Answer to Question 1
Answer: Diamond and graphite consist only of carbon atoms, but their physical properties differ. Carbon atoms in diamond are closely spaced and share strong covalent bonds in all directions; this configuration produces the hardest mineral on Earth. The smooth crystal faces defining the external form of diamond coincide with planes of carbon atoms in the crystal structure. In graphite, however, carbon atoms are more widely spaced than in diamond and are strongly bonded only in two dimensions. The covalently bonded carbon sheets are weakly held together by van der Waals forces. The weakly linked sheets readily separate in graphite, accounting for its softness and its tendency to cleave readily into thin, scaly plates.
Answer to Question 2
Answer: Only naturally formed substances are minerals; manufactured materials, such as synthetic gemstones (e.g., cubic zirconia), are not.
A mineral must be a solid with an orderly arrangement of atoms; it cannot be a gas or liquid. Opal, a popular gemstone, consists of silicon and oxygen, the same elements that compose quartz. Opal, however, lacks a highly ordered atomic structure and is not considered a mineral.
Organic compounds, those defined as containing mostly carbon and hydrogen atoms, are not minerals. Table sugar (C12H22O11), therefore, is not a mineral, even though it is a naturally occurring solid with an orderly atomic structure.
The chemical composition of a particular type of mineral can vary slightly (as in the colored varieties of quartz illustrated in Figure 2.10), but the principal constituents are common to all specimens.
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