Physical Properties of Minerals

  • Exhibition Text

    • Audio Transcript
      The physical properties of a mineral depend on the kind of atoms it is composed of and, more critically, the way these atoms fit together to form the mineral's crystal structure.
      Near the top of this case on the left, you will notice two models showing the atoms in the crystal structures of two minerals with actual specimens displayed above them. In both instances, the atoms involved are those of the element carbon. In the structure of the mineral diamond, on the left, the carbon atoms are spaced the same distance apart and are covalently bonded tightly together. This arrangement causes the great hardness of diamond and its inert chemical characteristics.

      In the other mineral, graphite, also made of carbon, the carbon atoms are quite strongly bonded, and closely packed, but only in a plane, as you can see in the model. As right angles to this plane, the atoms are weakly bound. Weakness along a plane allows graphite to be soft and easily split, or cleaved. This is why graphite pencil leads leave a mark on paper.

      The atomic structure of all minerals determines their physical properties, as illustrated by the large block of mica, and its well-developed cleavage. The minerals on the right in this case show important physical properties, which are useful not only in identifying minerals but may make them useful to us.

      In this series of 10 minerals at the top, each successive mineral is harder than the ones to its left. This series, first organized by the geologist Frederick Mohs, is known as the Mohs scale of hardness. The hardness of a mineral depends upon how strongly the atoms are bonded and packed within that mineral's crystal structure.

      However, the atoms forming the crystal structure of minerals are often less closely packed, or have weaker bonds, in some directions than in others. One can think of these directions of weaker bonding as planes of weakness along which the mineral will readily break when struck. This property is known as cleavage, and, in some cases, several directions of cleavage may occur in one mineral.

      The next group of specimens in this panel shows us minerals with different cleavages characteristic of their crystal structures. A more irregular type of breaking, called fracture, occurs in many minerals, having no clear orientation of weakness in bonding. This property can be seen in the next sequence of specimens.

      The way in which minerals deform is known as tenacity. For example, mica is flexible, copper is malleable, that is, it can be beaten into shapes, gypsum can be cut, or is sectile. These, and other similar properties, are displayed in the next group of specimens.

      The last sequence of minerals in this case portrays the varying density of minerals in terms of specific gravity, which represents the weight of a mineral compared with that of an equivalent volume of water. Two factors control the specific gravity of minerals. The first is the kind of atoms present in the mineral.

      Each atom has its own atomic weight. And if a mineral contains heavy atoms, such as those of lead, then one would logically expect the mineral to be relatively dense — that is, to have a high specific gravity. The second, and perhaps even more important, factor, in some instances, is the closeness of atomic packing within the mineral's crystal structure.

      As we saw in the display of graphite and diamond, both are made of carbon. But diamond, with more closely packed atoms, is considerably harder and has a much higher specific gravity than graphite.

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Models of Diamond and Graphite Atoms

Models of Diamond and Graphite Atoms

Both graphite and diamond are made of carbon. But diamond, with more closely packed atoms, is considerably harder and has a much higher specific gravity than graphite.