Determining the Hardness of a MineralLast Updated: 2nd Jan 2018
By Donald B Peck
Mineral hardness is defined as the relative ability of a mineral to resist scratching or abrasion. The first attempt to quantify the hardness of a mineral for which there is any record was made in 1812 by Friedrich Mohs, a German geologist and mineralogist. He chose 10 relatively common minerals that ranged in hardness from the softest known mineral, talc, to the hardest, diamond. Metallurgists use a penetration hardness (Vickers, Knoop, etc.) that is obtained by pressing a diamond point into a flat surface under a known load and measuring the area of indentation. Professional papers in mineralogy often report Vickers hardness applied to minerals, in which case it is considered to be the hardness of crystal deformation. The Mohs Scale, however, is used by amateur and professional mineralogists,geologists, and collectors. The Mohs Scale and its application to testing mineral properties is the subject of this article.
Mohs invented an ordinal scale, one through ten, with each numeral defined by the hardness of a specified mineral species. Although they are good approximations, the absolute differences in hardness between ordinal values are not equal. When compared to the Knoop Scale, each successive Index Mineral is 1.2 to 2.7 times harder than the previous one. The single major exception is between the hardness of corundum and diamond. Diamond is almost five times as hard as corundum.
The Mohs Scale of Mineral Hardness
No Intermediate Values
The Mohs Scale is an ordinal scale. Therefore, there are no intermediate values. That said, you will often see values like 3½, or 5½. Such designations do not mean that the hardness is halfway between 3 and 4 or 5 and 6. Instead, the collector or mineralogist is saying that the hardness is greater than 3 but less than 4; or similarly, greater than 5 but less than 6. You should not attempt to state any finer measurement, as it is meaningless. It is a fine point, but a hardness between 8 and 9 should be written as 8½ and not as 8.5. The decimal fraction implies a continuous range rather than the discrete ordinal values.
Tools You Will Need
A hardness set of index minerals can be purchased, but most are so common that you can build your own set. Ideally, each piece should be approximately 2 x 2 x 3 cm in size. Cleavage faces are ideal to scratch, corners are good to produce scratches so cleavage blocks are excellent when possible. When they are not, choose a crystal. Only the first nine index minerals are necessary for you know that a diamond will scratch all other minerals. A small box divided into nine compartments provides useful storage.
A set of pencil-like holders with sharp tips, each with one of the Mohs minerals, can be purchased. They are excellent for test scratching an unknown but, as will be discussed later, it is necessary also to discover whether the unknown mineral can scratch the index mineral. That is not possible with some sets of points. MineralLabs set of hardness points and test surfaces permit the complete protocol. The points' mounts are steel pencil-type holders and a carborundum sharpening stone is supplied for re-sharpening of the points as required. None of the points are mineral. Gypsum is replaced by a plastic of the same hardness. Calcite by copper. 4 - 9 are all steel alloys of the correct hardness to equal the minerals they replace. The points are useful, particularly with small specimens.
For close approximations a pocket knife (H=5 to 5½), a length of copper wire (H=3), a shard of quartz (H=7), a small piece of copper sheet metal (H=3), a square if window glass (H=6½), and a bright steel fender washer (H=5) will do. Another that you always have with you is your fingernail (H=2 to 2½). Use of them prior to using the points or hardness set saves wear and tear on the latter. The disadvantage is that in refining your estimate, turning to a hardness set requires making a second scratch.
Making & Observing a Scratch
When choosing a place to make a scratch on your recently acquired, valuable specimen, choose a fairly smooth but inconspicuous surface, preferably on the back or bottom of the piece. You do not want to mar a great crystal face with an ugly scar.
If you have no idea as to what the hardness might be, start in the middle . . . try 5. This is where a pocket knife, a small length of copper wire, etc., is handy. They allow you to find the approximate value without eroding your better tools.
In making the scratch, draw the point for only about 3 mm. And use a magnifier. A 3 mm scratch is just as easy to see as a 3 cm scratch. At first, use light pressure but if that produces no effect, increase to a firm pressure. After the "scratch" is made, wipe it with your finger or a cotton swab to make certain that the mark is in fact a scratch that incises the surface, and is not merely a chalky mark on it. If possible, draw your fingernail across the scratch to discover whether it is an incised scratch or merely a residual mark.
When using styli (points), hold the styli at approximately a 45o to 60o angle to the mineral surface and draw it towards yourself.
