflects a restricted depositional environment. The formation of sepiolite and palygorskite fibers, either as cement between/enclosing dolomite and/or as calcite crystals, reflects occasional changes in physicochemical conditions provided by fluctuations in the lake-water level and influx of groundwater in relation to climatic changes during and after dolomite precipitation. The positive correlations of ΣREE with Al2O3, Nb, high-field-strength elements, and transition elements are due to alteration of feldspar and hornblende in the volcanic units. The high values of Ba and Sr relative to Cr, Co, Ni, and V also indicate that felsic rather than ophiolitic rocks were the parent material. The crossplot of whole-rock SiO2vs. Al2O3+K2O+Na2O and V/Cr ratio suggests deposition of carbonate-dolomitic sepiolite-sepiolitic dolomite under arid climate and oxic conditions, whereas the Ni/Co and V/(V+Ni) ratios of the sediments indicate deposition of organic-bearing sepiolite/palygorskite under anoxic-dysoxic conditions. An enrichment in δ13C and δ18O values of dolomite with respect to calcite is probably due to differences in mineral fractionations. The δ34S and δ18O values and 87Sr/86Sr isotope ratios for gypsum suggest an intensely evaporitic lacustrine environment fed by an older marine evaporitic source. The Si, Al, Mg, Ca, and enhanced TOT/C required for periodic precipitation of organic-rich brown sepiolite/palygorskite characterize deposition in a swampy environment, while dolomitic sepiolite and sepiolitic dolomite formed in ponds by partial drying of the main alkaline lake.

Sepiolite and palygorskite are common as layers and nodules in the Neogene lacustrine sediments of the Eskişehir area. This study aims to determine their mineralogical and geochemical characteristics, plus the distribution of these economically important sepiolite and palygorskite deposits within the lacustrine environment. Using these data the research goes on to discuss the environmental conditions for their formation. Sepiolite and palygorskite layers are associated with dolomite, marlstone, and argillaceous limestone. The sepiolite nodules (meerschaum), which are former magnesite gravels, are observed in the Upper Miocene reddish-brown fluvial deposits derived from the ophiolite and its fractureinfills at the northern margin of the basin. Sepiolite and palygorskite are only sparsely associated with dolomite and accessory magnesite, quartz, feldspar, and amphibole. Sepiolite and palygorskite fibers formed as oriented platy fan, interwoven, and knitted aggregates in the absence of dolomite indicated precipitation from supersaturated solution. Sepiolite and palygorskite fibers edging dolomite crystals postdate dolomite and formed through precipitation in a vadose environment under semi-arid to arid climatic conditions. High values of Mg+Fe+Ni and enrichment of light rare earth elements (LREE) relative to middle rare earth elements (MREE) and heavy rare earth elements (HREE), Sr content, depletion of Rb+Ba and K, and negligible negative Eu anomalies all reflect the derivation from the Paleozoic metamorphic and Upper Cretaceous ophiolitic rocks. Locally, Upper Miocene to Lower Pliocene volcanic, volcanoclastic, and fluvio-lacustrine sedimentary rocks supplied the required Si, Mg, Al, and Fe for precipitation of Al-sepiolite and Mg-palygorskite with average structural formulae of Si11.91Al0.09O30Mg6.60Al0.78Fe0.13 (OH)4Na0.12K0.06(OH2)4·nH2O and Si7.74Al0.26O20Mg2.52Al1.13Fe0.38(OH)2(OH2)4Na0.32K0.14 Ca0.12·nH2O, respectively. In contrast to the layered sepiolites, the absence of Al and high Ni content in sepiolite nodules suggest formation through replacement of magnesite gravels at shallow burial in an alkal ine environment. The calculated meerschaum sepiol ite chemical formula i s: Si12.02O30Mg7.87Fe0.01(OH)4Na0.13K0.03(OH2)4·nH2O.

