minerals

All minerals are rocks, but not all rocks are minerals.

Weathering

Weathering is when rocks break down in place, that is, without moving the rock. This is usually done by water, but there are plenty of other physical and chemical processes that break down rocks without moving them. Physical weathering occurs when a tree root grows into a rock and breaks through, when a river cuts through a canyon, when particles carried by the wind abrade the rock, or during the process of frost wedging, which is when water fills a crack in a rock and freezes, then the ice expands and makes the crack deeper. Chemical weathering can be caused by acid rain or even regular rain, as minerals in the water weaken the rocks and make it easier for them to be eroded or broken later. Minerals can even react with chemicals in the air (such as iron and oxygen reacting to form rust, also known as iron oxide) or with other minerals nearby. Minerals are made of chemicals, after all, and there is nothing stopping them from reacting with one another.

There are a lot of interesting ways that minerals can change due to weathering, both physical and chemical. For example:

  • Limestone dissolves
  • Calcite dissolves
  • Gold may dissolve if manganese is present
  • Silver minerals can change to horn silver (cerargyrite) or dissolve
  • Feldspar changes to clay
  • Olivine and hornblende change to serpentine or chlorite
  • Pyrite changes to limonite and hematite
  • Rhodochrosite and rhodonite change to psilomelane or pyrolusite (manganese) minerals
  • Copper sulfide minerals change to malachite, azurite, cuprite, or metallic copper, or may dissolve entirely
  • Some copper minerals become partly limonite

Adapted from an article in Cycad, Flint Chips, Osage Hills Gems 11/1992

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Dyed Agates

Have you ever seen dyed agates and wondered how they get such brilliant colors? The process is more simple than you might think. A company buys banded agates in bulk and soaks the slabs in certain chemicals for a certain amount of time (in a fume hood of course). Heat treating may also be required.

But how do they preserve the white stripes that make the agates look like agates? This is simply because some bands are porous and will absorb the dye, but the denser layers and the quartz crystals will remain white because they are too dense to absorb the dye.

(Note: To discourage you from trying this at home, I’m not going to specify the concentrations.)

Red: Iron (II) nitrate for several weeks folllowed by heating to 300° C
Green: Potassium chromate followed by ammonium carbonate plus heating to 440° C
Blue: Potassium ferrocyanide followed by ferrous sulfate
Black: soak in sugar for 3 weeks followed by sulfuric acid for 3 weeks

Pyrope Garnet

This is a special type of garnet called pyrope garnet. The name comes from the Greek pyro, meaning fire. Pyrope and other members of the aluminum part of the garnet group have a higher specific gravity and hardness, and are usually red. Calcium garnets like the previously mentioned andradite and uvarovite are the ones that are usually green and have a lower hardness and specific gravity.

Pyrope garnet is difficult to distinguish from almandine, but pyrope usually has fewer flaws and inclusions. However, garnet jewelry is usually almandine garnet because almandine is much more common and inexpensive.

If you would like some pyrope garnet it can be found nearby in Kansas, all around the Nemaha Uplift (or Nemaha Ridge), which is in the area between Salina and Manhattan, and extending south into Oklahoma. Basically, garnets are found anywhere near previous volcanic activity. The one pictured is from Apache County, Arizona. They are also found in Africa and other places. For lots and lots of information about this particular specimen, see its page on the RRUFF here.

Andradite Garnet

andradite garnet

Credit: Aaron Palke/Gemological Institute of America

Since it’s January, it’s a good time to read about this garnet originally posted by Chemistry in Pictures.

“This gemstone isn’t pure andradite garnet [Ca₃Fe₂(SiO₄)₃], but its flaws produce its mesmerizing colors. Some gemologists think that this rainbow explosion arises because the garnet’s different elements aren’t regularly spaced from the core of the gemstone to the outside. For example, in some regions, aluminum atoms might have worked their way into the structure and replaced the iron atoms. These irregularities create mismatched sheets of atoms that then bend and stretch. This makes the stone birefringent, meaning that light travels through it at two different speeds. Under cross-polarized lighting conditions, rays of light that enter get misaligned by the time they exit, so they then interfere with each other and highlight some colors in certain spots, producing the spectrum seen here. The black flecks are tiny pieces of magnetite that were enveloped by the crystal as it grew.”

