Why dolomites do not directly precipitate from sea water
Answers
Explanation:
Dolomitization
Dolomite forms the bulk of the carbonates of Isla de Mona. Samples from a number of localities contain varying amounts of dolomite. A vertical transition from pure dolomite at the base of the seacliffs to pure limestone at their top is present along the eastern and northern sides of the island from Punta Este to Punta Capitán.
Fabric-retentive microcrystalline dolomite is the most common and widely distributed type of dolomite in the island. The fine grain size of these crystals makes it difficult to distinguish them from calcite in thin section without the aid of staining (Fig. 9-11C). This type of dolomite replaces both the abundant pelleted muds as well as skeletal grains that characterize the backreef facies of Isla de Mona. The extent of replacement is variable. Rocks from the lower part of the Isla de Mona Dolomite are pervasively dolomitized including originally high-Mg calcite components such as red algae. At higher elevations, dolomitization mostly affects matrix material whereas red algae remain partially calcitic. Dull luminescence is characteristic of replacive dolomite. Nonluminescent dolomite overgrowths around dolomitized echinoderm fragments are present within the limestone-dolomite transition zone (Fig. 9-11D). The absence of overgrowths on echinoid fragments in the lower part of the measured sections of the Isla de Mona Dolomite, contrasting with their abundance in the upper Isla de Mona Dolomite and the lower Lirio Limestone, suggests that dolomitic overgrowths resulted from replacement of a calcitic precursor.
Euhedral, limpid dolomite spar commonly fills interparticle porosity in the Isla de Mona Dolomite. It commonly fills late vugs and some moldic pores. This cement nucleated around dolomitized grains as well as around areas of dolomitized matrix and increases in size toward the center of the cavities. This nonluminescent cement is commonly associated with grainstones in the Isla de Mona Dolomite.
Dolomite rhombs with cloudy centers and clear rims are also common in the Isla de Mona Dolomite. Cloudy-centered dolomite rhombs are commonly found related to the limestone-dolomite transition zone (Fig. 9-11E). Staining indicates that the center of the rhombs are calcitic. Dull luminescence is characteristic of this type of dolomite.
Zoned dolomite cements are found lining secondary porosity in the Isla de Mona Dolomite. The distribution of this cement is restricted to the lower part of the unit. Its abundance decreases upward toward the zone of transition between dolomite and limestone where it is mostly absent. This type of dolomite consists of alternating dark and light zones that are visible under both transmitted light as well as under cathodoluminescence. Commonly this dolomite cement contains an early nonluminescent inner zone followed by a single brightly luminescent outer zone. Samples from various localities, including Punta Este and Punta Capitán, contain multiple generations that are locally well developed.
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