Aragonite hardness

Crystal Type Acicular Aragonite Hardness Mobs 4.0 Refractive Index 1.68... 

The most common forms of pure calcium carbonate are calcite and aragonite (hard, more dense, less abundant than calcite). 

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). 

Strontium Carbonate. Strontium carbonate, SrCO, occurs naturally as strontianite in orthorhombic crystals and as isomorphs with aragonite, CaCO, and witherite, BaCO. There are deposits in the United States in Schoharie County, New York in WestphaUa, Germany and smaller deposits in many other areas. None is economically workable. Strontianite has a specific gravity of 3.7, a Mohs hardness of 3.5, and it is colorless, gray, or reddish in color. 

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. 

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... 

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... 

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. 

The devices are further claimed to ensure that magnetically treated water keeps minerals in a soft amorphous powder form instead of the usual hard crystalline structure. This amorphous powder deposits a thin film of aragonite talc on the inside of pipes and the waterside of condensers which prevents free oxygen in the water from attacking the metal surfaces. ... 

Some mollusks produce a hard exoskeleton that is composed of calcium carbonate, sometimes mixed with conchiolin, a schleroprotein. Either or both of the two common polymorphs of calcium carbonate may exist in one shell (calcite and aragonite). The composition and structure of the shell varies according to the species of animal, its age, and health. 

At shallow depths carbonate cements may cause sands to become brittle and hard. Carbonate which precipitates on the sea floor may also form hard grounds in dominantly clastic sequences. Sandstones may become calcite cemented due to dissolution of biogenic aragonite at relatively shallow depth (less than a few hundred meters). Calcareous sediments flushed by meteoric water at shallow depth or exposed during regression may become rapidly ce-... 

Oceanic surface water is everywhere supersaturated with respect to the two solid calcium carbonate species calcite and aragonite. Nevertheless, calcium precipitation is exclusively controlled by biological processes, specifically the formation of hard parts (shells, skeletal parts, etc.). The very few existing amounts of spontaneous inorganic precipitation of CaC03(s) come from the Bahamas region of the Caribbean. 

The detrital rain of carbon-containing particles can be divided into two groups the hard parts comprised of calcite and aragonite and the soft tissue containing organic carbon. The composition of the soft tissue shows the average ratio of biophils as P N C Ca S = 1 15 131 26 50, with Cc Co ratio as 1 4. More details of carbon transformation in bottom sediments are presented in Box 2. 

Although marine plants and animals are numerous and diverse, only relatively few groups produce hard parts capable of contributing to the formation of sediments, and only very few groups occur in an abundance relevant for sediment formation (Table 1.2). Relevant for sediment formation are only carbonate minerals in the form of aragonite, Mg-calcite and calcite, as well as biogenic... 

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