The origins of dolocrete and associated palygorskite in the Çanakkale region of Turkey have been little studied, but are of fundamental importance for a more complete understanding of the mineralogy of this region. The present study was undertaken in order to narrow this gap. Siliciclastic red mudstones within alluvial-fan deposits of the Middle Miocene Sariyer Formation locally contain dolocretes in various forms (powdery, nodular, and fracture-filling) and scarce matte-brown, authigenic clay lenses. The mineralogical characteristics of dolocrete and authigenic clay lenses were examined using polarized-light microscopy, X-ray diffraction, differential thermal analysis and thermal gravimetry, scanning-electron microscopy, and infrared spectroscopy, as well as by chemical and isotopic methods. These analyses indicate that the dolocretes are indeed predominantly dolomite, coexisting with variable amounts of palygorskite. The authigenic clay lenses are composed mainly of palygorskite. Dolomite appears as euhedral crystals, whereas palygorskite developed authigenically as interwoven fibers on and between resorbed dolomite crystals, rimming euhedral crystals, and as fiber bundles (where dolomite ± magnesite is absent). The stable-isotope values and some petrographic features, such as alveolar texture and dolomite needles, support a pedogenic origin for the dolocretes. In the initial stage, dolomite formed by replacement of siliciclastic red mudstones and/or by precipitation from percolating soil-derived water in a near-surface setting. Subsequently, palygorskite either precipitated on the dolomite crystals from relatively more evaporative water, replaced the host-rock mudstone in the presence of Al + Fe, or formed directly from solution where the Ca/Mg ratio decreased and the Al + Fe increased. In view of the large Cr and Ni contents of the bulk-rock samples, the elements required for the crystallization of dolomite and palygorskite (namely Mg, Ca, Si, Al, and Fe) may have been supplied by weathering of ophiolitic rocks that crop out in the area.

The Konya region in central Anatolia is covered by Pliocene-Late Pleistocene sediments and volcanites related to the sediments NNW of Karapınar, Turkey. In the area, the Upper Miocene-Quaternary Üzecek Dağı and Karacadağ volcanites are generally of the same age and formed from magmas of similar composition. The Karapınar formation is brown to whitish-beige, partly fossiliferous and consists of limestone, marl, claystone and, locally, sandy layers. Silica-rich lenses, nodules and layers are observed in the upper strata which locally contain sepiolite-rich layers. The mineralogical composition of sepiolite samples taken from the area was determined by powder X-ray diffractometry, while the abundance of major-element oxides was measured by X-ray fluorescence spectrometry. The crystallographic and morphological properties of samples were determined by means of scanning electron microscopy and energy dispersive spectroscopy. Samples were taken from three sections and from random locations. Mineral assemblages in the same stratigraphic position are generally similar in the three sections, while the thickness of the individual beds varies between the sections. Dolomite and calcite are the main carbonate minerals in the sections. Sepiolite occurs primarily with dolomite and, locally, dolomite and calcite, and less commonly with just calcite. Generally, quartz, feldspar and mica are found, especially in the upper parts of the sections where tuff is abundant. CaO and MgO dominate the major-element oxides. The CaO content is between 1 and 30% while MgO is 3–21%. Al2O3 and SiO2 are generally higher in the sepiolitic and tuffitic layers. Al2O3 is <3% and SiO2 is between 15–18% in the sepiolitic layers. The average structural formula of sepiolite was calculated as: (Mg7.00Al0.44Fe0.18)(Si11.71Al0.29)O30 (OH)4(OH2)4Ca0.13K0.09Na0.01. Sepiolite occurs as fibers and dolomite as subhedral or euhedral crystals. It is considered that sepiolite was formed either by conversion of dolomite or by direct precipitation from solution under alkaline and saline conditions in the Karapınar paleolake. The paleolake was saturated with respect to Mg, Ca and Si derived from groundwater that percolated along fracture systems.

