Anhydrite: Warning! Keep Dry!

The word “anhydrous” means “lacking water.” So, when gypsum looses the water molecules that help make up its chemical composition “anhydrite” is an apt name for the new mineral that results.

Anhydrite commonly forms when gypsum dehydrates, but it can form in other ways. It is often found between layers of gypsum and even between gypsum and halite. Because it usually forms when gypsum dries out the layers of anhydrite are often contorted.

Anhydrite crystallizes in the orthorhombic crystal system, but it is usually found in massive or fibrous habits, the latter looking a bit like satin-spar selenite (a variety of gypsum that you can read about in my March 17, 2011post Gypsum: The Mineral About the House) Anhydrite can be distinguished from gypsum by the fact that it is a little harder. Anhydrite is usually white or colorless, but it can also be lavender, reddish, bluish, or gray.

I’ve already stated that gypsum becomes anhydrite when it looses its water, but if the anhydrite absorbs water then it will become gypsum once again. Because of this, anhydrite can be ground into a powder and used to make concrete and plaster of Paris. This property also makes anhydrite difficult to find. Since it so easily reverts to gypsum, anhydrite tends only to be found deep underground where it won’t be exposed to much moisture.

Anhydrite can be found in such places as: Mexico, Cuba, Peru, Chile, Canada, Zaire, and the American states of: Alabama, Michigan, Louisiana, and Texas.

I hope that you’ve enjoyed this piece about anhydrite. I have one final note for collectors which concerns anhydrite’s relationship with gypsum: if you want to collect the stuff it is a good idea to keep it in an airtight case with some silica gel. Otherwise your anhydrite specimen might revert to gypsum!

Aragonite, the Mineral With a Patriotic Name

    
     Aragonite is named for the Spanish province of Aragon. Aragonite crystals can be fibrous, acicular (needle-like) or prismatic, but they often display what is known as crystal twinning. Crystal twinning is a mineral phenomenon in which crystals grow together and sometimes appear as one. In the case of aragonite these crystals can form as columns or large needles that radiate from their base. Aragonite can also form as stalactites. As is the case in many mineral types aragonite is clear when pure, but impurities can cause it to become yellow, blue, pink, green, brown, orange, and rarely purple. Aragonite can be confused with the mineral calcite, but it can be easily distinguished by its greater specific gravity. The specific gravity loosely defined is the ratio of a mineral’s weight to its size. Minerals with a higher specific gravity feel heavy for their size. So a piece of calcite will weigh less than a piece of aragonite that is the same size.
     Aragonite has some use in jewelry, but due to aragonite’s softness this use is limited. Aragonite is used to make beads and small carvings. Aragonite is sometimes polished though I cannot elaborate on what it looks like polished because I have never seen it. However, I have seen a polished slice of an aragonite stalactite that was quite pretty. The picture that I saw had alternating layers of white, yellow, brown, and orange. The layers were uneven and looked as if they had been folded. Aragonite is also used in the making of cement and in glass-making applications. Aragonite is composed of a chemical that is called calcium carbonate, which is used to neutralize acid in industry. Aragonite is also used to make calcium-rich fertilizers for agriculture, animal feed, and white pigments.
     Aragonite is found in many places, but I personally doubt that any source can be considered more famous that that of Aragon, Spain. Aragonite is also found throughout the western Mediterranean region, England, Colorado, Mexico, Russia, the Czech Republic, and Australia. Though I did not come across this source an any of the books that I read I have known New Mexico to be a good source.
     Aragonite is certainly a captivating mineral and I am glad to have learned so much about it. Also aragonite inspires the idea of naming a mineral that one discovers after the region in which he or she lives. If ever you come across a blog about the mineral michiganite, you’ll known that I’ve done just that!

Amethyst--the Tie-dyed Quartz

        Amethyst is a purple-colored variety of the mineral quartz. In pure form quartz (silicon dioxide) is colorless, but impurities can alter its color. The purple color of amethyst is thought to be the product of tiny amounts of iron impurities. Amethyst can also be dichroic, which means that its color can appear to change when it is viewed at different angles (certain inks, such as the one used on the American $20 bill, do this too) Amethyst often has tiny drops of liquid trapped within its crystal structure, as well. The technical term for anything that is trapped within a crystal is an inclusion. In the case of amethyst these liquid inclusions are often called "tiger stripes" because the look like the stripes on a tiger's fur. It is because of all of these color phenomena that I call it the "tie-dyed quartz."
      Like quartz amethyst often forms within natural gas pockets and geodes. Quartz is the most common mineral in the world and is found everywhere on earth, so amethyst could theoretically be found anywhere. However there are certain areas where the mineral is found in abundance, most notably Brazil and the Ukraine. I have personally never found any in nature, but I do have three clusters of amethyst and one geode that contains the purple mineral. I think that amethyst is a beautiful mineral because its color is so uncommon.