Galena

Did you know that Americans use 11 pounds of lead per person each year, only some of which comes from recycling? Although lead is the most reclaimed industrial metal in the world, we still need to mine more, in the form of galena (lead sulfide), to keep up with our demand.

Lead is one of the first metals to have been used by humans, dating back to about 6500 BC. Today, the United States is the world's leading producer of lead, and galena (the primary ore of lead and silver) has been mined here in huge quantities since the 19th century, when the need for extensive plumbing arose from the influx of people to cities.

Galena has been such an important mineral for miners that they have named other minerals based on their resemblance. A similar metallic black mineral that is the primary ore of zinc was dubbed sphalerite by miners, which is Greek for treacherous rock, and is also called blende, which is German for blind or deceiving - because it was much less valuable than galena!

Unfortunately, it was later determined that lead, while easy to work and inexpensive, came with a hidden high price -- it was poisonous. Luckily (for rockhounds), in its sulfide mineral form (galena), lead is not harmful to handle.

We still use lead for anti-knock gasoline additives, car batteries, protection from radioactivity, solder, paints, varnishes, and glazes -- but no longer for plumbing or house paint. Galena is also a popular mineral for collectors both young and old, because it has some very interesting properties:

• a striking, silver-grey color, sometimes with a bluish tint
• a bright metallic luster on broken faces that dulls with weathering (due to the oxidation of the lead within)
• crystal habit that is most often cubical, octahedral, or a combination
• cleavage in four directions, forming perfect cubes
• hardness of 2.5 Mohs (about as hard as your fingernail)
• a specific gravity of 7.4 - 7.6 -- one cubic foot of galena weighs about 700 pounds!

Galena is, by far, the heaviest mineral that most of us will ever have the opportunity to hold a large chunk of. Only gold, iridium, platinum, and tungsten are heavier.

Specimens of galena are often found with gorgeous clusters of long, thin quartz crystals and glittering golden pyrite. If you would like to experience this fascinating mineral first-hand, we have several of these display pieces available in our One-of-a-Kind Rocks and Minerals category, at surprisingly affordable prices!

Petrified Wood and How It's Formed

In our holiday issue of the Stone Tabloid (our print newsletter), we began a discussion of the process of petrifaction -- how minerals replace the cells of a log or branch that is buried in mud very shortly after it dies. If you missed the first part of the story, download our newsletter or visit our LEARN MORE section to catch up. Here's the rest of the process:

As the wood itself disintegrates over many thousands of years, the second step of the process, known as replacement, occurs as minerals take the place of the original tissues, sometimes molecule by molecule. Because some portions of the wood -- such as the layers of the annual rings that grew in the spring -- are more vulnerable to decay than harder portions, they tend to be replaced by minerals first. This results in the preservation in stone of the distinctive concentric ring pattern you see in a freshly cut stump.

The final composition of petrified wood -- some form of quartz or calcite, commonly -- will depend on what minerals are in the water. A silica solution leads to an often vividly colorful agate, jasper, and/or opal specimen, as these are all varieties of quartz. This is generally the result of the original wood having been buried in mud containing volcanic ash. Excellent examples of this kind of petrified wood are found in the Petrified Forest State Park in Arizona. The different colors are the result of trace minerals in the solution. Copper, cobalt, and chromium give a green or blue color; iron creates red, brown, and yellow; and carbon makes the rock black. A calcified specimen will probably be white or gray when first exposed to the air, and results from a calcium carbonate solution. Sometimes a particular specimen will combine a quartz mineral, such as agate, with calcite, indicating that both silica and calcium carbonate were present in the groundwater solution, though perhaps not at the same time.

When a piece of wood rots away completely within its mud casing before petrifaction can take place, it leaves a mold behind. If this mold is then filled with crystallized minerals in a large-scale version of permineralization, a cast is created. The cast may preserve all the exterior details of bark and branch stubs but will show no annual rings or cellular structure. Paleontologists can still learn much about a particular tree's environment -- insect damage and the like -- from such casts. They also often make dramatic display specimens.

Curious about how other types of fossils form? Our About Fossils section has 5 more articles for you!

Visit our One-of-a-Kind Fossils and Meteorites category for photos of unique specimens. Our Fossils category contains fossil kits and inexpensive specimens. And wouldn't our "Trilobite Me" t-shirt make the perfect gift for someone you know?

