Dolomite Dissociation Research Articles

Features of Dissociation of Dolomite in the Composition of an Organomineral Mixture

Features of Dissociation of Dolomite in the Composition of an Organomineral Mixture

Dolomite dissociation indicates ultra-deep (>150 km) subduction of a garnet-bearing dunite block (the Sulu UHP terrane)

Dolomite dissociation indicates ultra-deep (>150 km) subduction of a garnet-bearing dunite block (the Sulu UHP terrane)

Experimental subsolidus phase relations in the system CaCO3–CaMg(CO3)2 up to 6.5 GPa and implications for subducted marbles

The dissociation of dolomite to aragonite + magnesite and the reaction of dolomite + aragonite to Mg-calcite have been experimentally determined by piston cylinder experiments in the range of 650–1000 °C and 2–6.5 GPa. Friction decay within the salt sleeves used as pressure medium was monitored by continuously logging the travel distance of the piston. Piston movement ceased after the first 24 h of the experiment, and all experiments were kept at the steady-state conditions for an additional 24–196 h. The reaction of dolomite to aragonite + magnesite has a positive slope and has been bracketed at 650 °C between 5.1 and 5.8 GPa and at 750 °C between 5.7 and 6.5 GPa. The reaction is sluggish with a 0.6-GPa interval where all three phases coexist. The reaction dolomite + aragonite to Mg-calcite has been determined in 0.5-GPa intervals from 660 °C at 2 GPa to 940 °C at 6.5 GPa. The composition of Mg-calcite at the reaction curve is buffered and changes systematically from X(Ca) of 0.90 at 2 GPa to 0.57 at 6.5 GPa. The solvus between dolomite and Mg-calcite shrinks slightly with increasing pressure, providing evidence for a small negative slope of the critical curve dolomite = Mg-calcite. The degree of disorder in dolomite has been determined in some of the retrieved samples by Rietveld refinement based on X-ray diffraction patterns. The order parameter S decreases gradually from 1 to 0.7 as a function of temperature as the critical curve is approached and then drops dramatically after crossing it. No ordering on quench has been observed. The experimental results permit the construction of a petrogenetic grid for the system dolomite, aragonite, Mg-calcite, and magnesite, which has an invariant point at ~7.5 GPa and 980 °C, representing the maximum pressure stability of dolomite. X(Ca) isopleths of Mg-calcite coexisting with dolomite serve mainly as a thermometer, whereas in the presence of aragonite they are a useful barometer. The experimental results have been applied to diamond-bearing marbles of the Kokchetav Massif, where Mg-calcite inclusions in garnet and zircon with X(Ca) of 0.76 most likely formed during prograde metamorphism close to the graphite–diamond transition.

Thermodynamics and kinetics of dolomite thermal dissociation

Here we present the results of studying thermodynamic principles and kinetics of thermal dissociation of dolomites in the Urals deposits. Dolomite samples vary by fractions and grain size of calcite inclusions that determines the differences in their dissociation kinetics. It was found that dolomite dissociation occurs based on the two-stage model limited at the first stage by the reaction on a three-dimensional surface and at the second stage – by the kinetic equation with a degree close to 0,3 and 0,6.

Ultrahigh‐pressure metamorphism in the magnesite + aragonite stability field: evidence from two impure marbles from the Dabie–Sulu UHPM belt

AbstractTwo magnesite‐bearing impure dolomitic marbles from the Dabie–Sulu ultrahigh‐pressure (UHP) region have been investigated to clarify if they had actually attained P–T conditions outside the dolomite stability field, limited by the reaction dolomite = aragonite + magnesite, and to test their potential for recording (U)HP conditions. In both cases, the silicate mineral assemblage records conditions around the terminal amphibole breakdown reaction: amphibole + aragonite ± quartz = clinopyroxene + talc, which is a good geobarometer between at least 2.0 and 2.6 GPa. At higher pressures, the terminal breakdown of talc to clinopyroxene + coesite is the last P–T milestone below 4 GPa recordable by the silicate assemblage, but evidence of former coesite is ambiguous. The dolomite dissociation curve becomes strongly divariant in Fe‐bearing marbles and may thus be attainable during cold subduction near the 5 °C km−1‘geotherm’. At least one of the samples (from Xinyan village, near Taihu, Dabie Shan) preserved relicts of both magnesite and aragonite and most likely attained conditions within the aragonite + magnesite stability field. For the second sample from Sanqingge village in the Sulu terrane, no certain evidence has been found in this study. Impure dolomitic marbles have considerable potential to preserve (ultra)high‐pressure relicts. Particularly, massive impure marbles that have not been pervasively infiltrated by fluids during exhumation are considered to preserve high‐pressure relicts better than metapelites, paragneisses and orthogneisses, and perhaps almost as well as eclogites or metaperidotites. The inconspicuous mineral assemblage clinopyroxene + talc or quartz (after former coesite) may in fact record UHP conditions.

