Dolomite Structure Research Articles

Thermal analysis of ground dolomites

The characteristics of ground dolomites which have undergone a change in crystal structure and of those almost free from structural change, were studied mainly by thermal analysis. Firstly, pulverized samples passed through a 325-mesh Tyler sieve were ground further for 1, 2, 3, 5 and 24 h in order to observe their change with progressive grinding. The X-ray diffraction (XRD) and IR patterns and TG-DTA curves obtained in flowing 100% CO 2 gas indicated a gradual loss of the cation ordering characteristic of dolomite structure up to a grinding period of 24 h when a new phase of lower ordering appeared; this confirmed the results of earlier investigators. To characterize further the gradually developing new phase which was not apparently distinguished by XRD or IR analysis, TG-DTA curves were obtained under varying CO 2 partial pressures. It was found that the new phase decomposed in a straightforward manner, similarly to that of magnesite and quite differently from that dolomite, its decomposition temperature and rate were dependent upon the CO 2 partial pressure. In addition, the influence of grain size and particle size distribution on the temperature and shape of the endothermic peak of the DTA curve was studied. It was found that the peak shape is affected by the grain size, and moreover that the temperature range of the endothermic peak is affected by the size and, more importantly, the size distribution of the particles.

Basinal Marine Dolomicrite from the Pennsylvanian/Wolfcampian Horquilla Formation, New Mexico

ABSTRACT A 6-m-thick sequence of black dolomicrite occurs within a shaley, basinal portion of the Pennsylvanian-Wolfcampian Horquilla Formation, New Mexico. Deep-water lithologic associations, absence of hypersaline and freshwater indicators, elevated Na and Sr contents, and marine-derived 18O values all indicate that the dolomite precipitated from marine-derived water during burial beneath the sediment-water interface. Trace and major element contents and oxygen isotopic compositions of the dolomites exhibit significant covariance. Sr, Na, and 18O all covary positively with excess Ca in the dolomite structure, and Fe and Mn covary negatively with excess Ca in the dolomite structure. Such variations are suggestive of a diagenetic alteration trend ranging from primary marine compositions preserved in calcian dolomite to diagenetic compositions in more stoichiometric dolomite. Moreover, dolomite which preserves primary 18O values exhibits only minor variation in 13C. This is in contrast to the highly variable 13C values observed in Tertiary deep-water dolomites where sulfate reduction during organic diagenesis plays a key role in promoting dolomitization. Thus, depletion of sulfate was not a prerequisite for dolomitization of basinal carbonates of the Hor uilla Formation. Rather, dolomitization was promoted by the metastability of the precursor mineralogy. While such a mechanism has been postulated for dolomitization of marginal slope facies of oceanic atolls, this study extends its application to burial-marine dolomitization of deep-water micritic sequences.

Composition-induced microstructures in rhombohedral carbonates

AbstractRecent TEM observations of two-phase microstructures and associated crystal defects in selected, rare dolomites have been extended to calcite-structured (R3̄c) carbonates and to other natural and synthetic carbonates that crystallize with the dolomite (R3̄) structure. The samples included siderites (FeCO3), smithsonites (ZnCO3), ankerites (Ca[Mg,Fe](CO3)2), and kutnahorites (Ca[Mn,Fe](CO3)2).TEM methods show that the forms of second phases which result from the presence of common, divalent, metallic impurities are morphologically similar in R3̄c and R3̄ carbonates and occur more widely than hitherto realized. The most common form consists of thin ribbons of second phase which are coherent with and have the same crystallographic orientation as the host carbonate. Another form of microstructure, manifest as modulations in diffraction contrast, appears to be associated with incipient breakdown of single-phase carbonate. The results of extensive TEM/EDS microanalyses show that in siderite and ankerite the formation of ribbons is promoted by Ca impurity or Ca excess (with respect to R3̄c stoichiometry). In smithsonite, Cu and Ca impurities can play similar roles in relation to modulated microstructures. In kutnahorites, the perfection of grains and the absence of second-phase effects is strongly dependent on the ratio of Fe to Mn but is also affected by Ca in excess of the stoichiometric requirement. Electron diffraction results from several of the minerals show c-type spots, which can be interpreted as indicating ordering within basal layers of cations.The results show that, by correlating analytical TEM data with the study of second phases and incipient two-phase microstructures, it should be possible to determine the limits of solid solubility in carbonate systems.

