Lunar Minerals

The lunar rocks described in the next chapter are unique to the Moon. Their special characteristics— especially the complete lack of water, the common presence of metallic iron, and the ratios of certain trace chemical elements—make it easy to distinguish them from terrestrial rocks. However, the minerals that make up lunar rocks are (with a few notable exceptions) minerals that are also found on Earth. Both lunar and terrestrial rocks are made up of minerals. A mineral is defined as a solid chemical compound that (1) occurs naturally; (2) has a definite chemical composition that varies either not at all or within a specific range; (3) has a definite ordered arrangement of atoms; and (4) can be mechanically separated from the other minerals in the rock. Glasses are solids that may have compositions similar to minerals, but they lack the ordered internal arrangement of atoms. Minerals have provided the keys to understanding lunar rocks because their compositions and atomic structures reflect the physical and chemical conditions under which the rocks formed. Analyses of lunar minerals, combined with the results of laboratory experiments and studies of terrestrial rocks, have enabled scientists to determine key parameters—temperature, pressure, cooling rate, and the partial pressures of such gases as oxygen, sulfur, and carbon monoxide—that existed during formation of the lunar rocks. The fact that minerals are mechanically separable from the rest of the rock is also a critical characteristic in considering the economic extraction of resources from lunar materials. For terrestrial resources, mechanical separation without further processing is rarely adequate to concentrate a potential resource to high value (placer gold deposits are a well-known exception). However, such separation is an essential initial step in concentrating many economic materials and, as described later (Chapter 11), mechanical separation could be important in obtaining lunar resources as well. A mineral may have a specific, virtually unvarying composition (e.g., quartz, SiO2), or the composition may vary in a regular manner between two or more endmember components. Most lunar and terrestrial minerals are of the latter type. An example is olivine, a mineral whose composition varies between the compounds Mg2SiO4 and Fe2SiO4. The intermediate members, produced by the variable substitution of Mg and Fe, are called solid solutions and can be represented by the formula (Mg,Fe)2SiO4, a notation indicating that the elements inside the parentheses may substitute for each other. The crystal structure of a mineral reflects the regular and ordered arrangement of atoms within it. Within this structure, positively charged cations (generally metals such as Si, Al, Mg, Ti, and Fe) are linked into complicated geometric networks with negatively charged anions (chiefly O). Within the crystal structure, each cation is shared (coordinated) with several anions. Coordination with four anions produces an arrangement of anions surrounding a cation in tetrahedral or fourfold coordination, while coordination with six anions is called octahedral or sixfold coordination. These tetrahedral and octahedral units combine to form 5