Читать книгу Earth Materials - John O'Brien - Страница 107

In any crystal, the three crystallographic axes have a characteristic axial ratio, typically grounded in the cell edge lengths of the unit cell. No matter how large the mineral becomes during growth, even if it experiences preferred growth in a particular direction or inhibited growth in another, the axial ratio remains constant and corresponds to the axial ratio implied by the properties of the unit cell.

In the growth of any mineral one can imagine the development of a crystal face (or any plane parallel to it) that intersects the positive ends of all three axes at lengths that correspond to the axial ratio of the mineral (Figure 4.19). For crystals with a center (i), such faces would intersect each axis at a distance from the center of the crystal that corresponds to the axial ratio. For the monoclinic (pseudo‐orthorhombic) mineral staurolite discussed in the previous section, such a face could cut the a‐axis at 0.47 mm, the b‐axis at 1.00 mm, and the c‐axis at 0.34 mm from the center, or, for a larger crystal, it could cut the three axes at 0.47, 1.00, and 0.34 cm (or 0.235, 0.50, and 0.17 cm) from the center. All these faces would be parallel to one another. Any face or plane that intersects all three axes at distances from the center that correspond to the axial ratio of the mineral is a unit face or unit plane. It is part of a set of parallel planes all of which are unit planes because they intersect the three crystallographic axes at lengths that correspond to the axial ratios.

Figure 4.19 Unit face (outlined in solid blue) in an orthorhombic crystal with three unequal unit cell edges and crystallographic axes that intersect at right angles. All parallel faces (e.g., outlined in dotted red) will have the same general relationship to the crystallographic axes and the same atomic content and properties.