57Fe Mössbauer effect in ankerite: Study of the electronic relaxation

The low-temperature ( T ⩽ 10 K) 57Fe Mössbauer spectra of (Ca 1.1Mg 0.5Fe 0.3Mn 0.1) (CO 3) 2 (ankerite) are a superposition of a slow- and a fast-relaxation component. The relaxation rates are determined using the Blume-Tjon model and from their temperature dependence the energy splittings of the ferrous ground-state doublet are estimated. The effect of applied magnetic fields of up to 60 kOe is presented. The relaxational behaviour is ascribed to small lattice distortions due to the excess of Ca in the dolomite structure.

Luminescent materials with dolomite structure

Dolomites of the type MZr(BO 3) 2 ( M = Ca, Sr, Ba) are reported. The Pb 2+ ion is an efficient luminescent activator in these compounds. The Stokes shift is large. The Tb 3+ ion is sensitized by Pb 2+ (R c ~ 9 Å). The Ti 4+ ion shows at 300 K medium efficiency due to thermal quenching.

Coherent transformations in calcian dolomites

Many sedimentary calcian dolomites commonly display a modulated microstructure when viewed with the transmission electron microscope. This structure has been attributed to a disordered incorporation of excess calcium in the dolomite structure. We describe here two more highly ordered metastable superstructures (γ and δ dolomite) that are considered intermediate between disordered calcian dolomite as formed by dissolution-reprecipitation processes and—probably as a result of diagenesis—decomposition to stable stoichiometric dolomite and calcite. Small platelike regions of coherent calcite are observed in dolomite from the Devonian Lost Burro Formation in southeastern California and in heat-treated calcian dolomite. These observations document the evolution of sedimentary carbonates from metastable to stable phases under favorable kinetic conditions.

The nature of ordering and ordering defects in dolomite

The apparent lack of transformation-induced domains and stacking disorder defects in natural dolomites is considered in light of the different types of order in the dolomite structure. Experimentally produced twin domain boundaries and basal stacking defects are documented in a dolomite using high-resolution electron microscopy and electron diffraction techniques. Results reveal that upon cation ordering to form the dolomite structure, a twin domain is favored over an antiphase domain. The twin domain boundaries closely resemble antiphase boundaries (APB's) and are in contrast for superlattice reflections. However, background contrast within domains is shown to be different when imaged using certain fundamental reflections. The results allow speculation about the nature of ordering at low temperatures. Observations of twin domain boundaries in samples annealed at different temperatures allow estimation of the critical ordering temperature at 1,100°–1,150° C in stoichiometric dolomite.

Thermal analysis studies of the dolomite, ferroan dolomite, ankerite series. Part 1. Iron content recognition and determination by variable atmosphere DTA

Under static air furnace atmosphere conditions, high Fe ferroan dolomites and ankerites give characteristic three-endothermic-peaked DTA curves. On dilution, peak resolution deteriorates and eventually the minerals give only a single broad peak of greatly decreased diagnostic value. In contrast, determinations in flowing carbon dioxide result in increased peak separation, definition and height, while individual peaks remain recognisable (without coalescence) down to the limits of detection, which are greatly improved. This complete separation of the three peaks of ankerite permits a true evaluation of the now unmodified middle endothermic peak which decreases with falling iron content. Based on this peak, the detection limit of iron, expressed as wt.% FeCO 3 component in the dolomite structure, is of the order of 1%. The presence of Fe in the mineral siderite may be clearly distinguished from the Fe present as ferroan dolomite/ankerite.

Solid State Chemical Model for the Solubility Behaviour of CaCO3  MgCO3 Solid Solutions

AbstractMg‐calcite can contain up to about 6 mol‐% MgCO3, dolomite between 40 and 51%. Solid solutions of CaCO3 and MgCO3 are unstable between 20 and 30 mol‐% MgCO3. The thermodynamic stability of dolomite with respect to calcite and magnesite amounts to about ‐6.8 kJ mol−1. It is shown that both the thermodynamical properties and the solubility behaviour of these solids can be explained with a solid‐state chemical model based on the theory of subregular solid solutions. Evaluation of the parameters of the model resulted in a critical cluster size of about one unit cell for the dolomite structure. The fact that normal sea water is close to equilibrium with both calcite and dolomite means that the concentrations of Ca2+, Mg2+ and CO32‐ ions in sea water are restricted by fixed sclubility relations.

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

The instability of transition metal dolomites [CaR 2+(CO 3) 2 where R 2+ 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 Ca 2+ 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 R 2+CO 3 becomes more positive with increasing octahedral distortion. Estimated ΔG o f(dolomite) as well as stabilities and solubility limits of R 2+ 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(CO 3) 2] in the system CaCO 3-MgCO 3-FeCO 3 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(CO 3) 2 is to be expected in the system CaCO 3-MgCO 3-ZnCO 3. The tendency for transition metal dolomites to contain excess CaCO 3 can also be accounted for in terms of octahedral distortion and AG o f.

