Manganese allotropes

Manganese(II), concentration formation constant of chelates, 5 717t Manganese 2B, 19 436 Manganese(IV), 24 415 Manganese(III) acetate dihydrate, 15 578 Manganese allotropes, properties of,... 

Table 2.46. Physical properties of four manganese allotropes...

The metal looks like iron it exists in four allotropic modifications, stable over various temperature ranges. Although not easily attacked by air. it is slowly attacked by water and dissolves readily in dilute acids to give manganese(II) salts. The stable form of the metal at ordinary temperatures is hard and brittle—hence man ganese is only of value in alloys, for example in steels (ferroalloys) and with aluminium, copper and nickel. 

There are four allotropic forms of manganese, which means each of its allotropes has a different crystal form and molecular structure. Therefore, each allotrope exhibits different chemical and physical properties (see the forms of carbon—diamond, carbon black, and graphite). The alpha (a) allotrope is stable at room temperature whereas the gamma (y) form is soft, bendable, and easy to cut. The delta A allotrope exists only at temperatures above 1,100°C. As a pure metal, it cannot be worked into different shapes because it is too brittle. Manganese is responsible for the color in amethyst crystals and is used to make amethyst-colored glass. 

Steel, as is well known, differs from iron by the presence of a certain amount of carbon, which induces the iron, when cold, to persist in its allotropic state. This appears to be due to a carbide of iron mixed with the excess of iron in the steel. The compound has been found as a meteoric mass it has been named cohenite, and has the formula FegC. On treating steel with dilute acetic acid, the same substance remains as a black powder. Its formula is similar to that of manganese carbide, MngC. 

Manganese exists in four allotropic forms. Allotropes are forms of an element with different physical and chemical properties. The element changes from one form to another as the temperature rises. The form that exists from room temperature up to about 1,300°F (700°C) is the most common form. 

The difference between the forms involves either (1) crystalline structure (2) the number of atoms in the molecule of a gas or (3) the molecular structure of a liquid. Carbon is a common example of (1), occurring in several crystal forms (diamond, carbon black, graphite) as well as several amorphous forms. Diatomic oxygen and diatomic ozone are instances of (2) and liquid sulfur and helium of (3). Uranium has three crystalline forms, manganese four, and plutonium no less than six. A number of other metals also have several allotropic forms which are often designated by Greek letters, e.g., a-, y-, and A-iron. 

Properties There are four allotropic forms of which a is most important. Brittle silvery metal, d 7.44, Mohs hardness 5, mp 1245C, bp 2097C, decomposes water. Readily dissolves in dilute mineral acids. Pure manganese cannot be fabricated. Manganese is considered essential for plant and animal life. 

The particular advantage of diffraction analysis is that it discloses the presence of a substance as that substance actually exists in the sample, and not in terms of its constituent chemical elements. For example, if a sample contains the compound A By, the diffraction method will disclose the presence of A B as such, whereas ordinary chemical analysis would show only the presence of elements A and B. Furthermore, if the sample contained both A B, and Aj Bjy, both of these compounds would be disclosed by the diffraction method, but chemical analysis would again indicate only the presence of A and B. To consider another example, chemical analysis of a plain carbon steel reveals only the amounts of iron, carbon, manganese, etc., which the steel contains, but gives no information regarding the phases present. Is the steel in question wholly martensitic, does it contain both martensite and austenite, or is it composed only of ferrite and cementite Questions such as these can be answered by the diffraction method. Another rather obvious application of diffraction analysis is in distinguishing between different allotropic modifications of the same substance solid silica, for example, exists in one amorphous and six crystalline modifications, and the diffraction patterns of these seven forms are all different. 

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