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In amorphous substances, gases, and liquids, as well as in minerals crystallising in the cubic system, hght entering from the outside is refracted equally in all directions. In all other minerals the refracted light is broken into two rays vibrating at right angles and travelling at different velocities. These minerals are said to be doubly refracting or birefringent. Double refraction can be particularly well demonstrated with a rhombohedral cleavage-fragment of the clear variety of calcite called Iceland spar. In the case of most minerals, however, the property of double refraction can only be observed with optical instruments. A mineral can be identified by the extent of its double refraction, the direction of the two refracted rays within the crystal, as well as by its other optical properties. It is not surprising, therefore, that the examination of thin sections of transparent minerals and polished sections of opaque ore minerals with the petrographic microscope is one of the most important aspects of the science of mineralogy.

Double refraction

The hardness of minerals has long been used as a means of identification. The scale of hardness in general use is known as Mohs’s Scale, which has ten grades. The minerals used as standards for this scale are as follows: i, talc; 2, gypsum; 3, calcite; 4, fluorspar; 5, apatite; 6, feldspar; 7, quartz; 8, topaz; 9, corundum; and 10, diamond. Any one mineral in this scale will scratch all the minerals which precede it, and will itself be scratched by all the succeeding minerals. Minerals of hardness 1 can be scratched with the fingernail; they usually also have a soapy feel like talc or graphite. Minerals of up to hardness 5 can be scratched with the blade of a pocket-knife, and those of hardness 7 and over can scratch window glass. It should be noted that the difference in hardness between successive grades is by no means uniform, and that that between the last two grades, corundum and diamond, is particularly great.

When minerals are cut and polished into gems or ornaments, only the harder ones resist abrasion in everyday use and escape the small scratches from minute quartz particles in the atmosphere. For this reason it has been customary to class minerals with a hardness above 7 (i.e. those which cannot be scratched by quartz) as precious stones. In certain minerals the hardness of the crystal varies according to the direction in which its face is scratched. Kyanite, for example, whose hardness ranges from 4- 5 to 7, can be scratched by a penknife in only one direction. Aggregates made up of more than one crystal usually appear to have a lower degree of hardness than a single crystal of the same mineral. It is, for instance, easy to scratch most sandstones with a knife, in spite of the fact that they consist essentially of quartz grains whose hardness is 7.

The streak of a mineral is the colour of its powder as seen on a white surface. It is often quite different from that of a larger piece of the mineral, and can be a diagnostic feature used in mineral identification. Pyrite crystals, for instance, are yellow, but the powder of pyrite is greenish-black. The streak of the black minerals, blende, limonite and haematite, is respectively leather-brown, yellowish-brown and blood-red. Many mineral identification tables are based on the colour of streak, as this provides a simple means of distinguishing between many superficially similar minerals. Streak is readily observed by rubbing the mineral on a tablet of unglazed porcelain.

Streak of a mineral