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2014

Mineral Hardness and Hardness Scales

The hardness of a mineral is an easy diagnostic tests to perform in the attempt to identify an unknown mineral. Hardness measures a mineral's resistance to scratching by another substance and reflects its atomic structure. For example, if mineral X scratches mineral Y, and mineral Y does not scratch mineral X, then mineral X is harder than mineral Y. If mineral X and Y both scratch each other, then their hardness is equal or very similar. Hardness is almost always rounded off to the nearest half number.

Mohs Hardness Scale
A scale to measure hardness was devised by Austrian mineralogist Frederick (Friedrich) Mohs in 1822, and is currently the standard scale for measuring hardness. The scale consists of numbers one through ten; 1 being the softest and 10 being the hardest. Each number represents a different mineral - each harder than the previous. The 10 minerals are:

TRADITIONAL
Mineral
OTHER
CHOICES
Hardness
VALUE
TRADITIONAL
Mineral
OTHER
CHOICES
Hardness
VALUE
Talc Sulphur, Graphine 1 Orthoclase Hematite, Pyrite 6
Gypsum Amber 2 Quartz Tourmaline 7
- Fingernail 2.5   - Hardened Steel File 7+
Calcite Copper Penny 3 Topaz Spinel 8
- Coral,  Pearl 3-4   -    
Fluorite Malachite, Platinum 4 Corundum   9
Apatite Dioptase 5 Diamond   10
- Glass, Knife Blade 5.5        

Mohs scale is very useful but it is not linear. The minerals chosen were only selected because of their popularity. Number 10 on the scale (diamond) is 140 times harder then number 9 (corundum), whereas 4 (fluorite) is only 1.11 times harder than 3 (calcite). A proportional measurement, called absolute hardness, was recently devised, but is only used by scientists who need accurate results. The Mohs scale is the standard used by mineral collectors.

How to Perform the Test

  1. Select a fresh surface The scratch should not be done on a coated, chipped, or weathered surface for it will give inaccurate results. It also should not be done on a visible surface since a bad scratch on the face of a mineral can diminish its value.
  2. Making the swipe Testing is done by "swiping" one mineral with the other. The swipe should be strong enough to make a scratch, but not so much as to damage the specimen.  Hold the sample and attempt to scratch it with the point of the object of known hardness by pressing the object firmly but lightly against the unknown sample. If the know object is harder, you should see and feel a definite "bite" into the sample
  3. Inspect for an etched line When a mineral is scratched, a permanent indentation is created and powder of the softer mineral will come off. This powder must be brushed away to see if the mineral really got scratched, or if the powder of the softer mineral that was swiped across the specimen being tested created a scratch-like marking. When minerals of similar hardness are scratched together, it is difficult to tell which mineral (if not both of them) is really getting scratched because of this.

Absolute Hardness
The Mohs Hardness Scale is relative. Fluorite at 4 is not twice as hard as gypsum at 2; nor is the difference between calcite and fluorite similar to the difference between corundum and diamond. An absolute hardness scale looks a little different than the relative scale. Using a piece of sensitive equipment called a sclerometer, a comparison of the absolute hardness of minerals can be measured. Most minerals are close in hardness. But as hardness increases, the difference in hardness greatly increases as seen in this  absolute hardness scale:

Mineral Hardness Mineral Hardness
Talc 1 Orthoclase 72
Gypsum 3 Quartz 100
Calcite 9 Topaz 200
Fluorite 21 Corundum 400
Apatite 48 Diamond 1600

Using an absolute scale you can say that corundum is actually 4 times softer than diamond, not half as soft as Mohs relative scale leads you to believe.

Knoop and Vickers Hardness Scales
Microhardness testing is an indentation method for measuring the hardness of a material on a microscopic scale. A precision diamond indenter is impressed into the material at loads from 15 to 1000 gf. The impression length, measured microscopically, and the test load are used to calculate a hardness value.

The indentations are made using a square-based pyramid indenter (Vickers hardness scale) or an elongated, rhombohedral-shaped indenter (Knoop hardness scale). The hardness values obtained are useful as an indicator of materials properties and expected service behavior. For example, corundum has an Knoop absolute hardness of between 2000 and 2050. Generally speaking, any material will scratch another material that is lower on the scale. 

This information compiled from sources including www.minerals.net and www.galleries.com