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Crystal Structure is the fundamental building block of a mineral. It describes the simplest alignment of atoms needed to define the structure of a mineral. There are seven systems or structures that are further characterized by their axis length, the angles between their axis, and finally the number of symmetry centers contained within the group. A mineral's atomic structure actually is responsible for many of its physical properties. An understanding of the crystal structure and the particular elements used in the creation of a mineral provides a good deal of insight to its physical properties. Density (specific gravity), hardness, cleavage, fracture, toughness, and color are all strongly related to the structure and elements contained within a mineral. Density, the measure of a minerals mass to volume, is related to the specific element's that make up the mineral, and their individual atomic weights. High atomic weight elements mean higher density. A compact crystal structure will aid in driving the density up as a tighter packing pattern means more atoms per unit cell. Hence minerals containing lead, barium, or any elements in the 5th row of the periodic table will show much higher density due to their atomic numbers. Minerals comprised mainly of Si, O, Al, Na, K, Mg or other 2nd and 3rd row elements will tend to have average density values. Density is thus a function of elemental make-up and the crystal structure of the mineral. |
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Lanthanides * |
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Actinides ** |
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Hardness is the ability of a mineral to resist surface scratching. It is not related to toughness or the ability of a mineral to withstand a blow. Hardness is related to the crystal structure of a mineral, and the type of chemical bonding that is shared by the specific elements in the structure. Minerals are ranked in hardness from 1 to 10 (Moh's scale), but it is a very subjective scale. A harness of 2 does not necessarily indicate that it is twice as hard as mineral with a hardness of one. The hardness scale is defined using 10 minerals. The table shows the list, and provides the Moh's hardness (1-10) and the true (relative) hardness. It also provides some simple rules of thumb for everyday items. Each mineral on the table can scratch itself or the minerals below, but is unable to scratch anything higher on the list. |
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Hardness |
Equivalents |
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0.03 |
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1.25 |
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4.50 |
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5.00 |
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6.50 |
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37.00 |
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120.00 |
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175.00 |
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1,000.00 |
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140,000.00 |
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Typically there are two types of bonding possible in most chemical compounds, ionic bonding and covalent bonding. Let's examine the periodic table. Elements located at the far left tend to be metallic and tend to give up their electrons very easily; hence they form most ionic-based compounds. These are shown in the first two columns and are colored white. At the opposite side of the table is another column that contains the column of elements called halides. These elements are totally non-metallic in character, and will capture an electron and hold it fast. This right-hand column is also highlighted in white. When combined, the metals on the left and the halides on the right create a set of minerals with strong ionic bonding, and thus are low on the hardness scale. The minerals include NaCl (sodium chloride, mineral:halite), KCl (potassium chloride, mineral:sylvite), CaF2 (calcium flouride, mineral:fluorite), and Na3AlF6 (sodium alumino fluorite, mineral: cryolite). They also tend to easy cleavage or part, and many are soluble in water. Want to know more
about the periodic arrangement of the elements, then check
out these sites: |