Here is some fluorite (purple) and calcite (yellow) on sphalerite (silver). No further comments, I just thought this was pretty.
At Mark Sherwood’s talk “Earth Science… Facts, Frauds and Scams” he mentioned carborundum (also spelled carborundrum). It is made of silicon carbide, but it is not a natural mineral that you can find in the ground. If you want to find some carborundum, look in a chimney. At an iron foundry, the carbon and silicon in the smoke rise and precipitate on the inside of the chimney. When the chimney is cleaned, they find these nice silicon carbide deposits. They are iridescent and pretty enough to buy, but don’t be fooled. Some sellers will say that carborundrum or moissanite and pretend like it is from some secret mine or even a meteorite, but it is really a man-made mineral.
Note: Moissanite is a naturally occurring silicon carbide, but it is very rare and it doesn’t look like the specimen pictured above. It actually looks like tiny green glass crystals. They are usually heat treated to increase clarity. If so, the seller needs to disclose that the specimen has been heated or they are being fraudulent. Buyer beware.
This hematite and magnetite specimen is from Patagonia, Argentina. Bruce got it at the Denver show and gave it to Sharon Penner. It’s about 4 inches long and pretty shiny.
In November 2016, we went to see Marv Dahmen’s collection of vintage Joplin/Tri-State mining equipment and minerals. He talked about it for 5 hours but there was never a dull moment. We managed to record some of it, although it was so long Stephanie and David ran out of space on their phones. Here are some photos.
Thank you Marv for inviting us on your property and into your home to see your amazing collection!
Article by Dr. Bill Cordua, University of Wisconsin-River Falls
Have you ever been to a show and seen enormous amethyst geodes or crystals 3-5 feet or more in height? The tubular geodes are lined with deep purple gemmy amethyst crystals. How do such wonders form?
These excellent geodes come from a region along the Brazil-Uruguay border. The genesis of deposits on the Brazil side of the border has recently been extensively researched by an international team of geochemists lead by H. Albert Gilg of Techniche University Munchen in Germany (Gilg, et. al., 2003). The geodes are mined from several lava flows belonging to the Parana Continental Flood Basalt Province. This was one of the largest outpourings of basalt lava known. An estimated 800,000 cubic kilometers of lava extruded over an 11 million year time span. For comparison, this would be enough to cover Minnesota with a pile of basalt lava over 2 miles high. The lava outburst occurred as part of the opening of the South Atlantic Ocean during Cretaceous time about 130 million years ago. Of all these flows, however, only a few are known to host amethyst cathedral geodes.
Gilg et al. proposed a 2-stage model for their formation. In the first stage the large hollows form. This was caused as volcanic gases were released from certain lavas as they cooled. Not every lava has enough dissolved gas to form such big openings. As gas bubbles emerged from the congealing lava (much as bubbles emerge when beer or soda pop is poured) they coalesced as they rose. The lava was cooling fast too, and soon became so thick and sticky that bubbles quite rising and were trapped. The bulbous to tubular shapes thus point towards the top of the flow, a fact easily seen when the geodes are in place in the mines. These cavities, though, were empty of crystals.
The second stage was the formation of the amethyst, plus celadonite, calcite and gypsum fillings. An important clue to this event is the presence of small gas and liquid bubbles (called fluid inclusions) trapped within these minerals. These are samples of the mineral-forming liquids caught as the crystals grew. Fluid inclusions are treasure troves of information when studied with sophisticated instruments. Analyses of the fluid inclusions in the amethyst, calcite and gypsum show them to be filled with slightly salty water. This water had a temperature of no more than 100 degrees C, and possible less than 50 degrees C, during mineral formation. These cannot be fluids related to the magma that formed the lavas.
What was the source of these fluids? An amazing story unfolds from the radiometric dating of the minerals. The basalts formed about 130 million years ago, but the green celadonite, which makes up the rinds of the geodes, formed about 70 million years ago. For 60 million years these enormous cavities sat empty of crystals. Trace element data from the fluid inclusions gives another important clue to the source of the mineral-forming fluid. Below the lavas is a large aquifer (the Botucatu aquifer) filled with ground water that closely resembles the fluid inclusion liquids. Uplift and tilting of the area about 70 million years ago would force water out of the aquifer into the porous areas of the overlying lava. In the lava flow these waters would have found volcanic glass. Glass breaks down over geologic time and makes silica and other chemicals available in a form that is readily soluble in water soaking through the rocks. The water carried these chemicals into the cavities, where the amethyst and other minerals grew due to cooling and pressure release.
The special combination of geologic circumstances, unfolding over millions of years, is not often duplicated. Understanding the process gives geologist tools to prospect more efficiently for these wonders.
Gilg, H. et. al, 2003, “Genesis of amethyst geodes in basaltic rocks of the Serra Geral Formation (Ametista do Sul, Rio Grande do Sul, Brazil): a fluid inclusion, REE, oxygen, carbon, and Sr isotope study on basalt, quartz and calcite” Mineralium Deposita vol. 38, p. 1009-1025.
The Glacial Drifter 08/2011, The Gemrock 06/2015
Special guest article from Show-Me Rockhounds club members Dan and Connie Snow
Fairburn agates are a form of microcrystalline chalcedony, 100% silicon dioxide with a hardness of 6 ½ to 7 on the Mohs scale. They are also called fortification agates because of their banding. They were formed approximately 300 million years ago in an ancient limestone bed of an inland sea. To hunt Fairburn agates requires looking at every rock and turning many with a rock pick. It is strictly surface hunting no digging, mining, cracking or breaking rocks. The photos shown are exactly the way the agates were found, with no cutting, polishing or tumbling having been done.
Fairburn Agates found by Dan and Connie Snow. Collected from the Fairburn Agate beds of South Dakota and the Oglala National Grasslands in Nebraska.
Labradorite has become a popular gemstone because of the unique iridescent play of color that many specimens exhibit. Labradorite is a feldspar mineral of the plagioclase series that is most often found in mafic igneous rocks such as basalt, gabbro and norite. Some specimens of labradorite exhibit a Schiller effect, which is a strong play of iridescent blue, green, red, orange, and yellow colors as shown in the photographs above. The Schiller effect is also seen in fire agate and mother of pearl. Labradorite is so well known for these spectacular displays of color that the phenomenon is known as “labradorescence.” Specimens with the highest quality labradorescence are often selected for use as gemstones. Labradorescence is not a display of colors reflected from the surface of a specimen. Instead, light enters the stone, strikes a twinning surface within the stone, and reflects from it. The color seen by the observer is the color of light reflected from that twinning surface. Different twinning surfaces within the stone reflect different colors of light. Light reflecting from different twinning surfaces in various parts of the stone can give the stone a multi-colored appearance.
The 4th of July fireworks that we saw last night would not be possible without minerals. Fireworks mainly contain gunpowder, which is a combination of charcoal, sulfur, and the mineral potassium nitrate. In order to create the pretty colors we are used to seeing in fireworks, mineral salts are added. This infographic from Compound Interest explains which mineral salts create which colors. If you go to the website, you can read a lot more about the chemistry of fireworks and a brief explanation of why different minerals make different colored flames.
I learned that blue fireworks are very difficult to produce because copper chloride breaks down at high temperatures, so they have to somehow keep the temperature hot enough to ignite but not so hot that the blue color vanishes. Thus, you almost never see purple fireworks because it is a combination of red and blue.
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
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