Cryptocrystalline

Chalk is a soft, fine-grained, fossiliferous form of calcium carbonate that varies widely in color, hardness, and purity. Its grain size is so minute that it appears amorphous, but actually it is cryptocrystalline with a very high surface area. 

Texture. All limestones are crystalline, but there is tremendous variance in the size, uniformity, and arrangement of their crystal lattices. The crystals of the minerals calcite, magnesite, and dolomite are rhombohedral those of aragonite are orthorhombic. The crystals of chalk and of most quick and hydrated limes are so minute that these products appear amorphous, but high powered microscopy proves them to be cryptocrystalline. Hydrated lime is invariably a white, fluffy powder of micrometer and submicrometer particle size. Commercial quicklime is used in lump, pebble, ground, and pulverized forms. 

MicrocrystaUine Silicas. Various microcrystalline (cryptocrystalline) materials such as flint, chert, and diatomaceous earth are found ia nature (see Diatomite). These may arise from amorphous silica, often of biogenic origin, which undergoes compaction and microcrysta11i2ation over geologic time. 

AH graphite has crystal stmcture but only certain kinds and sizes of natural graphites are commercially classified as crystalline, a term used for import duty purposes. Throughout this article reference is made separately to dake, vein (lump or high crystalline), and amorphous forms, all of which are essentially the same crystalline form of carbon. However, fine stmctured graphites (cryptocrystalline (2)) have been classified as amorphous. 

Vitreous, colourless form of free silica. Formed when quartz is heated to 870°C (1598°F). Aporous siliceous rock resulting from the decomposition of chertorsiliceous limestone. Used as a base in soap and scouring powders, in metal polishing, as a filtering agent, and in wood and paint fillers. A cryptocrystalline form of free silica. 

Quartz is found in several forms in all three major kinds of rocks—igneous, metamorphic, and sedimentary. It is one of the hardest minerals known. Geologist often divides quartz into two main groupings—course crystalline and cryptocrystalline quartz. Course crystalline quartz includes six-sided quartz crystals and massive granular clumps. Some colored varieties of coarse crystalline quartz crystals, amethyst and citrine, are cut into gem stones. 

Cryptocrystalline forms contain microscopic quartz crystals and include the chalcedony grouping of rocks such as chert, agate, jasper, and flint. 

In addition to the above crystalline phases sdica also exists in a few micro-crystadine forms. Such micro crystalline or cryptocrystalline silicas occur in nature and include diatomaceous earth, flint, and chert. They are mostly of biogenic origin forming from compaction of amorphous silica over geologic time. 

Various names, such as amethyst, citrine, and smolqr or black quartz, have been used for colored rock-crystal, whereas for the cryptocrystalline aggregate of quartz, names such as chalcedony and jasper are used. Agate and cornelian, for example, are types of chalcedony that have specific textures or colors. In this chapter, we analyze how a variety of morphologies of high-temperature and low-temperature quartz appear, and how textures of polycrystalline aggregate seen in agate and other crystals are formed. 

Recent non-oceanic sediments where zeolites are known to form most often contain cryptocrystalline silica (Brown, et al , 1969 Heath, 1969, Moiola, 1970 Hay, 1964). 

It would appear from the above summary of natural occurrences that quartz is the most stable form of silica at near-surface conditions but that other metastable phases, representing initially poorly organized material, predominate in the natural occurrences or newly formed silica. Experiments demonstrate the persistence of metastable amorphous or cryptocrystalline hydrated Si02 at low temperature (Kittrick, 1969 Krauskopf, 1956, 1959) and slow conversion at higher temperatures (above 100 bars) (Frondel, 1962 Heydemann, 1964 Carr and Fyfe, 1958 Mlzutanl, 1970). 

Most commonly, zeolites are found in series of sedimentary rocks which contain pyroclastic material and are formed during the devitrification of this material. If the rocks are silica-rich, the zeolite species formed seems dependent upon the bulk composition and burial depth or temperature of formation (Hay, 1966). They are most frequently accompanied by silica in an amorphous or cryptocrystalline form (opal, chalcedony). Analcite and all other compositional intermediates up to the silica-rich clinoptilolite are found in this association. The most comifton clay mineral in such tuffs is montmorillonite. Zeolites are sometimes found with glauconite (Brown, et al . 1969) or celadonite (Hay, 1966 Iijima, 1970 Read and Eisenbacher, 1974) in pelitic layers or acidic eruptive rocks... 

In renal stones, apatite (Ca5(P04)3 OH, 1/2 C03) with a hexagonal shape has been observed however, this mineral is difficult to identify because of its cryptocrystalline appearance. Struvite (MgNH4P04 6 H20) orthorhombic, newberyte (MgHP04 3 H20) orthorhombic, whit-ockite (Ca3(P04)2) hexagonal, and also brushite have been reported to occur in renal stones82. ... 

Next to materials of the glass-ceramics type, many varieties of chalcedony, such as agate, carneol, onyx, sardonyx, heliotrope and jasper, exhibit similar changes in hardness resulting from different consolidation of the cryptocrystalline structure of silica among mineral individuals. 

The lower abrasiveness of chalcedony as compared with quartz, roughly four times less than that of the latter, is due to the difference in brittleness of the two materials. The high elasticity of cryptocrystalline materials has consequences for the results of other hardness measurement methods. 

It was stated that hydrated calcium monohydrogen phosphate in amorphous or cryptocrystalline form is a potential precursor in the formation of hydroxyapatite because the structural position of Ca2+ on (010) and (110) crystal planes of both minerals essentially correspond to one another492. These planes of calcium ions could easily serve as transition boundaries with little distortion of crystal structure the same holds true for octacalcium phosphate or defect apatites. Thus apatite may form from amorphous or microcrystalline calcium monohydrogen phosphate possible via octacalcium phosphate or defect apatites. This process may already start inside the matrix vesicles and continue during extravesicular activities. 

Meyers J.H. (1987) Marine vadose beachrock cementation by cryptocrystalline magnesian calcite—Maui, Hawaii. J. Sediment. Petrol. 57,558-570. 

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