Duncan F. Sibley

Classification of Dolomite Rock Textures

ABSTRACT Dolomite rock textures can be classified according to crystal size distribution and crystal boundary shape. The classification scheme presented here is largely descriptive but carries genetic implications because size distribution is controlled by both nucleation and growth kinetics, and crystal boundary shape is controlled by growth kinetics. Size distributions are classified as unimodal or polymodal. Crystal boundary shapes are classified as planar or nonplanar. If the evidence permits, a complete classification includes a description of recognizable allochems, matrix, and void filling. Allochems and preexisting cements may be unreplaced, partially replaced, replaced mimically, or replaced nonmimically. Allochems may be dissolved, leaving molds. Matrix can be unreplace, partially replaced, or replaced by a unimodal or polymodal size dolomite. Unimodal size distributions generally indicated a single nucleation event on a unimodal substrate. Polymodal sizes can be formed by multiple nucleation events on a unimodal or polymodal substrate or differential nucleation on an originally polymodal substrate. Planar crystal boundaries develop when crystals undergo faceted growth, and nonplanar boundaries develop when crystals undergo nonfaceted growth. Nonplanar boundaries are characteristic of growth at elevated temperature (> 50°C) and/or high supersaturation. Both planar and nonplanar dolomite can form as a cement, replacement of CaCO3, or neomorphism of a precursor dolomite.

Using web-based instruction to improve large undergraduate biology courses: an evaluation of a hybrid course format

We developed a hybrid course format (part online, part face-to-face) to deliver a high-enrollment, introductory environmental biology course to resident (living on or near campus), non-science majors at a large, public university. The hybrid course was structured to include bi-weekly online assignments and weekly meetings in the lecture hall focused on active-learning exercises. To evaluate the effectiveness of the web-based component of the hybrid course, we taught the hybrid course simultaneously with a traditional course in which we used passive lectures to cover material in the online assignments. Both courses received the same active-learning activities in class. Students in the hybrid course reported that the quality of interaction with the instructor was high, that they read the text more often and studied in groups more frequently. Performance on a post-course assessment test indicated that the hybrid course format was better or equivalent to the traditional course. Specifically, online assignments were equivalent to or better than passive lectures, and that active-learning exercises were more effective when coupled with online activities. Performance gains were greater for upperclassmen than for freshmen, indicating that hybrid course formats might be a superior option for upperclassmen when satisfying general science requirements.

The origin of common dolomite fabrics; clues from the Pliocene

ABSTRACT Characteristic features common to dolomites of all ages are found in Pliocene dolomites from the Netherlands Antilles. These features include 1) pseudomorphic replacement, 2) selective replacement, 3) fossil moldic porosity, 4) cloudy-centered, clear-rimmed dolomite, 5) sucrosic dolomite, and 6) dolomite cement. The development of a particular feature is controlled by 1) mineralogy of the material being replaced, 2) the degree of saturation of the bulk solution with respect to the mineral being replaced, and 3) the availability of dolomite nucleation sites. Low-Mg calcite (LMC) cement and fossils were often neither replaced by dolomite nor dissolved but aragonite and high-Mg calcite (HMC) were both replaced and dissolved. The resistence of LMC to dolomitization was due to solutions saturated with respect to LMC and the lack of nucleation sites. Cryptocrystalline LMC was more readily replaced by dolomite because it provides more nucleation sites. Cloudy-centered, clear-rimmed dolomite and sucrosic dolomite formed in sediments with abundant LMC and few nucleation sites. Fossil moldic porosity developed concurrent with pseudomorphic replacement, suggesting that the dolomitizing solution was undersaturated with respect to the mineral being replaced. Dolomite cement commonly lines pores and fills intraparticle pore space. Dolomite cement occasionally has inclusions of a precursor LMC cement. Dolomite cement crystals are larger than replacement crystals because there are fewer nucleation sites for the cement crystals.

Pressure solution features in a shallow buried limestone

ABSTRACT Three distinct types of pressure solution features are found in the Alpena limestone (Devonian, Michigan): stylolites, solution seams, and fitted fabric texture. The style of pressure solution is different in grainstones, packstones, and wackestones. The difference is interpreted to be the result of preferential cementation of the grainstones. Well-cemented grainstones typically have stylolites, whereas solution seams and fitted fabric texture are more common in poorly cemented grainstones, packstones, and wackestones. Pressure solution occurs at e80 percent of the lithologic transitions. This is probably due to competency contrasts between adjacent units. That material dissolved at pressure solution surfaces is not locally reprecipitated is indicated by porous allochems which abut against stylolites and units of abundant intergranular pressure solution (fitted fabric texture) which lack cement. The style of pressure solution in the Alpena limestone and its relationship to cementation is also observed in the Tuscarora quartz arenite (Silurian, Appalachian Basin).

