Oolitic form

Calcitic oolite A pavement of calcitic ooids formed by continuing precipitation of calcium carbonate between the calcitic ooids. 

Oolite A hydrogenous precipitate commonly found in carbonate sediments of continental shelves located in the tropics. They are composed primarily of calcium carbonate and are thought to be an abiogenic precipitate formed from warm seawater supersaturated with respect to calcite and aragonite. 

Figure 8.4. Forms of polycrystalline aggregate (a) botryoidal (b) mamillary (c) spherulitic (d) oolitic (see p. 156).

PERLITE (or Pearlstone). An unusual form of siliceous lava composed of small spherules of about the size of bird shot or peas. It is grayish in color with a soft pearly luster. The spherules often show a concentric structure and are believed to be funned as a result of a peculiar spherical cracking developed while cooling. They may be confused with oolites, which are classified as concretions. 

In nature, a series of minerals are known to interact with organic substances, notably carbohydrates and amino compounds103-108. In the context of this discussion the dominance of acidic amino acids in oolites is significant109-111. Oolites are spherical bodies up to 1—2 mm in diameter which are chemically precipitated in tropical lagoons, e.g. Bahamas, Bermudas and Red Sea in the form of aragonite and accumulate as fine sand. The process of precipitation is a slow one and may proceed over a period of a few hundred years. 

Finally, this tripartite cyclicity is also seen in the frequency of occurrence of Phanerozoic ironstones and oolites (Figure 10.18). As sea level withdrew from the continents and continental freeboard increased, shallow-water areas with the requisite environmental conditions necessary to form oolite and ironstone deposits decreased in extent. Thus, as calcium carbonate deposidon increased on slopes and in the deep sea, carbonate oolite and ironstone deposition on shelves and banks nearly ceased. 

The high calcium content of the younger coals has led to the formation and deposition of calcium carbonate in the liquefaction reactor in the form of wall scale and oolites which were first observed in German operations (10). These deposits form as calcium salts of humic acids in the coal decompose under liquefaction conditions. The deposits continue to grow with time and could lead to unwanted solids accumulation in the reactor itself as well as fouling of downstream equipment (11). Data shown in Figure 7 indicate the accumulation rate of the calcium carbonate in the liquefaction reactor for different coals under typical EDS conditions as well as two methods for controlling the solids build-up. 

Under the microscope the ore has an appearance suggestive of oolitic limestone, from which it has very probably been formed by molecular replacement of the calcium carbonate by ferrous carbonate, through the infiltration of waters containing the latter m solution.2... 

Transport of iron in carbonate waters, mainly in the form of Fe " bicarbonate, is more common. The decrease in COj due to the overall reduction in pressure when ground waters come to the surface, when carbon dioxide is consumed as a result of photosynthetic activity of plants or even, as Mokiyevskaya (1959) mentions, when the temperature rises, leads to deposition of FeCOj. In Strakhov s opinion such a process could lead to the formation of oolitic hydrogoethite-chamosite-siderite ores. The iron migrated in mobile form as Fe, which accumulated in solution in a reducing environment. Formation of the ores was related to the draining of high-iron waters formed in swampy regions. The near-shore parts of the sea with... 

A) Horizontal beds forming a transition from floodplain deposits to a palustro-lacustrine environment and lacustrine limestones. (B) Lacustrine deposits with stromatolitic bioherms (C) Palustrine limestone with abundant root traces. (D) Lacustrine bottom-set sediments enriched in organic matter and showing thin turbiditic layers. (E) Palustrine limestone with a well developed palaeosol at the top. (F) Various types of crushed shell fragments in a lacustrine mud. (G) Lacustrine bioclastic and oolitic sand deposited near a shore. 

Inorganic precipitation of calcium carbonate occurs from both sea and inland waters (as used by geologists, precipitation refers to the relatively slow process of crystal growth on surfaces). This route has resulted in some commercially significant deposits, the most common of which are oolitic limestone and travertine (see section 2.2.1). Some minor dolomite sediments have been formed by direct precipitation from sea and lake waters. 

Crystals can also aggregate in masses that have less easily recognized crystalline forms. These masses include shapes such as botryoidal (grape-like), oolitic (pea-shaped), globular (nearly spherical), stalactitic, lamellar (platy), and dendritic (finely extended structures). Figure 105 shows botryoidal forms of both goethite and lepidolite. 

Chemical sediments are created by a precipitation of low temperature/pressure minerals from water solution onto a depositional surface or within sediment pores. Depending on the acidity, oxidation, temperature, or salinity, a variety of chanical sedimentary rock may result. Examples of chemical sediments include carbonates, evaporites, opal, chert (may form in other ways too), iron oxides, and aluminum oxides. Chanical textures are usually o-ystaUine with some special terms, such as oolitic or pisolitic. Sandstone is a clastic sedimentary rock made up mainly of sand-size (1/16 to 2 mm diameter) weathering debris. 

Chemical sedimentary rocks are formed as precipitates from supersaturated solutions precipitating out around. Oolitic limestone is an example of rock in this category. 

A fairly common variety of limestones in this category are the oolitic limestones, which are composed of so-called ooliths, i. e., more or less spherical rock particles grown by accretion around a nucleus and of the order of 1 mm in diameter. These calcareous ooliths are formed in shallow water (less than about 2 m depth) subject to considerable motion. When a certain amount of lime has been deposited around the nucleus (which may be a grain of sand or a shell fragment), the oolith sinks to the bottom by gravity. Portland limestone belongs to this type. 

All niarine phosphorites consist mostly of microcrystalline apatite (carbonate fluorapatite) in the form of laminae, pellets, oolites, nodules and skeletal or shell debris. Uranium, considered syngenetic, may be present in carbonate fluorapatite as a substitute for calcium. Uranium in sea water was probably incorporated during or shortly after precipitation, and it is usually disseminated rather uniformly throughout a given bed or horizon. Primary uranium minerals are rarely present, but secondary uranium minerals (tyuyamunite, autunite, torber-nite) have been identified in a few localities. 

Results are presented here for two types of stone, namely Portland limestone and Monks Park limestone. The structure of these materials has been characterised in the past (Honeyborne and Harris 1959). Both of these rock types are composed of small spherical CaCOa particles, known as oolites, in a calcium carbonate rich cement. The oolites themselves have been formed by nucleation in a CaCOa rich oceanic environment. Both of the stones are carboniferous in origin. Tiles 50x50x8 mm were supplied by the Building Research Station (BRS) in the following condition ... 

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