Signature of coseismic decarbonation in dolomitic fault rocks of the Naukluft Thrust, Namibia

Unequivocal geological signatures of seismic slip are rare, exceptionally so in carbonate-hosted faults where carbonate minerals dissociate at temperatures lower than those required for producing a friction melt. This thermal dissociation leads to significant fault weakening by increased fluid pressure and/or nanoparticle lubrication, preventing further heating of the fault surface. Pseudotachylyte is therefore unlikely to form in carbonate-hosted faults, and other evidence for seismic slip must be identified.We studied the lower Cambrian Naukluft Thrust which crops out in central Namibia. It contains a cataclastic dolomite fault rock, referred to as “gritty dolomite”, which we interpret as a signature of coseismic carbonate dissociation and subsequent fluid–rock interactions. The fault was active at ambient temperatures below 200°C.“Gritty dolomite” contains: rounded, low aspect ratio dolomite clasts with a uniform Fe-rich dolomite coating, euhedral to subhedral magnetite, quartz, and K-feldspar in a fine-grained, massive to laminated carbonate matrix of particulate dolomite and crystalline calcite cement. The fault rock texture, combined with evidence of injectites of gritty dolomite into the wallrock, indicates the cataclasite deformed as a fluidized granular flow. At seismic slip velocities, frictional heating caused dissociation of dolomite to CO2 and Ca-, Fe- and Mg-oxides. This release of CO2 decreased the pH of the pore fluid in the fault, causing dissolution and rounding of dolomite clasts within an inertial grain flow, and precipitation of carbonate coatings and euhedral silicates and oxides during subsequent cooling and CO2 escape.Examples of similar rocks having some, if not all of these characteristics have been described from other carbonate-hosted faults. The geological setting of the Naukluft Thrust is unique in spatial extent and quality of exposure, allowing us to eliminate alternative hypotheses for sources of CO2 to drive fluidization.

A Kinetic Study of Thermal Decomposition of Limestone UsingIn SituHigh TemperatureX‐Ray Powder Diffraction

The kinetics of the thermal decomposition of a limestone and, simultaneously, of the lime growth have been investigated during isothermal treatments by means ofin situHigh TemperatureX‐Ray Powder Diffraction. In particular, four grain sizes of a quarry limestone with differences in terms of mineralogical composition (dolomite content), chemical composition (FeandMgamounts), and texture, have been studied. The lime produced during the isothermal treatments was then examined by using Scanning Electron Microscopy. In all samples, limestone decomposition starts with dolomite dissociation, followed by calcite decomposition. Rather than particle size, limestone texture, and dolomite content seem to be the most important factors controlling the starting temperature of calcination and its kinetic behavior.

Time-resolved remote Raman study of minerals under supercritical CO 2 and high temperatures relevant to Venus exploration

