Fluorite hardness

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). 

Intermediate Hardness (4) Fluorite, soft phosphate, magnesite, limestone (5) apatite, hard phosphate, hard limestone, chromite, bauxite (6) feldspar, ilmenite, orthoclase, hornblendes. 

The correlation of phosphate precipitation with decrease of conductivity (Wilson Kent, 1968), increase in pH (Kent Wilson, 1969) and hardness (Wilson et al, 1972) is shown in Figure 6.16. These results demonstrate the relationship between the development of physical properties and the underlying chemical changes, but there are no sharp changes at the gel point. Evidence from infrared spectroscopy (Wilson Mesley, 1968) and electron probe microanalysis (Kent, Fletcher Wilson, 1970 Wilson et al, 1972) indicates that the main reaction product is an amorphous aluminophosphate. Also formed in the matrix were fluorite (CaF ) and sodium acid phosphates. 

Figure 9.8 Proportionality between hardness and bond modulus for alkaline earth fluorite crystals.

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. 

To substantiate the need for indicating the crystallographic direction along with hardness test results, we calculate the hardness of one of the fundamental standard blocks of the Mohs scale, i.e., fluorite (H = 4), in three directions... 

Fig. 6.2.4. Image of deformation of hardness rosette in testing fluorite (111) face (P = 981 mN) after polishing its surface.

BARYTOCALCITE. This mineral is a carbonate of barium and calcium il crystallizes in the monoclinic system but occurs massive as well. Il lias a perfect cleavage parallel to the prism and one, less perfect, parallel to the base fracture, sub-conchoidal brittle hardness, 4 specific gravity, 3.66-3.71 luster, vitreous color, white or gray or may be greenish or yellowish transparent to translucent. Barytocalcite is found in Cumberland. England, associated with barite and fluorite. 

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. 

Rather hard, very brittle, slate-blue crystals. Crystallizes in structure type Cl (fluorite type), d 1. 94. Stable to alkalis. Decomposed by acids, forming silicon hydrides and hydrogen. 

The most frequently used scale is the one proposed in I8l0 by Mohs. This scale assigns integer values (frohi 1 to 10) to usual minerals (see Table 21). A mineral that streaks fluorite, but that is scratched by apatite, has a hardness conventionally noted as 4.5. 

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. 

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