Friday, November 11, 2016

Geological State Symbols Across the US - #6 Colorado

The next state up in our review of the Geological State Symbols across the US is:

 Colorado

Here are the stats:

                                                                                        Year Established
State Rock: Yule Marble                                                          2004
State Mineral: Rhodochrosite                                                  2002
State Gemstone: Aquamarine                                                  1971
State Fossil: Stegosaurus                                                         1982

State Rock: Yule Marble

Marble is produced when limestone is metamorphosed (subjected to increased heat and/or pressure). Marble is primarily composed of calcite, which during metamorphism is recrystallized to form interlocking crystals. However, marble often has impurities in it, such as clay, mica, quartz, pyrite, and iron oxide (Geology.com). The unique feature of the Yule Marble, found near the town of Marble, Colorado, though is that it is one of the purest marbles ever quarried (meaning it lacks most of those impurities). For this reason it was chosen as the decorative stone on the Lincoln Memorial in Washington DC.
Tomb of the Unknown Soldier.
Photo by Thomas Loker Photography (tloker.wordpress.com)

The Yule Marble started out as the dark-blue Mississippian age Leadville Limestone. During the Tertiary, parts of the Leadville Limestone underwent contact metamorphism along the edges in contact with the uplifted granitic Treasure Mountain dome. After recrystallization, the Leadville Limestone was metamorphosed into the distinctive white marble known today (USGS).

Along with the Lincoln Memorial, Yule Marble has been used for the Tomb of the Unknown Soldier, which when quarried was the largest single piece of marble ever quarried at the time. They also supplied some of the national cemetery headstones. However due to problems financing and running the mine, not many other notable uses have been made of the Yule Marble.

State Mineral: Rhodochrosite 

A note about minerals and gemstones -
Gems are valued for their use in jewelry; because rhodochrosite has a much lower hardness than most gemstones, its use in jewelry is very limited (Colorado Geological Survey).
Rhodochrosite is a manganese carbonate mineral (chemical formula (Mn, Fe, Mg, Ca)CO3) with a light pink to bright red color. As you can see with the chemical formula the manganese is frequently replaced with other elements like iron, magnesium, and/or calcium. These substitutions present problems with providing definitive mineral properties since the substitutions cause the specific gravity, hardness, and color of the mineral to vary. However, generally, rhodochrosite has a hardness of 3.5-4 with 3 directions of perfect, rhombohedral cleavage. Rhodochrosite is often found in association with silver mines and is sometimes mined as a byproduct in those mines (geology.com).

Rhodochrosite from Sweet Home Mine, Alma, CO
Rhodochrosite, however is extremely rare in well-formed crystals. That is the reason that the Sweet Home Mine, near Alma, Colorado is so unique. This is one of the few mines in the world to produce specimen quality rhodochrosite samples. Formerly a silver mine that opened in 1873 the mine was leased to a rhodochrosite company who started excavating specifically for high grade rhodochrosite samples. They mine by blasting holes in the rock that are 2 meters by 2 meters by 2 meters, and continue the process forming tunnels. Once a potential rhodochrosite pocket is located, special tools are brought to carefully extract the specimens, where they are then taken to be cleaned and prepared for sale (collectorsedge.com).

State Gemstone: Aquamarine

Aquamarine from Mt. Antero, CO.Photo by Kevin Dixon.
Aquamarine is a greenish blue variety of beryl (Be3Al2Si6O18), named for the seawater in which it looks like. It is also March's birthstone. It has a hardness of 7.5-8 and often forms perfect, six-sided prismatic crystals. Although it can be found as duller colors, when heated to very high temperatures, the mineral attain a sky-blue color (minerals.net). 

Aquamarine began to be prospected for in the late 1800's in Colorado in the Mt. Antero area. Many aquamarine crystals have been found in vugs (holes) within the granite pegmatites of the Tertiary Princeton batholith. The veins and pegmatites where the aquamarine has been found found are mostly contained within an area roughly 3 miles across. The summits of Mt. Antero and White Mountain are contained within this region. These pegmatites formed at temperatures that ranged from 200°C up to 600°C (American Mineralogist).





State Fossil: Stegosaurus

 
Stegosaurus at the Field Museum, Chicago, IL
Photo by Jim Lehane
The Stegosaurus was one of the early dinosaur finds during the infamous Cope and Marsh Bone Wars (if you don't know about these I recommend looking them up). The first Stegosaurus fossils were discovered in 1876 in the Morrison Formation of Colorado by M.P. Felch and were named by Marsh in 1877. The original fossils were discovered north of the town of Morrison, CO.

Stegosaurus was an herbivore, that lived about 150 million years ago during the Jurassic period. The animal had a twin row of bony plates that lined its back and a set of spikes on its tail. The use of the plates has been rather controversial, ranging from defense, to display, to possible heating/cooling pads (blood vessels ran through the plates allowing the air/sun to heat or cool the plates as needed), however it is pretty universally accepted that the spikes at the end of the tail were for defensive purposes (livescience.com). Most commonly known about Stegosaurus was that it had a very small brain for the size of the animal (it is about the size of a bus, 30 ft in length, with a brain the size of a hot dog).





References
https://www.colorado.gov/pacific/archives/symbols-emblems
http://geology.com/rocks/marble.shtml
https://pubs.usgs.gov/pdf/bulletin/b2162/b2162.pdf
http://geology.com/minerals/rhodochrosite.shtml
https://www.collectorsedge.com/t-sweethome.aspx
http://coloradogeologicalsurvey.org/education/state-symbols/state-mineral/
http://www.minerals.net/mineral/aquamarine.aspx
http://geology.com/gemstones/states/colorado.shtml
http://www.minsocam.org/MSA/collectors_corner/arc/comtantero.htm
http://www.the-vug.com/vug/collectors_edge_lees_collection.html#.WCX3JGorKHs
http://www.livescience.com/24184-stegosaurus-facts.html
http://www.ereferencedesk.com/resources/state-fossil/colorado.html

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