Calcite hardness

Along with the silicate debris carried to the sea by rivers and wind, the calcitic hard parts manufactured by marine organisms constimte the most prominent constituent of deep-sea sediments. On high-standing open-ocean ridges and plateaus, these calcitic remains dominate. Only in the deepest portions of the ocean floor, where dissolution takes its toll, are sediments calcite-free. The foraminifera shells preserved in marine sediments are the primary carriers of paleoceano-graphic information. Mg/Ca ratios in these shells record past surface water temperatures temperature corrected 0/ 0 ratios record the volume of continental ice ratios yield information... 

One might ask what controls the depth of the transition zone. The answer lies in chemical economics. In today s ocean, marine organisms manufacture calcitic hard parts at a rate several times faster than CO2 is being added to the ocean-atmosphere system (via planetary outgassing and weathering of continental rocks) (see Figure 3). While the state of samration in the ocean is set by... 

A mineral s hardness is defined as its ability to scratch another mineral. This is usually measured using a comparative scale devised about 200 years ago by Friedrich Mohs. The Mohs scale lists ten common minerals, assigning to each a hardness from 1 (talc) to 10 (diamond). A mineral can scratch all those minerals having a lower Mohs hardness number. For example, calcite (hardness three) can scratch gypsum (hardness two) and talc (hardness one), but it cannot scratch fluorite (hardness four). 

Hardness. The hardness (qv), or related property abrasiveness, is an important filler property. Hardness is determined by comparison to materials of known hardness on the Mohs scale. On this nonlinear scale, diamond is rated 10, quartz 7, calcite 3, and talc 1. The abrasiveness of a filler is also dependent on psd and the presence of impurities, eg, ka olin clay (Mohs hardness of 3) can be quite abrasive because of the presence of quartz impurities. 

Hardness. Most limestone is soft enough to be readily scratched with a knife. Pure calcite is standardized on Mohs scale at 3 aragonite is harder, 3.5—4. Dolomitic limestone is generally harder than high calcium. Dead-burned or sintered limes are 3—4 on this scale, whereas most commercial soft-burned quicklimes are 2—3 (see Hardness). 

Calcium carbonate (calcite) scale formation in hard water can be prevented by the addition of a small amount of soluble polyphosphate in a process known as threshold treatment. The polyphosphate sorbs to the face of the calcite nuclei and further growth is blocked. Polyphosphates can also inhibit the corrosion of metals by the sorption of the phosphate onto a thin calcite film that deposits onto the metal surface. When the polyphosphate is present, a protective anodic polarization results. 

The hardness of a mineral as measured by the Mohs scale is a criterion of its resistance to crushing [Fahrenwald, Trans. Am. In.st. Min. Metall. Pet. Eng., 112, 88 (1934)]. It is a fairly good indication of the abrasive character of the mineral, a factor that determines the wear on the grinding media. Arranged in increasing order or hardness, the Mohs scale is as fohows 1, talc 2, gypsum 3, calcite 4, fluoride 5, apatite 6, feldspar 7, quartz 8, topaz 9, corundum and 10, diamond. 

FIG. 20-79 Effect of pelleting pressure on axial crushing strength of compacted calcite particles of different sizes demonstrating existence of a critical yield pressure. Inset shows the effect of hardness on critical yield pressure. [Benhow, Enlargement and Compaction of Particulate Solids, Stanley-Wood (ed.), Butteixooiihs, 169 (1.9S3),]... 

In areas of the system where the heat gradient is less severe, calcium carbonate precipitates in both crystalline and amorphous forms. It may precipitate as a calcite or aragonite sludge, but more usually an aragonite scale is produced. Aragonite is hard and adherent, depositing in FW lines and various boiler surface components such as boiler tubes. 

Catalytic devices These nonmagnetic devices use a perforated non-ferrous tube to encourage small calcite seed crystals to form and reduce the risk of bulk water scaling. They are promoted for use in hard waters under conditions where supersaturation can easily occur. 

Carbonate in boilers usually is present as a hard, dense, white to tan or brown calcite scale (CaC03). A tan to brown color usually indicates the presence of iron. Samples of scales and deposits normally fizz when tested with concentrated muriatic acid (hydrochloric acid, HC1) if carbonate is present, although some preliminary heating may be required. 

The detrital rain of carbon-containing particles can be divided into two groups the hard parts comprising calcite and aragonite and the soft tissue containing organic carbon. The composition of the soft tissue shows surprising... 

Vinegar is recommended for cleaning a variety of appliances and other items that may be stained by hard water deposits. Automatic coffee makers, steam irons, dishwashers, teapots, faucet heads, and shower heads — over time, all accumulate calcium deposits from hard water. Groundwater, that is, water that travels through soil and rocks, accumulates dissolved calcium ions as a consequence of the natural weathering of minerals that contain calcium such as limestone and calcite, shells, and coral. At the same time, carbon dioxide in the air dissolves in water to form carbonate ions that combine with calcium ions to form a white solid, calcium... 

A large hard rock rutile deposit was discovered in central Chile. This ore is relatively complex with variable head grade of rutile ranging from 2% to 4% Ti02. The liberation of rutile occurs at about 100 mesh nominal size. The major gangue minerals present in this ore include feldspars, calcite and some silicates. 

Abrasives (DW, HC) Provide smoothing, scrubbing, or polishing action Hardness Calcite Feldspar Quartz Sand 0-55% 60-90% 0-15%... 

Major structural components of hard parts Ca, C, SI, 0, P, F, Sr, S Calcite, aragonite, opaline silica, celestite, apatite, fluoroapatite Components of frustules and tests, bone, teeth... 

The clay minerals of aeolian origin comprise 25 to 75% of the mass of pelagic sediments. The large range in composition reflects the latitudinal nature of the dust belt as well as dilution by other locally important particle types such as clay minerals of volcanogenic origin and biogenic hard parts (calcite and opaline silica). 

In the present-day ocean, about half of the PIC exported to oceanic sediments consists of the remains of foraminiferans. These microorganisms are protozoans. They are widespread in the marine environment with some species having a pelagic lifestyle and others benthic. As shown in Figure 15.1a, their calcareous structures have the appearance of a chambered snail shell and are composed of the mineral calcite. Since this hard part is covered by tissue, it is technically a type of skeleton. These detrital remains are referred to as tests or forams. Among the present-day and extinct species of foraminiferans, considerable variation exists in the size, shape, and density of their tests. 

About 25% of the carbonates deposited in shallow water are eventually eroded and carried downslope by bottom and turbidity currents to become part of the shelf and pelagic sediments. Shallow-water carbonates are also notable for their mineral composition. In addition to calcite and aragonite, some shallow-water calcifiers deposit hard parts containing high percentages of magnesium. These are referred to as magnesium-rich calcites. 

Although surfece waters are supersaturated with respect to calcium carbonate, abiogenic precipitation is imcommon, probably because of unfevorable kinetics. (The relatively rare formation of abiogenic calcite is discussed further in Chapter 18.) Marine organisms are able to overcome this kinetic barrier because they have enzymes that catalyze the precipitation reaction. Because fl declines with depth, organisms that deposit calcareous shells in deep waters, such as benthic foraminiferans, must expend more energy to create their hard parts as compared to surfece dwellers. 

Foraminifera Protozoans that deposit hard parts, called forams, which are composed of calcite. 

Calcium carbonate has a number of crystal modifications,but the calcite form is the one that is principally used for filler appHcations. Pure calcite is a relatively soft material (Moh hardness 3.0) with a specific gravity of 2.7. 

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