Changes

Hydrothermal experiments<ref name=Grafandgoldsmith_1956 /><ref name=Goldsmithandheard_1961>Goldsmith, Jr., and H. C. Heard, 1961, Subsolidus phase relations in the system CaCO₃-MgCO₃: Jour. Geology, v. 69, p. 45-74.</ref> extrapolated to low temperature, demonstrate that calcite and dolomite are essentially ideal in composition at 25°C ([[:file:CN24FG2.jpg|Figure 2]]). In other words, any double carbonate crystal of Ca and Mg at 25°C which is not essentially pure dolomite is either metastable or unstable with respect to a mixture of pure calcite plus pure dolomite. The same thing is true with respect to ideal dolomite plus magnesite. The composition of phases which we observe at Earth's surface define the range of metastability. Unstable phases are only observed as transient states in the laboratory. In the case of dolomite, few phases containing more than about 8% excess calcium (on a molar basis) have been reported to date, although the data are admittedly sparse.

Hydrothermal experiments<ref name=Grafandgoldsmith_1956 /><ref name=Goldsmithandheard_1961>Goldsmith, Jr., and H. C. Heard, 1961, Subsolidus phase relations in the system CaCO₃-MgCO₃: Jour. Geology, v. 69, p. 45-74.</ref> extrapolated to low temperature, demonstrate that calcite and dolomite are essentially ideal in composition at 25°C ([[:file:CN24FG2.jpg|Figure 2]]). In other words, any double carbonate crystal of Ca and Mg at 25°C which is not essentially pure dolomite is either metastable or unstable with respect to a mixture of pure calcite plus pure dolomite. The same thing is true with respect to ideal dolomite plus magnesite. The composition of phases which we observe at Earth's surface define the range of metastability. Unstable phases are only observed as transient states in the laboratory. In the case of dolomite, few phases containing more than about 8% excess calcium (on a molar basis) have been reported to date, although the data are admittedly sparse.

Reeder<ref name=Reeder_1981>Reeder, R. J., 1981, Electron optical investigation of sedimentary dolomites: Contr. Mineralogy Petrology, v. 76. p. 148-157.</ref> has shown that the structure of various kinds of dolomite revealed by transmission electron microscopy and electron diffraction can be classified into at least three types. All structures are ordered, although the degree of order is variable and difficult to quantify. The first, characteristic only of Holocene dolomite, consists of irregular "mosaics" on a scale of tens or hundreds of Angstroms. The crystals are characterized by extremely high densities of crystallographic faults and dislocations, and can be thought of as an aggregate of "micro-crystals" whose compositions may vary, forming a very discontinuous lattice. This leads to many unsatisfied or strained chemical bonds and to X-ray diffraction patterns with broad, generally weak reflections. This kind of dolomite is also characterized by large trace element substitutions, especially strontium<ref name=Behrensandland_1972>Behrens E. W., and L. S. Land, 1972, Subtidal Holocene dolomite, Baffin Bay, Texas: Jour. Sed. Petrology, v. 42, p. 155-161.</ref> and sodium.<ref name=Landandhoops_1973>Land L. S., and G. K. Hoops, 1973, Sodium in carbonate sediments and rocks; a possible index to the salinity of diagenetic solutions: Jour. Sed. Petrology, v. 43, p. 614-617.</ref> Qualitative data suggest that this material is extremely soluble compared to better ordered forms of dolomite. Land's<ref name=Land_1982>Land, Lyndon S., 1982, Dolomitization: AAPG Course Notes 24, p. 1-20.</ref> attempts to beneficiate samples composed of mixtures of this kind of dolomite and aragonite (for example, supratidal crusts from Florida and the Bahamas) by slow leaching in acetic acid resulted in only slight concentration of the dolomite by selective solution of aragonite. CO₂ for isotopic analyses of Holocene dolomite is evolved much faster than from finely ground ancient dolomite. All evidence suggests that Holocene dolomite is a unique, highly soluble material. It is clearly a metastable phase, unknown (in an unmodified form) in ancient rocks.

Reeder<ref name=Reeder_1981>Reeder, R. J., 1981, Electron optical investigation of sedimentary dolomites: Contr. Mineralogy Petrology, v. 76. p. 148-157.</ref> has shown that the structure of various kinds of dolomite revealed by transmission electron microscopy and electron diffraction can be classified into at least three types. All structures are ordered, although the degree of order is variable and difficult to quantify. The first, characteristic only of [[Holocene]] dolomite, consists of irregular "mosaics" on a scale of tens or hundreds of Angstroms. The crystals are characterized by extremely high densities of crystallographic faults and dislocations, and can be thought of as an aggregate of "micro-crystals" whose compositions may vary, forming a very discontinuous lattice. This leads to many unsatisfied or strained chemical bonds and to X-ray diffraction patterns with broad, generally weak reflections. This kind of dolomite is also characterized by large trace element substitutions, especially strontium<ref name=Behrensandland_1972>Behrens E. W., and L. S. Land, 1972, Subtidal Holocene dolomite, Baffin Bay, Texas: Jour. Sed. Petrology, v. 42, p. 155-161.</ref> and sodium.<ref name=Landandhoops_1973>Land L. S., and G. K. Hoops, 1973, Sodium in carbonate sediments and rocks; a possible index to the salinity of diagenetic solutions: Jour. Sed. Petrology, v. 43, p. 614-617.</ref> Qualitative data suggest that this material is extremely soluble compared to better ordered forms of dolomite. Land's<ref name=Land_1982>Land, Lyndon S., 1982, Dolomitization: AAPG Course Notes 24, p. 1-20.</ref> attempts to beneficiate samples composed of mixtures of this kind of dolomite and aragonite (for example, supratidal crusts from Florida and the Bahamas) by slow leaching in acetic acid resulted in only slight concentration of the dolomite by selective solution of aragonite. CO₂ for isotopic analyses of Holocene dolomite is evolved much faster than from finely ground ancient dolomite. All evidence suggests that Holocene dolomite is a unique, highly soluble material. It is clearly a metastable phase, unknown (in an unmodified form) in ancient rocks.

[[file:CN24FG3.jpg|thumb|300px|{{figure number|3}}Dark field transmission electron micrograph of a calcium dolomite (Ca_1.12Mg_0.88(CO₂)₂) of Eocene age. The prominent modulated structure is typical of sedimentary dolomite, and such crystals are metastable with respect to ideal stoichiometric dolomite. Photograph by Richard Reeder.]]

[[file:CN24FG3.jpg|thumb|300px|{{figure number|3}}Dark field transmission electron micrograph of a calcium dolomite (Ca_1.12Mg_0.88(CO₂)₂) of Eocene age. The prominent modulated structure is typical of sedimentary dolomite, and such crystals are metastable with respect to ideal stoichiometric dolomite. Photograph by Richard Reeder.]]