Gypsum hardness scale

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). 

Mohs scale A hardness scale in which a series of ten minerals are arranged in order, each mineral listed being scratched by and therefore softer than those below it. The minerals are (1) talc (2) gypsum (3) calcite (4) fluorite (5) apatite (6) orthoclase (7) quartz (8) topaz (9) corundum (10) diamond. As a rough guide a mineral with a value up to 2.5 on this scale can be scratched by a fingernail, up to 4 can be scratched by a coin, and up to 6 by a knife. The scale was devised by Friedrich Mohs (1773-1839). 

In 1822, Friedrich Mohs developed a hardness scale for minerals, using ten minerals. From hardest to softest, they are diamond, corundum, topaz, quartz, potassium feldspar, apatite, fluorite, calcite, gypsum, and talc. 

Gypsum is very soft at 2 on hardness scale of Moho (Deer et al., 1992). Gypsum is so soft that a fingernail can easily scratch it. It is characterized by a monoclinic crystal system and a perfect cleave. The specific gravity of gypsum is 2,3. It has a white streak and a vitreous luster. 

Mohs hardness scale Empirical scale by which the hardness of solids can be determined by comparison with 10 reference minerals ranked from 1 to 10 1, talc 2, gypsum 3, calcite 4, fluorite 5, apatite 6, orthoclase 7, quartz 8, topaz 9, corundum and 10, diamond. 

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. 

Gypsum, 4 582-601 5 467, 785t 23 576 forms and composition, 4 583t hardness in various scales, 7 3t in Portland cement, 5 467 in Portland cement hydration, 5 477t thermal reduction of, 23 577 thermodynamics and kinetics of formation- decomposition, 4 586-588 Gypsum board, 4 600-601 Gypsum processes, obtaining sulfur from, 23 576-577... 

Calcium sulphate hemihydrate [ 10034-76-1 ] M 145.2. Sol in H2O (0.2 parts/100 at 18.75°). Completely dehydrated >650°. Dry below 300° to give a solid with estimated pore size ca 38% of vol. Anhydrous CaS04 has high affinity for H2O and will absorb 6.6% of its weight of H2O to form the hemihydrate (gypsum). It sets to a hard mass with H2O, hence should be kept in a tightly scaled container. 

Wallerius in 1747 used a finger nail, knife, file or diamond powder for hardness determination. Werner in 1774 rubbed a mineral against a finger nail, knife or piece of steel to determine the quantities of powder thus derived. Hatty (1801) used calcite and quartz as well as glass for this purpose. The first arbitrarily chosen scale of hardness, containing exclusively minerals, was devised in Sweden by Kvist in 1768. It covered diamond—20, topaz—15, zeolite—13, quartz—11, fluorite—7, calcite—6, gypsum—5 and chalk—2. 

In specifying the results of measurements taken with a Rockwell hardness tester, both standard type and Super, the measuring scale is indicated, e.g., //RA, Hrc, Hr45N, and so on. Scale N is applicable for hard materials and the remaining special scales, for soft materials (brass, lead, zinc, organic materials, gypsum and lime-based materials). 

In cooling water deposits, the sulfate scale is normally present as gypsum (CaSC>4 2H2O). Sulfate scales are hard and difficult to remove and therefore treatment techniques employed must be carefully controlled to avoid scaling risks. There are specific polymeric chemical treatments now available to control sulfate deposition in high sulfate waters. 

The softest mineral, talc, can be used in body powder. The hardest, diamond, is used in drill bits to cut through the most dense crustal materials. Mohs scale is a relative index scale, meaning that a determination of Mohs hardness number for a mineral is based upon scratch tests. For example, gypsum (Mohs hardness number = 2) will scratch talc (Mohs hardness number = 1). Talc, however, will not scratch gypsum. Glass is assigned a Mohs hardness number of 5.5 because it will scratch apatite (Mohs hardness number = 5) but will not scratch orthoclase feldspar (Mohs hardness number = 6). 

This hardness, which can be removed by heating, is called temporary or carbonate hardness. Temporary hardness is derived from contact with carbonate (limestone and dolomite). Hardness which cannot be removed by boiling is called permanent or non-carbonate hardness and it is due to anions, such as chloride, nitrate, sulfate and silicate. This hardness does not contribute to scale formation. Contact with gypsum would result in permanent hardness. Calcium hardness is that due to Ca only, while magnesium hardness is due to Mg only. Magnesium hardness can be calculated from a determination of total and calcium hardness ... 

As for other physical properties, mica appeared as term 3 on a 12-term scale of hardness proposed by Breithaupt (1836). This scale was not very different from the preceding one of Mohs. Indeed, there were only two differences, namely mica between gypsum and calcite and sodalite between apatite and adularia. 

Mohs scale of hardness is a scale of scratch resistance, graduated in terms of the relative hardness of 10 common minerals, which include talc (1), gypsum (2), calcite (3), quartz (7) and diamond (10). 

In the above scale, each mineral scratches the previous. It is used to make reference to fhis charf to evaluate the hardness of some ofher known materials. For example, the hardness of ordinary glass is 5.8. The fragility is a property represenhng how easy a substance may be crumbled or broken by impact. It does not necessarily have a direct relation with hardness. Gypsum, for insfance, is soff buf nof fragile. Finally, fhe tenacity is known as the property the metals and alloys present to resist collisions. 

Alabaster is a variety of gypsum, and is known as a decorative stone. It is white but sometimes beautifully shaded with light-colored tones of yellow or red. Its usefulness is limited by its softness (hardness 2 on Mohs scale). A gypsum curiosity is the yellow Desert rose, found in the Sahara. 

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