Mohs Hardness Scale

Mineral Hardness: Mohs Hardness Scale

History of the Mohs Scale

Mohs' scale of mineral hardness quantifies the scratch resistance of minerals by comparing the ability of a harder material to scratch a softer material. The Mohs scale was invented in 1812, by the German mineralogist Friedrich Mohs. Mohs based his scale on ten minerals.

With the Mohs Scale, the hardness of a material is measured against the scale by finding the hardest material that it can scratch, and/or by identifying the softest material that can scratch it. If a given material can be scratched by topaz, but not by quartz, its hardness on Mohs scale is 7.5.

Mohs Hardness vs True Hardness

The table above shows a comparison with absolute hardness measures using a sclerometer. The Mohs scale is a ordinal or successive scale and therefor, does not measure or compare actual hardness. For instance, corundum is twice as hard as topaz, but diamond is almost four times as hard as corundum yet there is only one step between each of these three minerals.

Minerals in Order of Hardness

	1. Talc Talc is the world's softest mineral and the lowest mineral on the Mohs scale. Talc is a hydrated magnesium sheet silicate which is highly insoluble in water. Talc is translucent to opaque with a iridescent or pearly luster. Talc is used in cosmetics such as talcum powder, as a lubricant, and in paper manufacturing. Absolute Hardness: 1 Chemical Composition: Mg3Si4O10(OH)2
	2. Gypsum Gypsum is a soft mineral composed of calcium sulfate dihydrate. Gypsum occurs in nature as flattened or twinned crystals and transparent cleavable masses called selenite. When Gypsum has a silky and fibrous texture it is called Satin Spar. Absolute Hardness: 2 Chemical Composition: CaSO4á2H2O
	3. Calcite Calcite is an anhydrous carbonate, and one of the most widely distributed minerals on the Earth's surface. It is a common constituent of sedimentary rocks. In crystallized form, Calcite has a vitreous luster. Absolute Hardness: 9 Chemical Composition: CaCO3
	4. Fluorite Fluorite (fluor-spar) is a mineral composed of calcium fluoride. It is an isometric mineral with a cubic crystal habit. Fluorite is named for its property of fluorescence, or its ability to fluoresce under ultraviolet light. Absolute Hardness: 21 Chemical Composition: CaF2
	5. Apatite Apatite (hydroxylapatite, fluorapatite, chlorapatite) is a group of phosphate minerals and is one of few minerals that are produced by biological organisms. Hydroxylapatite is the major component of tooth enamel. Absolute Hardness: 48 Chemical Composition: Ca5(PO4)3(OH-,Cl-,F-)
	6. Orthoclase Orthoclase (aka feldspar) in an igneous rock forming tectosilicate (silicate) mineral and is a key component in granite. Orthoclase derives its name form the Greek word for "straight fracture" because of its two cleavages at right angles to each other. Orthoclase crystallizes in the monoclinic crystal system. Absolute Hardness: 72 Chemical Composition: KAlSi3O8
	7. Quartz Quartz is one of the most common minerals found in the Earth's crust. It has a hexagonal crystal structure made of trigonal crystallized silica (silicon dioxide). The typical shape of a Quartz crystal is a six-sided prism that ends in six-sided pyramids. Absolute Hardness: 100 Chemical Composition: SiO2
	8. Topaz Topaz is a silicate or "nesosilicate" mineral created from a combination of aluminium and fluorine. It crystallizes in the orthorhombic system and it's crystals are prismatic in form. Absolute Hardness: 200 Chemical Composition: Al2SiO4(OH-,F-)2
	9. Corundum Corundum is the crystalline form of aluminium oxide and one of the basic rock-forming minerals. Corundum is naturally clear or colored by impurities. Due to its hardness, Corundum is used as an abrasive in sandpaper. Emery is an impure and less abrasive variety of Corundum. Absolute Hardness: 400 Chemical Composition: Al2O3
	10. Diamond Diamond is the hardest natural occurring material. Diamond is a natural allotrope of carbon. The crystal bond structure of diamonds give the stone its hardness and differentiates it from graphite, which is the main allotrope of carbon. Absolute Hardness: 1500 Chemical Composition: C

Crystal Forms

Crystal Forms: Open, Closed & Isometric Crystal Forms

Types of Crystal Forms

Closed & Open Crystal Forms

Any grouping of crystal faces or facets that are arranged in the same symmetry is referred to as a crystal's "form." There are approximately 48 unique crystal forms which are divided into either "open," "closed Isometric," or "closed non-Isometric" form categories.