We report time-resolved (TR) remote Raman spectra of minerals under supercritical CO(2) (approx. 95 atm pressure and 423 K) and under atmospheric pressure and high temperature up to 1003 K at distances of 1.5 and 9 m, respectively. The TR Raman spectra of hydrous and anhydrous sulphates, carbonate and silicate minerals (e.g. talc, olivine, pyroxenes and feldspars) under supercritical CO(2) (approx. 95 atm pressure and 423 K) clearly show the well-defined Raman fingerprints of each mineral along with the Fermi resonance doublet of CO(2). Besides the CO(2) doublet and the effect of the viewing window, the main differences in the Raman spectra under Venus conditions are the phase transitions, the dehydration and decarbonation of various minerals, along with a slight shift in the peak positions and an increase in line-widths. The dehydration of melanterite (FeSO(4).7H(2)O) at 423 K under approximately 95 atm CO(2) is detected by the presence of the Raman fingerprints of rozenite (FeSO(4).4H(2)O) in the spectrum. Similarly, the high-temperature Raman spectra under ambient pressure of gypsum (CaSO(4).2H(2)O) and talc (Mg(3)Si(4)O(10)(OH)(2)) indicate that gypsum dehydrates at 518 K, but talc remains stable up to 1003 K. Partial dissociation of dolomite (CaMg(CO(3))(2)) is observed at 973 K. The TR remote Raman spectra of olivine, alpha-spodumene (LiAlSi(2)O(6)) and clino-enstatite (MgSiO(3)) pyroxenes and of albite (NaAlSi(3)O(8)) and microcline (KAlSi(3)O(8)) feldspars at high temperatures also show that the Raman lines remain sharp and well defined in the high-temperature spectra. The results of this study show that TR remote Raman spectroscopy could be a potential tool for exploring the surface mineralogy of Venus during both daytime and nighttime at short and long distances.

Carbon recycled into deep Earth: Evidence from dolomite dissociation in subduction-zone rocks: Comment and Reply

I appreciate the interest that Hermann has shown in our study of carbon recycled into deep Earth ([Zhu and Ogasawara, 2002a][1]). The work reported in that paper was accomplished in Ogasawara's laboratory at Waseda University; all samples were collected by Ogasawara, and the first author

Carbon recycled into deep Earth: Evidence from dolomite dissociation in subduction-zone rocks: Comment and Reply

[Zhu and Ogasawara (2002)][1] interpret the textures and mineralogy of dolomitic marbles from the Kokchetav massif in terms of pressure-induced dissociation of dolomite (dol) to aragonite (ara) + magnesite (mag). On the basis of this assumption they postulate a minimum metamorphic pressure of 7.85

Carbon recycled into deep Earth: Evidence from dolomite dissociation in subduction-zone rocks

The dolomite-dissociation textures documented here in rocks from the Kokchetav ultrahigh-pressure massif suggest that the experimentally expected dolomite dissociation happened in the subducted slabs represented by these rocks. Two reactions, magnesite 5 C 1 MgO 1 O2, and majoritic garnet 1 MgO 1 H2O 5 garnet 1 clinochlore, recorded in carbonate inclusions and the host majoritic garnet are responsible for generation of graphite and clinochlore during the exhumation. The dolomite dissociation indicates that carbonate materials were subducted to depths of .250 km below Earth’s surface. Such deep subduction evidently brings abundant carbon and carbonate into deep Earth.

Experimental investigation on dolomite dissociation into aragonite+magnesite up to 8.5 GPa

We carried out high-pressure and high-temperature experiments on the composition CaMg(CO 3 ) 2 to determine the equilibrium boundary of dolomite=aragonite+magnesite up to 8.5 GPa and 1200°C, using a 6–8 type multianvil high-pressure apparatus employing a quenching technique. Dolomite dissociates into aragonite and magnesite under high P – T conditions and the phase boundary of dissociation is described by a linear equation P (GPa)=1.75+0.0061 T (°C). At P >6.3 GPa and T >800°C, the equilibrium boundary was established by reverse and backward experiments to grow dolomite from mixtures of CaCO 3 and MgCO 3 with or without dolomite, respectively. This study suggests that the equilibrium line of the dolomite=aragonite+magnesite reaction could be very useful in addition to phase transformation of quartz–coesite and graphite–diamond to constrain P – T conditions for the metamorphism of dolomite-bearing ultrahigh-pressure rocks.

