Mineral Hardness No. 109
November 1949

TOS home

This unit includes:

The specimens are samples of the first nine index minerals of the ten-step Mohs' scale of mineral hardness (the tenth is diamond). Step 6 is properly orthoclase feldspar (potassium aluminum silicate, KAlSi3O8); the feldspars also include many other aluminum silicate minerals, with Mohs' hardness between 6 and 6.5.

The booklet includes a common hardness test (experiment 3), using a copper penny to help identify calcite. In 1812, when Frederich Mohs devised his hardness scale, copper pennies in Britain and the US were made of pure copper (hardness 3.0), as were German pfennigs. As Andrew Alden has demonstrated, the hardness of the US penny has varied between 2.5 and 3.5 depending on its composition, which has changed several times in the past century. The pennies current in 1949 were bronze and do not scratch calcite easily, but are readily scratched by calcite, and current copper-clad zinc pennies are similar in hardness to 1949 pennies. Alden concludes that most US pennies in circulation should be considered to be of hardness 2.5.

My 10-year old daughter and I spent an enjoyable afternoon identifying the specimens shown here. We changed our minds several times about the identities of the quartz and topaz. Using only the methods described in the booklet, it is difficult to tell the difference between these two minerals of similar hardness and appearance in specimens of this size (about a centimeter across).

TOS home