A 2 m thick brecciated zone containing magnesian minerals is present at the contact of tectonites and cumulates. Tectonites below this zone comprise serpentinized orthopyroxenite and serpentinite. An alteration zone with vein-type bedding comprises four different levels; from bottom to top they are: (1) green-brown serpentinite with dolomite (0.9 m), (2) light greenish-white dolomite with serpentine (0.5 m), (3) white dolomite with sepiolite (0.4 m), and (4) greenish-white dolomite with smectite-chlorite (0.2 m). The first level has a mineral association of serpentine + dolomite ± calcite ± aragonite, the second level consists of dolomite + serpentine ± calcite or dolomite + magnesite + serpentine, the third level comprises dolomite + sepiolite, and the fourth level is made up of dolomite + chlorite + smectite + serpentine. Dolomite, the main mineral of the alteration zone, occurs as coarse crystals (microsparitic-sparitic) in fractures and as small crystals (microsparitic-micritic) in the matrix, which includes serpentine and gabbro relics. Sepiolite developed at the edges and surfaces of dolomite and as fibrous forms in voids. Cumulate rocks above this zone comprise uralitic gabbros. The occurrences of magnesian minerals developed in three stages: the first stage was the serpentinization of olivine; the second stage was the dissolution of serpentine by groundwater and/or meteoric water containing carbon dioxide; and the last stage was the synthesis of neoformed minerals.

The Middle—Upper Miocene—Pliocene sediments near Polatlı contain commercial sepiolitic clay deposits. The sepiolite-rich Polatlı basin sediments were studied to describe the sepiolitic clay deposits of the area and to assess the environments of formation using X-ray diffraction, optical and scanning electron microscopy, and chemical analysis. The Polatlı basin is an elongated, rift-related graben trending NE—SW in central Turkey, filled with continental Late Miocene to Early Pliocene sediments. The sediments which comprise claystone, marl and limestone, dolostone, and evaporites are characteristic deposits of low-salinity, playa-lake depositional environments. These sepiolite-rich deposits include sepiolite, dolomite, and calcite, with minor amounts of palygorskite, quartz, moganite, amorphous silica (opal-CT), and feldspar.

The sepiolite shows all the characteristic X-ray diffraction reflections of that mineral, whereas amorphous silica containing sepiolite shows some of the characteristic reflections of sepiolite, but with somewhat broader and less intense basal reflections. In the siliceous deposits, the long, fibrous, and filamentous aggregates of the sepiolite were converted to thick, short fibers, low in Mg, and showing transition to amorphous silica.

Major and trace elements (e.g. Si, Al, Fe, Mg, Sr, Ba, etc.) were found almost exclusively in Mg-rich smectitic claystone and detrital silicate-rich rocks, whereas Mg, Ca, and some Si were concentrated in the neoformed minerals in the basin. The rare-earth elements (REE) and some of the high-field strength elements (HFSE), large ion lithophile elements (LILE), and transition elements (TRE) patterns were similar for detrital silicate-rich rocks and formed from neoformed mineral lithologies. The REE, TRE, LILE, and some of the HFSE contents of limestone, dolostone, and sepiolitic claystone were similar while those of detrital silicate-rich rocks and Mg-rich smectitic claystones were similar to each other. PAAS-normalized REE and other trace-element patterns were typically subparallel and depleted in neoformed minerals. All sample groups had positive Eu* anomalies, except Mg-rich smectite (0.80). Limestone, dolostone, and amorphous silica compounds showed slightly negative Ce* anomalies, whereas sepiolitic claystones, Mg-rich smectitic claystones, and detrital silicate-rich rocks had a slightly positive Ce* anomaly.

