Halide Mineral Fluorite

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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Vein containing pitchblende (uranium ore) in Colorado, USA
Photographed by Jonathan Caine, U.S. Geological Survey

About Hydrothermal Veins

Deep underground, magma (molten rock) heats water, which is less dense than cold water, so it tends to rise toward the surface. The hot water can carry dissolved minerals into fissures and cracks in the Earth's crust. When minerals are dissolved in a liquid, scientists say that the minerals are in solution. Eventually, the water cools and the materials in the water crystallize out of solution in their new location, and the resulting minerals look different from the surrounding rocks. Hydrothermal comes from the Greek words for water and heat; these structures are called hydrothermal veins because they resemble the blood vessels in the bodies of animals. Many ores of economically important metals, such as lead, zinc, copper, and gold, are found in these veins.

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Foliated mica (variety: muscovite) with an embedded hexagonal crystal of quartz
Photo courtesy of iRocks.com/The 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).

Fluorite, a source of fluoride for toothpaste
Photo by Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution

Minerals for Human Consumption

Have you eaten any minerals lately? Almost certainly! Halite is the formal name of the mineral we use as table salt. Humans use salt not just to season food, but also to cure (preserve) meats. Clay minerals, part of the silicate group, serve as mild abrasives in toothpaste, while the fluoride comes from the mineral fluorite. Finely ground silicon dioxide is an anti-caking agent in many powdered foods such as gravy mixes and non-dairy coffee "creamer." The mineral trona is the primary source of sodium carbonate, which helps baked goods rise and gives toothpaste that foamy feeling in your mouth. Calcium sulfate from the mineral gypsum coagulates (or solidifies) tofu, a soybean-based food. Many of the plants we eat depend on mineral-based fertilizers for robust growth.