If a point on apatite (H=5) does not scratch your specimen, try feldspar (H=6). If the feldspar does not scratch your sample, try quartz (H=7). If the quartz produces a scratch, then It is important to try to scratch the quartz with an inconspicuous point on your specimen.
While the hardness of most minerals is very nearly the same in all directions, small differences do exist. Thus, if your specimen permits, without defacing it, try scratches in different directions (lengthwise of the crystal and crosswise). The mineral best known for differential hardness is kyanite. Its hardness parallel to the length of the crystal is 5½ while perpendicular to the length the hardness is 7. With diamonds, the octahedral surface is the hardest and without differences in directional hardness a diamond could not be cut.
Interpreting the Results
Let us say that your unknown mineral specimen was not scratched by feldspar (H=6), was scratched by quartz (H=7), and did itself scratch quartz. Then the unknown must have a hardness equal to that of quartz; or H=7.
If your unknown specimen was not scratched by feldspar (H=6), was scratched by quartz (H=7), and did not itself scratch quartz. Then its hardness must be less than quartz but greater than feldspar ( 6 < H < 7). This value is expressed often as 6½, meaning "between" 6 and 7.
If the index scratches the unknown, does the unknown scratch the index? It is important to test the scratching both ways. This is the only way you can determine whether the hardness of the unknown is equal to, or less than, the index mineral that has the greater hardness.
Hardness is a function of bonding strength between atoms and/or ions. While the bond strength between atoms in a molecule (e.g. between hydrogen and oxygen in water) is essentially constant, bond strength between ions (e.g. Fe2+ and (CO3)2-) varies depending upon electrostatic charge on the ions, distance between them, and packing pattern. Since the distance between planes of ions is different in different directions, so is the bond strength. The plane with maximum hardness is the plane with the highest point density. That is the plane with the greatest number of ions in the least area. For diamonds, the greatest point density and the plane with the maximum hardness is the octahedral plane.
In general, smaller ions produce harder minerals. The cations (positively charged metal ions) of the carbonate minerals calcite Ca2+, magnesite Mg2+, siderite Fe2+, and rhodochrocite Mn2+ all have the same packing pattern, or crystal structure; and the same electrostatic charge. They are nearly the same size with the exception of the calcium ion, which is significantly larger. Calcite has a hardness of 3 while magnesite, siderite and rhodochrosite have a hardness of 4.
The carbonates calcite and nitratine have the same packing pattern (crystal structure) and about the same size ions. However, the charge on the calcium ion in calcite is twice the charge on the sodium ion in nitratine. This makes the electrostatic attraction between the calcium and carbonate ions in calcite stronger than the attraction between sodium and carbonate ions in nitratine. The hardenss of calcite, as we have seen, is 3. The hardness of nitratine is 1½ or 2.
Closer packing of ions in the crystal structure produces a greater hardness. Aragonite and calcite are both calcium carbonate, CaCO3. Calcite crystallizes in the trigonal crystal system, aragonite in the orthorhombic system; and, the ions are closer packed for aragonite than for calcite . The hardness of calcite, as you know, is 3. That of aragonite is 3½ or 4.
Minerals with covalent bonds between atoms are generally harder than those with ionic bonds. Diamond has covalent bonds between carbon atoms. Native copper has ionic bonds between copper ions. Both crystallize in the isometric (cubic) system. While the copper ion is considerably larger than the carbon atom, the strength of the covalent bond between carbon atoms is hugely greater than that of the electrostatic bonds between copper ions.
Mason, Brian and Berry, L.G. (1968) Elements of Mineralogy. W. H. Freeman and Company, San Francisco.
Peck, Donald B (2007) Mineral Identification: A Practical Guide for the Amateur Mineralogist. The Mineralogical Record, Tucson Arizona.
Pough, Frederick H, (1996) A Field Guide to Rocks and Minerals. Houghton Mifflin Company, Boston.
Website of the Mineralogical Society of America, http://www.minsocam.org/msa/collectors_corner/id/mineral_id_keyi1.htm , Mineral Identification Key II. Plante, Alan; Peck, Donald; Von Bargen, David.
The photo, "Mohs Hardness Set" is by courtesy of Wikpedia and Wikimedia Commons. Copyright by Hannes Grobe and shown under Creative Commons Attribution 3.0 Unported.
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Well done Don, I think this article will help and encourage all the posters looking to ID minerals add some hardness info.
27th Dec 2017 3:53pm
27th Dec 2017 3:53pm
Very nice, Don. Articles like this are much needed on Mindat for folks who visit and want to identify their specimen.
6th Jan 2018 3:59pm
6th Jan 2018 3:59pm
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