Liquid immiscibility has become the preferred mode of genesis for the carbonatite rocks, which commonly, but not exclusively, accompany silicate rocks in alkaline-rock complexes. This concept has been universally based on the presumption that nephelinitic and phonolitic magmas can evolve to a stage where two conjugate immiscible liquids separate. It is assumed that these two liquids separate quickly, or even instantaneously, into discrete bodies of magma capable of being intruded or extruded with subsequent independent crystallization. Supporting evidence generally given is: alleged consanguinity as discrete occurrence of the two rock types; similarity of radiogenic isotope ratios; trace element contents similar to those predicted from experimentally derived partition coefficients. We do not accept that a general case for liquid immiscibility has been demonstrated; although we do accept that silicate and carbonate liquids are inherently immiscible, we maintain that they are not conjugate in a petrogenetic context. We have reviewed and critically examined the experimental data purporting to establish liquid immiscibility and find that when applied to natural rocks, they are based on inappropriate experimental designs, which are not relevant to the genesis of calcite or dolomite carbonatites, although they might have some relevance to Oldoinyo Lengai nyerereite–gregoryite lavas. The design of these experiments guarantees immiscibility and ensures that the carbonate liquids formed will be calcitic or sodium-rich. We dispute the validity of comparing the trace element contents of natural rocks, which in many instances do not represent liquid compositions, to experimentally determine partition coefficients. We consider that experimental design inadequacies, principally assuming but not proving, that the liquids involved are conjugate, indicate that these coefficients are merely an expression of the preference of certain elements for particular liquids, regardless of how the liquids formed. Proof of consanguinity in alkaline complexes requires more accurate age determinations on the relevant rock types than has generally been the case, and in most complexes, consanguinity can be discounted. We dispute the contention that melt inclusions represent parental melts, although they might elucidate the character of magmas undergoing fractional crystallization from magmatic to carbothermal stages. Radiogenic isotope data are shown to be too widely variable to support a case for liquid immiscibility. We address the contention that calcite cannot crystallize from a dolomitic liquid formed by direct mantle melting, and must therefore have crystallized from a calcite carbonate liquid generated by liquid immiscibility, and demonstrate that it is an unsupported hypothesis as calcite can readily crystallize from dolomitic liquids. We observe that, because immiscible dolomite liquids have never been produced experimentally, the liquid immiscibility proposition could at best be applied only to calcite carbonatites, thus leaving unexplained the large number of dolomite carbonatites and those of either type, which are not accompanied by alkaline silicate rocks. The assumed bimodality of alkaline-rock carbonatite complexes is considered to be fallacious and no actual geological or petrographic evidence for immiscibility processes is evident in these complexes. Several examples of alkaline rock carbonatite complexes for which immiscibility has been proposed are evaluated critically and shown to fail in attempts to establish them as exemplifying immiscibility. We conclude that no actual geological or experimental data exist to establish liquid immiscibility being involved in the genesis of calcite or dolomite carbonatite-forming magmas.

The monastery of St. John in Müstair, a UNESCO world heritage site, preserves archaeological remains and stone structures dated from the 8th century to the present. It has been extensively studied archaeologically so that numerous samples of historical materials, including mortar, are available for study. In addition to that, some of the structures have been precisely dated with dendrochronology. The monastery is located in a region characterized by dolomite rocks and the mortars are therefore of dolomitic nature, being perfectly suited to test the possibility of being dated with 14C. Furthermore, the presence of embedded carbon fragments has provided additional independent data to support or deny the results of mortar dating. A comparison of the results obtained from radiocarbon (14C) dating of bulk mortars, sieved fractions enriched in binder, lime lumps and carbon fragments, for two samples is presented, in relation to the petrographic characterization and the mineralogical phase content. This preliminary study shows that the dating of 14C can potentially be applied to the mortar of Müstair, as results in accordance with the established chronologies have been obtained for one sample. However, if the dolomitic sand contamination is very high, further studies are needed to develop a specific sample preparation technique.