A Great Geology Book

Rocks and Fossils of the Central United States with Special Emphasis on the Greater Kansas City Area by Richard Gentile

The front cover of Richard Gentile's book, Rocks and Fossils of the Central United States with Special Emphasis on the Greater Kansas City Area

Front cover

The back cover of Richard Gentile's book, Rocks and Fossils of the Central United States with Special Emphasis on the Greater Kansas City Area

Back cover

Review by David Reed:

This book is great! It has beautiful pictures of the fossils that can be found in Kansas City and clear stratographic sections explaining the geology of the area. It also shows locations for picking up the fossils. Everything you might wish to know about Kansas City is in this book. Well worth the money and you can ONLY get it at UMKC (Amazon doesn’t have it). We purchased one when we visited the Sutton Museum.

Anthracite

Q: What makes rockhounds different from non-rockhounds?

A: They are happy to receive coal for Christmas.

Rockhounds love coal, and they love anthracite even more. Anthracite is a type of coal. It is very hard and burns slowly and cleanly due to its high carbon content and few impurities. It is rarer than bituminous coal (the soft, most common form of coal); in fact, less than 2% of the coal in the United States is anthracite. Also unlike bituminous coal, anthracite won’t leave soot on your fingers when you touch it.

There are four types of coal in all. The last two we haven’t covered yet are lignite coal and subbituminous coal, which have the lowest carbon content and are even softer then bituminous coal. Anthracite is the hardest and has the highest carbon content. Most of the coal in the United States is found in Colorado and Illinois, and is used primarily for making electricity and coke (coke is used by foundries to make iron and steel).

Missouri’s State Mineral

On July 21, 1967, the mineral galena was adopted as the official mineral of Missouri. Galena is the major source of lead ore, and the recognition of this mineral by the state legislature was to emphasize Missouri’s status as the nation’s top producer of lead. Galena is dark gray in color and breaks into small cubes. Mining of galena has flourished in the Joplin-Granby area of southwest Missouri, and rich deposits have been located in such places as Crawford, Washington, Iron and Reynolds counties. (RSMo 10.047)

Source: http://sos.mo.gov/symbols/symbols.asp?symbol=mineral

The specimen pictured is from the Southeast Missouri Mining District in Reynolds County, MO.

Hematite

A rock that looks like a cluster of small gray bits of shiny steel with spots of red rust.

Photo by Stephanie Reed

Hematite (Fe2O3) is a type of iron ore, which means that it contains iron which can be smelted out and used. It has a distinctive red color which means it can be used as red pigment, and is why hematite is sometimes called bloodstone. This is the main way you can identify hematite: it looks silver, but it produces a red streak. It was used in cave paintings when they wanted a red color. Hematite also can be polished and made into cabochons, but mostly, it is mined for its iron content and used for industrial purposes, such as for making steel or for X-ray shielding. Hematite is found all over the world, but primarily in Minas Gerais (Brazil), Cumbria (England), Morocco, Lake Superior, Utah (Thomas Range), and Arizona. This hematite specimen is from the UMKC Sutton Museum.

By the way, magnetic hematite jewelry is NOT made of hematite. The jewelry is a manmade ceramic barium-strontium ferrite magnet. See here: http://www.mindat.org/min-35948.html Hematite by itself is not magnetic, but some people think it is because it is frequently found with magnetite, which is magnetic.

Chemical Composition of Gemstones

Here’s a neat infographic from Compound Interest (one of my favorite websites) that describes 16 different gemstones and why they have different colors. It also includes their chemical formulas and hardness on the Mohs scale.

Many gemstones would be colorless or a different color if not for the presence of small amounts of transition metals such as chromium or titanium. For example, you can see that aquamarine and emerald both have the same chemical formula Be3Al2(SiO3)6, but emeralds are green because of chromium ions replacing some of the aluminum ions and aquamarines are blue because of iron 2+ or 3+ ions replacing some of the aluminum ions. Click through to read the whole article, because there are many other ways that gems and minerals get their colors!