Topaz hardness

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. 

Hard Materials (7) Quartz, granite (8) topaz (9) corundum, sapphire, emeiy (10) diamond. 

On ihe Mohs scale diamond is 10 and quartz 7. An abemative measure is the Knoop hardness (kgmm ) as measured with a 100-g load typical values on this scale are diamond 7000, boron carbide 2750, corundum 2100. topaz 1340, quartz 820. hardened tool steel 740. 

Hardness It is not possible to obtain a reliable figure for the hardness of anodic coatings with either the indentation or scratch methods, because of the influence of the relatively soft metal beneath the anodic film, and the presence of a soft outer layer on thick films. On Moh s Scale, the hardness of normal anodic films lies between 7 and 8, i.e. between quartz and topaz. 

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. 

Hardness of various brittle minerals, rocks and materials determined with the Mackensen blower with the use of various materials as abrasive grain. Hardness values for quartz, topaz and corundum have been taken as 7, 8 and 9 respectively on the Mohs scale... 

Finding a correlation between Young s modulus and material hardness required the use of hardness standards on the Mohs scale (quartz, topaz, corundum) whose hardness calculated using various methods was shown in Table 9.3. These data helped us to define the relation between Young s modulus and hardness of standard materials (Fig. 9.12), and then to read the degree of hardness of various tested materials from a chart (Table 9.4, Fig. 9.13). 

DANBIIRITE. The mineral danburile, CaBjSijO. calcium-boron silicate, crystallizes in the orthorhombic system in prismatic forms somewhat resembling the mineral topaz. Its fracture is subconchoidal brittle hardness, 7 specific gravity. 2.97-3.02 color, colorless, yellowish-white, yellow, dark wine yellow and brownish-yellow luster, vitreous to greasy translucent to transparent. It is found at Danbury. Connecticut, from whence its name was derived. Saint Lawrence County, New York. Switzerland, Japan, and Madagascar. 

Topaz is a rather rare silicate mineral with a Mohs hardness of 8. It comes in many colors from yellow to pink to purple to blue, depending on what ions are present in the crystal. It can even be colorless. However, the favorite variety is orange to brown in color and called imperial topaz. Much of the best topaz comes from Brazil. The gem called London blue topaz can be made from the colorless variety by treatment with heat and radiation. 

Diamonds are the hardest natural substance, but they can cleave easily. They occur in every color, but shades of yellow and brown are most common. Today s faceted diamonds are cut and polished with tools embedded with tiny diamond chips or dust. Before the advent of modern methods, diamonds were used in their natural state, or roughly shaped and polished by hand. The hardness of this mineral led many early jewelers to do a minimum of work before setting diamonds in jewelry or regalia. Common imitations of diamond are colorless spinel, sapphire, zircon, topaz, quartz, and many synthetics. 

In 1812, German geologist Frierich Mohs (1773-1839) devised a scale with specimen minerals that offered comparison of hardness qualities that allows the assignment of a Mohs hardness number to a mineral. Mohs scale utilizes ten specific representative materials that are arranged numerically from the softest (1) to the hardest (10). The reference minerals are (1) talc, (2) gypsum, (3) calcite, (4) fluorite, (5) apatite, (6) orthoclase feldspar, (7) quartz, (8) topaz, (9) corundum, and (10) diamond. 

The Mohs scale is a comparative index rather than a linear scale. In fact, Mohs scale has a near logarithmic relationship to absolute hardness. At the lower, softer end of the scale, the difference in hardness is close to linear, but at the extremes of hardness, there are much greater increases in absolute hardness (e.g., a greater increase in the hardness between corundum and diamond than between quartz and topaz). 

L. Blanc found that the oxide calcined from 500° to 800° has a sp. gr. 5-033 at 820°, 5-110 at 1080°, 5-130 and when fused, 6-145. H. P. Walmsley gave 5-238 for the sp. gr. of the dispersed oxide and W. H. Zachariasen, 5-25, calculated from the X-radiogram data. W. Biltz and co-workers found the mol. vol. of chromic oxide in the spinels to be 28-9. F. Wohler found that crystalline chromic oxide has a hardness great enough for it to scratch quartz, topaz, and hyacinth and G. Rose, and W. P. Blake found that it is as hard as corundum. 0. Ruff and A. Riebeth discussed the plasticity of mixtures of the oxide with water, etc. 

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