There are thirty closed Isometric, or closed non-Isometric forms, and eighteen open crystal forms. Closed crystal forms are those groupings of facets that are related by symmetry, and completely enclosing a volume of space.

Open Forms

The eighteen open-forms are those facet groupings that are related by symmetry, but do not completely enclose a volume of space. A crystal with open-form faces also requires some additional closed-form facets to complete a structure. Open-forms include:

• Pedion
• Pinacoid
• Dome
• Sphenoid
• Pyramid
• Prism

A Pedion is a flat face that is not parallel, or geometrically linked to any other faces. A Pinacoid is composed of only two parallel faces, forming tabular crystals such as ruby. A Dome is found in monoclinic and orthorhombic minerals.

Two intersecting faces that are caused by mirroring (topaz) commonly forms domes. Sphenoids are found in monoclinic and orthorhombic minerals, and have two-fold rotational axes. A Pyramid's multiple facets converge on a single crystallographic axis, and pyramid forms are not possible on minerals from the isometric, monoclinic or triclinic systems.

Open Hexagonal & Triangular Prisms

Prisms have a set of facets that run parallel to an axis of a crystal, yet never converge with it. Quartz forms two sets of three sided prisms. Prisms are not possible in isometric or triclinicminerals.

A Hexagonal (trigonal) prism is comprised of two hexagonal bases connected by a set of six rectangular faces that run parallel to, and never converge with an axes in the crystal.

A triangular (trigonal) prism is comprised of two triangular bases connected by a set of three rectangular faces that run parallel to, and never converge with an axes in the crystal. This form is similar to a light-refracting 60¼ prism.

Closed Isometric Forms

There are several crystal forms in the cubic crystal system that are common in diamond,garnet, spinel and other "symmetrical" gemstones. These forms include:

• Hexahedron (Pyrite)
• Octahedron (Diamond, Spinel)
• Tetrahedron (Tetrahedrite)
• Dodecahedron (Rhombic - Garnet)
• Hexoctahedron (Diamond)

A hexahedron (cube) has eight points, six faces, and twelve edges that are perpendicular to each other, forming 90 degree angles. An octahedron has two four sided pyramids lying base to base, and is totally symmetrical with no top, or bottom and has eight faces. A tetrahedronhas four equilateral triangular faces.

A dodecahedron has 12 faces and there are four types of dodecahedrons listed in order of descending symmetry: 1. Symmetrical pentagonal (five edged polygons) dodecahedrons, 2. Asymmetrical (tetartoid) pentagonal dodecahedrons, 3. Delta (four edged polygons) dodecahedrons, and 4. Rhombic dodecahedrons. A Hexoctahedron is a multi-faceted dodecahedron with 48 triangular faces.

Closed Non-Isometric Forms

1. Hexagonal (Trigonal) Closed Forms

• Hexagonal Pyramid
• Hexagonal Bipyramid (Apatite)
• Dihexagonal bipyramid (Beryl)
• Hexagonal Trapezohedron
• Hexagonal Scalenohedron
• Tetrahexahedron

2. Tetragonal Closed Forms

• Tetragonal Disphenoid
• Tetragonal Scalenohedron
• Tetragonal Trapezohedron
• Tetragonal Trapezohedral Trisoctahedron
• Tetragonal Ditetragonal Bipyramidal (Rutile)

3. Rhombohedral Closed Forms

• Rhombohedral Trapezohedral (Quartz)
• Rhombohedral Hemimorphic (Tourmaline)
• Rhombohedral Holohedra (Calcite)
• Rhombohedral Dodecahedron (Garnet, Fluorite)
• Rhombohedral Trisoctahedron

4. Orthorhombic Closed Forms

• Rhombic Prism
• Rhombic Pyramid
• Rhombic Dipyramid
• Rhombic Hemimorphic
• Rhombic Sphenoid
• Rhombic Pyramid

5. Monoclinic Closed Forms

• Prism
• Monoclinic Clinopinacoid

6. Triclinic Closed Forms

• Prism
• Triclinic Dipyramid