Intergranular pressure solution and cementation of the Tuscarora orthoquartzite

ABSTRACT Intergranular pressure solution is believed by many sandstone petrologists to be the major source of silica for the cementation of orthoquartzites, although there is no quantitative supporting evidence from thin-section petrology. The white Tuscarora orthoquartzite, often cited as a good example of cementation by pressure solution, has been studied using luminescence petrology to evaluate this hypothesis. Point counts were made to determine the amount of pressure solution and authigenic silica in 183 thin sections of samples of the Tuscarora orthoquartzite from Pennsylvania, Virginia, and West Virginia. Luminoscope measurements show large amounts of pressure solution to be an uncommon phenomenon in the Tuscarora throughout the study area. Pressure solution can account for only 30-35% of the pore-filling quartz cement. Proximity to folds, grain size, sorting and clay content are factors that have been suggested as controls on the amount of pressure solution in orthoquartzites. Of these, only clay appears related to the occurrence of pressure solution in the Tuscarora. Furthermore, the well-cemented samples show less pressure solution than more friable samples, but contain more pore-filling cement. We conclude that initiation of cementation by silica during early diagenesis has prevented widespread development of intergranular pressure solution by equalizing the distribution of stress along grain boundaries. Of the many possible sources of silica other than pressure solution, transport of H4SiO4 in surface-derived ground water seems the most likely source of the majority of the cement found in the Tuscarora.

Dolomitization kinetics of hydrothermal bombs and natural settings

ABSTRACT During high temperature (150-300°C) dolomitization experiments the rate of dolomitization increases with temperature, reactant surface area, reactant solubility, ionic strength and Mg2+/Ca2+ of the solution. One would predict these results from simple kinetic theory. The discoveries that SO42-slows the rate, Li+ increases the rate and the induction period is long for dolomitization could not have been predicted. Dolomitization is a three-step reaction: (1) Nucleation: Nucleation of very high-Mg calcite (35-40 mole % MgCO3, VHMC) or nonstoichiometric dolomite is followed by nucleation of more stoichiometric dolomite on CaCO3. The composition of the VHMC or nonstoichiometric dolomite is a function of Mg2+/Ca2+ of the solution. Other variables such as dissolved Fe can affect the composition of the nuclei. (2) Induction period: Nucleation occurs during the induction period but the most of this period is post-nucleation growth of VHMC, nonstoichiometric and stoichiometric dolomite nuclei. Changes in the solution and substrate surface area affect the duration of the induction period. (3) Replacement period: (a) CaCO3 is replaced by VHMC or nonstoichiometric dolomite. VHMC nucleates faster than stoichiometric dolomite and therefore begins to replace the reactant first. (b) CaCO3 and VHMC and/or nonstoichiometric dolomite are rapidly replaced by stoichiometric dolomite. This phase of the reaction is sensitive to solution variables such as Mg2+/Ca2+ and ionic strength. The three-phase reaction model is consistent with eight characteristics of natural dolomites. (1) Very Ca-rich dolomite is common only in modern dolomites. (2) There is a direct relationship between Mg2+/Ca2+00 solution and Mg/Ca ratio in the dolomite. (3) The stoichiometry of dolomites in some ancient rocks is directly related to the percentage of dolomite in the rock. (4) Suppression of stoichiometric dolomite nucleation allows the persistence of metastable Ca-Mg-CO3 phases. (5) Dolomite-limestone contacts are often sharp. (6) Dolomite selectively replaces fine-grained CaCO3. (7) Dolomite crystals often have cloudy centers and clear rims. (8) Dolomite textures are mainly determined by the crystal size of the reactant.

Kinetics of dolomitization

Dolomitization of CaCO3 in the laboratory at ≥175 °C demonstrates that the transformation has two distinct stages: an induction stage during which no detectable products form and a nucleation-growth stage during which dolomite forms at the expense of the reactant CaCO3. Experiments at 218 °C demonstrate that as the Mg/Ca ratio of the solution increases, the length of the induction stage decreases. Addition of CO2 to the solution decreases the induction stage and also changes the overall transformation mechanism. Previously published data show that the induction stage is affected by the temperature of the reaction and the surface area and mineralogy of the reactant. The induction stage is often the slowest part of the overall transformation. Characteristics of natural dolomites that are consistent with the experimental results include the following: (1) Dolomite selectively replaces fine crystalline CaCO3; (2) most carbonate rocks are 100% dolomite or 100% calcite; (3) limestone-dolomite contacts are usually sharp and involve a decrease in the number of dolomite rhombs, but the size of the rhombs remains constant; (4) dolomite in completely dolomitized rocks is more stoichiometric than dolomite in partially dolomitized rocks; (5) aragonite is selectively dolomitized in modern sabkha sediments; and (6) modern dolomite is most abundant in areas of elevated Mg/Ca ratio or HCO3−.

Unstable to Stable Transformations during Dolomitization

Dolomitization of calcite at 218°C in hydrothermal bombs resulted in a series of unstable precursors to the final product, stoichiometric, ordered dolomite. The first phase formed was high-Mg calcite with approximately

Secular changes in the amount and texture of dolomite

35 mole \% MgCO_{3}