The zeta potential and yield-stress behaviour of gels prepared from as-received SHCa-1 hectorite (Ht) containing 50 wt.% impurities, mainly of calcite, dolomite and quartz, were characterized. The zeta potential is negative in the pH range 4–12. At pH <5, reaction of CaCO3 producing CO2 and Ca2+ increased the solution conductivity significantly and reduced the zeta potential. The buffering effect due to this chemical reaction was also observed in the yield stress–pH characterization below pH 8. Low yield stress was observed below pH 6 and above pH 12. The yield stress peaked at pH ~8, where Ht displayed a zeta potential of –20 mV. The relationship between maximum yield stress and Ht content (wt.%) was linear and was not affected by impurities. The ageing behaviour was characterized by an initial phase of rapid yield-stress increase and then by a phase of gradual increase. The Leong model performed better than the two-parameter glassy polymer ageing model in describing this ageing behaviour. However, the empirical power law model was more accurate. The timescale of the ageing process is long, lasting several days. An open cellular microstructure was observed to form in the gels. Impurities appeared not to interact with or bind to the Ht platelets.

The present occurrence is the only deposit among those found in Eskişehir province containing both sepiolite and palygorskite in economic amounts. To describe the geology, mineralogy and genetic relationships, two representative measured sections were examined by field work and XRD, XRF and SEM analyses. Sepiolite and palygorskite occur in a ∼40 m thick part of Pliocene sequence where saponite is also found. These minerals appear in ascending order as: palygorskite, saponite, saponite + palygorskite, sepiolite and sepiolite + palygorskite, frequently in association with dolomite. Chemical precipitation from the alkaline lake environment under semiarid or arid conditions was the main process resulting in sepiolite, palygorskite and saponite formation. Of these, saponite and palygorskite were formed when the lake water was rich-in Al and Fe. Changes in pH may have controlled whether saponite or palygorskite could be formed. Palygorskite was also developed by transformation from saponite during diagenesis.

The saponite examined occurs as two 0.1 m thick layers in a Pliocene sequence consisting of dolomite and dolomitic marl. To characterize this material, mineralogical and structural analyses (XRD, SEM and FTIR), thermal analyses (DTA, TG) and chemical analyses (ICP-ES) were performed. From XRD patterns of randomly-oriented powder samples, the first basal reflection appears as an asymmetric and broad peak with d001 values varying between 16.55 and 17.32 Å. In oriented and air-dried samples, this reflection occurs between 14.45 and 16.42 Å and is fairly symmetrical with FWHM of 2.7º2θ. Oriented and ethylene glycol-solvated samples produce a rational series of basal reflections, where 001 occurs at ~17.8 Å as an intense, narrow (1.1º2θ) and fairly symmetrical reflection. Upon solvation with glycerol, the 001 reflection shifts to ~18.7 Å.

The chemical composition of this saponite is similar to stevensite. However, the structural formula of Na0.114Ca0.013K0.003(Mg2.957Al0.004Fe0.028Ti0.004)(Si3.826Al0.174)O10(OH)2 indicates that vacancies in the octahedral sheet do not exist. The negative layer charge arises nearly entirely from the substitutions in the tetrahedral sheet, with the net layer charge of –0.148, smaller than for common smectites.

Due to the XRD characteristics and particularly the layer-charge distribution, it was concluded that this mineral is a Mg-rich saponite with low layer charge. The saponite was formed by direct precipitation in an alkaline lake environment from Mg- and Si-rich solutions at high pH.

The clay fraction of the deposits of the Otjomongwa Pan, southwestern Kalahari, is dominated by Mg-smectite. This mineral formed as an authigenic precipitate in a groundwater-fed lake, without involving the transformation of a precursor mineral. During the earliest recorded period, Mg-smectite formed together with sepiolite and dolomite in a shallow lake. This period was ended by a change in environmental conditions that led to an increase in the depth of the lake and a decrease in salinity and Mg content of the water. This resulted in a transition to a mineral association without sepiolite and including calcite, together with Mg-smectite and dolomite. The depth of the lake gradually decreased after this event, while the lake water progressively became depleted in Mg.