The perturbing effect of a railroad tunnel on the hydrogeological equilibrium of a dolomite structure covering a whole region : 2F, 2T. Proc. 2nd Int. Congress, IAEG, Sao Paulo, paper III-28, V1 1974, 7P

The perturbing effect of a railroad tunnel on the hydrogeological equilibrium of a dolomite structure covering a whole region : 2F, 2T. Proc. 2nd Int. Congress, IAEG, Sao Paulo, paper III-28, V1 1974, 7P

Etude de borates MeMe′ (BO 3) 2 de structure dolomite

We describe 14 new borates MeMe′ (BO 3) 2 with dolomite structure (Me IV = Sn, Me′ II = Mg, Ni, Co, Mn, Ca, Cd, Sr and Ba; Me IV = Ti, Me′ II = Ba; Me III = Cr, Me′ III = Lu, Yb, Er, Y and Dy). We discuss more particularly the stability domain for the Me IVMe II(BO 3) 2 compounds. The refinement of the oxygen positions in the coumpounds SnMg(BO 3) 2 and SnSr(BO 3) 2 shows evidence for a small rotation of the (BO 3) 3− triangles with respect to the ideal positions, when the ionic radius of Me′ increases.

Trace element composition of dolostones and dolomites and its bearing on the dolomite problem

300 specimens of “primary” and “secondary” dolostones and 150 specimens of dolomite quantitatively analyzed in triplicate for 20 trace and minor elements statistically yield separate populations of the two major lithologic varieties of dolomitic carbonate rocks for certain trace elements. Histograms, probability plots and measure of the coefficient of variation suggest a lognormal distribution for the trace element data, which when tested statistically at the 1 per cent level of significance, indicate higher concentrations of Al, Ba, Fe, K, Li, Zn and Na in the “primary” dolostones. Sr is significantly concentrated in the secondary dolomites separated from the dolostones. On the basis of carbon-oxygen isotope ratios and crystallo-chemical considerations, the dolomite is considered to be a replacement mineral after calcite or aragonite and the abundance of Sr in “secondary” dolomite is interpreted as a minor impurity originally derived from metastable aragonite and entrapped in the dolomite structure. For the samples studied, trace element patterns appear to suggest that “primary” dolostones (characterized by very fine and uniform grain size; complete absence of fauna, relict textures and phantom structures; lack of appreciable porosity; relatively light color; frequent fine lamination; conchoidal fracture; and association with evaporitic sequences) may represent the early replacement of predominantly calcitic limestones under conditions of somewhat above normal salinity, whereas “secondary” dolostones (characterized by relatively coarse and non-uniform grain size; euhedral dolomite rhombs; frequent oolith, pellet and fossil textures; or organic fossil remains) may represent the early replacement of predominantly aragonitic limestones under normal marine conditions. A specific example of Ordovician Nittany dolostone comprising alternating zones of “primary” and “secondary” dolostones confirms the relationship evident in samples differing widely in age and geographic location.

Trace Element Composition of Dolostones and Dolomites and Its Bearing on the Dolomite Problem: ABSTRACT

Three hundred specimens of primary and dolostones and 150 specimens of dolomite quantitatively analyzed in triplicate for 20 trace and minor elements statistically yield separate populations of the two major lithologic varieties of dolomitic carbonate rocks for certain trace elements. Histograms, probability plots and measure of the coefficient of variation suggest a lognormal distribution for the trace-element data, which when tested statistically at the 1 per cent level of significance, indicate higher concentrations of Al, Ba, Fe, K, Li, Zn, and Na in the primary dolostones. Sr is significantly concentrated in the secondary dolomites separated from the dolostones. On the basis of carbon-oxygen isotope ratios and crystallo-chemical considerations, the do omite is considered to be a replacement mineral after calcite or aragonite, and the abundance of Sr in dolomite is interpreted as a minor impurity originally derived from metastable aragonite and entrapped in the dolomite structure. For the samples studied, trace element patterns appear to suggest that primary dolostones (characterized by very fine and uniform grain size; complete absence of fauna, relict textures and phantom structures; lack of appreciable porosity; relatively light color; frequent fine lamination; conchoidal fracture; and association with evaporitic sequences) may represent the early replacement of predominantly calcitic limestones under conditions of somewhat above normal salinity; dolostones (characterized by relatively coarse and non-unifor grain size; euhedral dolomite rhombs; frequent oolith, pellet, and fossil relict textures; or organic fossil remains) may represent the early replacement of predominantly aragonitic limestones under normal marine conditions. A specific example of Ordovician Nittany dolostone comprising alternating zones of primary and dolostones confirms the relationship evident in samples differing widely in age and geographic location. End_of_Article - Last_Page 375------------