The origin of dolomitic carbonatites: field and experimental constraints

Carbonatites are most commonly either calcitic or dolomitic/ankeritic with very few types in between - what is generally referred to as the bimodal distribution. There is a widely held view that dolomitic carbonatites are less abundant than calcitic carbonatites and that these arise as sub-solidus alteration products of primary calcitic carbonatites. It is demonstrated here that dolomitic types are far more common than is sometimes appreciated and that they are particularly abundant in old Precambrian cratonic regions such as the Zimbabwean and Kaapvaal Cratons and the Archaean parts of the Canadian Shield. The field, petrographic and chemical features favour these dolomitic carbonatites being magmatic rather than arising from sub-solidus replacement of calcitic carbonatites. Experimental studies show that partial melting of carbonated mantle peridotite produces a carbonate liquid with high Mg# and MgO content and an alkali content of up to 6%. On ascent through the mantle from its original generation site this primitive carbonatite will be destroyed by reaction with lherzolite and harzburgite if it remains in equilibrium with the surrounding mantle. If, however, the melt is shielded from the surrounding mantle by a lining of metasomatic wehrlite on the conduits or if it rises too rapidly for equilibrium to be maintained, it is able to escape the mantle and rise into the crust. Reaction between primitive magnesian carbonate melt and wall-rock wehrlite shifts the composition of the melt to more Ca-rich (“calcitic”) compositions. It is argued that such liquids are capable of generating the complete range of carbonatite compositions recognised at the surface. Dolomite melts incongruently at low pressures and so will only crystallise from a magnesian carbonate magma at temperatures below the dolomite dissociation reaction. These conditions are dictated by the P-T trajectory of the ascending melt as well as the nature and concentration of minor “fluxing” constituents in the melt such as fluorine and alkalis. As a result calcite is the liquidus phase over a wide range of P-T-X conditions. Several authors have suggested that many calcite-rich carbonatites formed as cumulate-enriched crystal mushes. Such calcite mushes could be readily generated from magnesian, essentially “dolomitic”, parental magmas. It is argued that no reasonable petrogenetic mechanism exists whereby magnesian carbonatite magmas could be generated from calcitic parental melts: it is argued that the reverse is true.

Predicting the rate of thermal decomposition of dolomite

This communication presents experimental data on the thermal decomposition of a high-grade dolomite. The objective is to determine and describe the rate of thermal dissociation of dolomite at moderate temperatures as a function of temperature and conversion

Regularly Interstratified Chlorite/Vermiculite in Contact Metamorphosed Red Beds, Newark Group, Connecticut Valley

AbstractA regularly interstratified chlorite/vermiculite occurs in red beds of the East Berlin Formation (Early Jurassic age) in the Connecticut Valley. The mineral is restricted to a 2.5-m wide zone of contact metamorphosed strata adjacent to and underlying the Hampden Basalt. Chemical and X-ray powder diffraction data indicate that the chlorite/vermiculite formed in response to lava-induced elevated temperatures and the availability of magnesium in the muds during and shortly after emplacement of the lava flow. Near the contact, hydrothermal fluids originating from the lava and from the synchronal weathering of basalt fragments by superheated pore waters provided a source of Mg. Further from the contact, magnesium was primarily derived from the thermal dissociation of dolomite. K2O concentrations and the distribution of clay minerals in the red mudstone suggest that the interstratified chlorite/vermiculite formed from preexisting illite or vermiculite as potassium was released and brucitic sheets were incorporated into interlayer positions.

BRUCITE DEPOSITS IN THE RUTHERGLEN DISTRICT, ONTARIO

Some deposits of brucitic limestone with a ratio of MgO: CaO higher than that of dolomite are described. Evidence is given to show that the excess magnesia derived from dissociation of dolomite by the heat of the adjacent granite. Selective transfer of the products of dissociation into the carbonate rocks is attributed to igneous emanations. The bulk of the brucite is believed to have been deposited by direct replacement of carbonates, rather than by hydration of previously formed periclase, as has been suggested for similar occurrences elsewhere.

The dissociation of dolomite

That dolomite (CaCO3 . MgCO3) on being heated behaves as though it were a mixture of the two carbonates by dissociating in two distinct stages, appears to be the widely accepted opinion. Thus it is staled (Mem. Geol. Survey, Special Reports on Min. Resources, London, 1920, vol. 16, ‘Refractory Minerals’, p. 102) ‘On calcination, dolomite undergoes a progressive dissociation which may be considered chemically as covering two distinct stages. The first effect of rising temperature is the expulsion of carbon dioxide from the magnesium carbonate contained in the dolomite and the formation of a mixture of magnesium oxide and calcium carbonate, accompanied by a considerable reduction in volume… . Complete calcination results in the expulsion of the remaining carbon dioxide, a still further reduction in volume, and the production of a dense crystalline aggregate of magnesium and calcium oxides.’