Active geothermal systems in fluvial-deltaic sediment of the Salton Trough typically develop a thick, carbonate-cemented sandstone caprock which shows a regular progression of carbonate minerals, mineral reactions, rock-fluid mass transfer, and physical properties on increasing temperature. The Sinclair 3 well contains calcian ankerite at < 175 °C, ankerite from 175 to 195 °C, calcite+minor dolomite from 195 to about 250°C, and calcite at higher temperatures. The carbonate content of sandstone decreases on increasing temperature from > 45% at < 140 °C to about 10% at > 250 °C, The most abrupt decrease occurs in the 140 to 170°C range where significant compaction of sandstone occurs as carbonate is reduced to < 25%, and kaolinite reacts to form chlorite. The overall result is the loss of significant Ca, Fe, Mg, and CO2 from sandstone to the fluid phase on increasing temperature. Reaction ofcalcian ankerite to ankerite near 175 °C, and of ankerite to calcite and minor dolomite in the 195 to 245°C range, takes place on a constant carbonate-volume basis by direct replacement of one carbonate by another. The latter reaction produces significant chlorite, with Al derived from solution of detrital feldspar and from the smectite-to-illite transformation. The equilibrium coexistence of calcite with dolomite and ankerite near 200 °C has allowed construction of an isothermal section in the Ca-Mg-(Fe+Mn) carbonate phase diagram and provided a low-temperature constraint on the calcite limb of the calcite-dolomite solvus.

Recognition of widespread carbonate volcanism in central Spain has led to another case in France, of similar age (23–0 Ma) but with entirely new features. More than 100 new carbonate volcanoes are indicated already, adding a wholly unexpected dimension to this form of activity. Eruptions form layers, mostly of glassy nephelinite fragments in a dolomitic matrix, but some layers are largely dolomite. Major new findings are phenocrysts of dolomite, magnesite and calcite in silicate glass, and spectacular dolomite-nephelinite melt immiscibility, neither recorded previously. Most volcanic carbonatites are Ca rich, and dolomite is rare. The Limagne dolomites share links with those in Spain and Zambia, with chromite a hallmark in all three. Limagne is exceptional in being the first case where dolomite has erupted with co-genetic silicate melt. Mantle debris and magnesite indicate a source within ∼ 100–150 km. Chromite in the dolomite globules, and in the enclosing silicate glass, is similar to that in high-temperature kimberlites, indicating immiscibility in the deep mantle. Recognition of two large, previously undetected provinces of carbonate volcanism in Europe, where there has been active research for >200 y, must lead to the inference that similar cases may await discovery on other continents.

A carbonatite sample from Phalaborwa, South Africa, consists of apatite, magnetite and a calcitedolomite ‘perthite’ which is interpreted as being due to exsolution of dolomite from a high-Mg calcite precursor. Carbon and oxygen isotope data indicate that the carbonates are equilibrated. In situ ionmicroprobe analyses for Fe, Mn, Na, Si, Y, the REEs, Pb, Th and U give the following average concentrations (in ppm) in the sequence apatite, calcite, dolomite: Fe 98, 1680, 8190; Mn 61, 510, 615; Na 1171, 627, 125; Si 368; 1.6, 0.2; Sr 4447, 5418, 2393; Ba 37, 2189, 75; La 1245, 300, 67; Y 121, 50, 5.8; Pb 16, 5.4, 1.4; Th 20, 0.02, 0; U 2.4, 0, 0.01. The concentrations are reasonably uniform in both apatite and dolomite, but in calcite are more variable. Na, Si, Y, the REEs, Pb, Th and U partition into apatite relative to both carbonates (and, hence, the precursor carbonate); KD ap/cc for REE decreases from ∽4 for La to ∽2 for Tm. There is almost equal partitioning of Sr between apatite and calcite. During separation of dolomite from calcite, Sr and Ba partition strongly into calcite and all the other analysed elements, except Fe and Mn, also preferentially enter calcite. The REEs prefer calcite relative to dolomite, and the KD dol/cc is reasonably constant, only varying from 0.23 to 0.17. Sr, Ba and Pb in the carbonates, and their partitioning between the calcite and dolomite, differ from other carbonatite carbonates reported in the literature.