Collecting Micrometeorites

This makes an excellent family project or science fair idea!

Meteorites can be either rock, metal (nickel and iron), or a mixture of both. Stony meteorites are difficult to identify and outnumber metals, but metallic meteorites are easier to find. Rarely are chunks of metal found lying about. A metal detector can be used to search for metallic meteorites. Dry barren areas where there is little vegetation to cover up the ground and turn over the soil are the best areas to look. Dry lake beds are good places to search since wind can blow dust off of the surface leaving the meteorites exposed. Many meteorites are found on the Antarctic ice sheet.

There is an easy way to collect meteorites, but we must be satisfied with finding small metal ones. They are actually microscopic and are known as micrometeorites. Tons of these fall on Earth each day. To collect micrometeorites you need to find a place where they can become concentrated. The drains of a house or building work well since rainwater can wash particles off of an entire roof and collect them at the drain spout. Tile roofs are best since they drain very well and do not produce many other sorts of particles or debris.

To find the metallic micrometeorites, collect and dry some of the material from a deep bowl at the base of the drain spout. After removing leaves and other large debris, dump the remaining material on a piece of paper and place a magnet under the paper. Tilt and tap the paper so that all of the non-metallic particles fall off. The remaining metallic particles are likely pieces of space dust! To examine them, place the paper under a microscope. High power will be required to see them clearly. The micrometeorites will show signs of their fiery trip through the atmosphere -- they will be rounded and may have small pits on their surfaces.

Much of what you've collected are particles that date from the formation of the solar system around 4.6 billion years ago! They are the debris remaining from the raw materials that formed into the nine known planets and the asteroids. Most particles have been broken off or ground down from larger objects.

After you've finished this project, if you'd like to see a substantial piece of a meteorite (one that wouldn't fall unnoticed on your roof, but might end up in your living room!), visit our One-of-a-Kind Fossils and Meteorites section. We carry both iron and stony meteorites from a variety of locales in all price ranges. Meteorites make great gifts for curious kids who love to look at the stars!

Geology Word Puzzler

What is the common name for the cryptocrystalline quartz that fills fissures, seams and cavities in rocks, replaces wood fibers in petrified wood, is found at the center of geodes, and may be red, white, blue, grey, green, brown, purple, pink, black or clear?

Chalcedony: (kal-SEHd-nee) a translucent grey or milky-colored quartz having a crystalline structure and a waxy luster (Wordsmyth.net). Of uncertain origin -- from the Greek khalkedon, a mystical stone, perhaps originating with Chalcedon, a port in Turkey where the first deposits were found.

While the dictionary definition makes this seem fairly simple, we actually use the term chalcedony for a huge range of minerals with the same chemical composition as quartz, but with more impurities. The impurities result in a wide range of colors -- iron oxides result in a clear red chalcedony is known as carnelian or sard; a green variety colored by nickel oxide is called chrysoprase; a bright to emerald-green chalcedony with drops of red jasper is known as bloodstone or heliotrope; when layered, we call it onyx. Chalcedony also may be used as a catch-all synonym for agate or jasper.

We'd love to hear from you! Have questions or comments about the new site, or just want to tell us about your latest rockhounding adventure? Email us at we_rock@mamasminerals.com!

Read about rose quartz, geodes, and identification of rocks and minerals in the Fall 2006 issue of RockZ NewZ

Read about rose quartz, geodes, and identification of rocks and minerals in the Summer 2006 issue of RockZ NewZ

Read about fluorite, fluorescent minerals, and cleaning quartz in the Summer 2005 issue of RockZ NewZ.

Read about vanadinite, building fountains, and identifying meteorites in the Winter 2004 issue of RockZ NewZ.

Read about celestite, pearls, and how to choose the right tumbler in the Fall 2004 issue of RockZ NewZ.

Read about malachite, tumbling grits, and how to pan for gold in the Summer 2004 issue of RockZ NewZ.

Read about amethyst, fossil preparation, and field tools for the rockhound in the Spring 2004 issue of RockZ NewZ.

Read about iron pyrite, tumbling seaglass, and stony meteorites in the last issue of RockZ NewZ, Summer 2003.

Sign up to get each new issue delivered to your inbox!