Phyllosilicate Mineral Phlogopite

Associated Smithsonian Expert: Jeffrey E. Post, Ph.D.

Jeffrey Post

Photograph by Cara Santelli, Smithsonian Institution

Dr. Jeffrey Post is the curator of the National Gem and Mineral Collection at the Smithsonian National Museum of Natural History. As far back as he can remember in childhood, Post collected rocks and fossils around his home near Madison, Wis. The symmetry of mineral crystals fascinated him, and experiments with a large chemistry set helped develop his interest in science. He earned a Ph.D. from Arizona State University in 1981 and joined the Smithsonian in 1984. Post’s research projects include the physical and chemical properties of fine-grained, environmentally significant minerals such as clays, manganese oxides, and iron oxides. He also uses powerful X-ray beams at the National Synchrotron Light Source at Brookhaven National Laboratory (Upton, N.Y.) to study the crystal structures of these minerals. With his Smithsonian colleagues, Post is always seeking new gem and mineral acquisitions for the Smithsonian. He analyzes specimens to resolve curatorial questions, oversees loans of Smithsonian gems to other museums, supervises the team that is building the collection website, meets with donors, and answers public inquiries about the Smithsonian mineral collection.

Meet our associated expert

This image was obtained from the Smithsonian Institution. The image or its contents may be protected by international copyright laws.

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Artist interpretation of the Laramide Orogeny, which led to the creation of the Rocky Mountains in the Western United States.
Image created by Karen Carr Studio, Inc., used with permission

About Silicate Minerals

Silicon and oxygen are two of the most common elements on Earth. Together, they make up nearly 75 percent of the Earth's crust, so it is no surprise that they play major roles in forming many of the minerals that we see in rocks. The silicon atom, which has four electrons in its outermost region, likes to form chemical bonds with oxygen atoms, which are attracted to extra electrons. Thus, silicon and oxygen, together with some of the metallic elements, can combine to make hundreds of different minerals. For example, quartz has two oxygen atoms for every silicon atom, and feldspar has two or three silicon atoms grouped with eight oxygen atoms and a few metal atoms. About half of the most common minerals found on Earth belong to the silicate group, as do some beautiful gemstones such as amethyst, opal, and topaz.

Foliated mica (variety: muscovite) with an embedded hexagonal crystal of quartz
Photo courtesy of Arkenstone

Crystal Shapes and Crystal Habits

Inside a mineral, atoms arrange themselves into a specific, repeating pattern called a crystal lattice or crystal structure. The smallest three-dimensional arrangement within the lattice is called a "unit cell," which is duplicated over and over again symmetrically. At the level of the everyday world, minerals that are growing without outside interference tend to form crystals that resemble their underlying crystal structures. Scientists call that kind of general, typical appearance a "crystal habit." Of course, conditions that existed during a mineral's formation or crystal growth may change its habit, but geologists still find this attribute to be a useful tool for identifying minerals. Scientists use more than three dozen adjectives to describe crystal habits. For example, natrolite and rutile can be acicular, or needlelike; quartz often forms hexagonal prisms; pyrite and halite typically crystallize as cubes; and mica is foliated or lamellar (layered).