Mar 142017
 

We just found out about this year’s Cumbres & Toltec Geology Train adventure in southwest Colorado/northwest New Mexico — 18 June 2017. It’s a special opportunity to enjoy some of that Rio Grande Rift, Brazos Uplift, Tusas Mountains, San Luis Basin, and San Juan Sag scenery.

Our very own Peter Barkmann, geologist extraordinaire and veteran Geology Train guide, will be on board for an informative and energized day in the high country.

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On June 18th, a special train will depart to traverse spectacular geology along the 64 miles of Cumbres & Toltec track. But simply experiencing the incredible overviews of the Rio Grande Rift, the eruptive evidence of the San Juan Volcanic field, the Precambrian core of the Tusas Mountains, recent glacial deposits, and snapshots of the Jurassic, will not be enough. This special train will stop at many outcrops and rail cuts along the right of way, to mingle, marvel and collect photographs, samples and experiences only accessible on the train route.

ALL ABOARD!

Feb 192017
 

Diamonds are formed from pure carbon, one of the most abundant elements on planet Earth. Diamonds, even from ancient times, have been sought for their extraordinary hardness (they are the hardest substance known) and their brilliance, especially in the colorless transparent gemstone variety. Ironically the other form of pure carbon is graphite, which is very soft with a soapy feel and a dull gray color. Graphite is commonly the “lead” in a pencil.

A diamond's crystal structure: tetrahedrally bonded carbon atoms crystallized into the diamond lattice, a variation of the face-centered cubic structure.

A diamond's crystal structure: tetrahedrally bonded carbon atoms crystallized into the diamond lattice, a variation of the face-centered cubic structure.

The Mohs Hardness Scale of minerals starts at 1 (talc) and ranges to 10 (diamond). That does not mean that diamonds are ten times harder than talc; mineral number 9 on the Mohs scale is corundum, a class of minerals which includes rubies and sapphires. Diamonds can be from ten to hundreds times harder than corundum. Diamonds themselves vary in hardness; for example, stones from Australia are harder than those found in South Africa.

The four main optical characteristics of diamonds are transparency, luster, dispersion of light, and color. In its pure carbon form, diamond is completely clear and transparent. As in all natural substances, perfection is nearly impossible to find. Inclusions of other minerals and elements cause varying degrees of opacity. The surface of a diamond can be clouded by natural processes, such as the constant tumbling and scraping in the bed of a river.

Luster is the general appearance of a crystal surface in reflected light. Luster of a smooth crystal face of diamond is strong and brilliant. It is intermediate between glass and metal and has its own special term — adamantine.

Relative size of octahedral diamond crystals from 1 to 500 carats. Credit: Modified from Bauer, 1968.

Relative size of octahedral diamond crystals from 1 to 500 carats. Credit: Modified from Bauer, 1968.

The process of white light breaking up into its constituent colors is called dispersion. Diamonds have strong dispersion, which along with their strong luster, causes the beautiful play of colors so often referred to as the “fire” of a diamond.

Gemstone varieties of diamond and imperfections. Yellow or yellowish-brown and even brilliant yellow diamonds have been found. Very rarely, diamonds are blue, black, pale green, pink, violet, and even reddish.

The most famous blue diamond, the Hope Diamond, is intertwined with Colorado’s mining history. Thomas Walsh, discoverer of the rich Camp Bird Mine near Ouray, purchased the Hope Diamond for his wife in the early 1900s; it was later given to his daughter, Evelyn Walsh McLean who wore it almost continuously until the 1940s. The 45.5-carat Hope Diamond now resides at the National Museum of Natural History in Washington, D.C.

Diamonds, in their perfect cubic crystal form, occur as isolated octahedral (eight-sided) crystals. Many variations on the cubic form are found in are usually clear and colorless, often containing minor inclusions nature, including twelve-sided crystals and a flattened triangular shape known as a macle. Gemologists recognize three main varieties of diamonds: ordinary, bort, and carbonado. Ordinary diamonds occur as crystals often with rounded faces, from colorless and free from flaws (“the first water” [1]) to stones of variable color and full of flaws. Bort diamonds occur in rounded forms without a good crystal structure. They are generally of inferior quality as a gemstone. Carbonados are black opaque diamonds usually from the Bahia Province of Brazil. They are crystalline but do not possess the mineral cleavage found in ordinary diamonds.

[1] An expression which refers to the highest quality diamonds and has come to mean the highest quality of just about anything. The comparison of diamonds with water dates back to at least the early 17th century, and Shakespeare alludes to it in Pericles, 1607:

Heavenly jewels which Pericles hath lost, Begin to part their fringes of bright gold.
The diamonds of a most praisèd water Doth appear, to make the world twice rich.

Diamonds in the rough, note the regular octahedral forms and trigons (of positive and negative relief) formed by natural chemical etching.

Diamonds in the rough, note the regular octahedral forms and trigons (of positive and negative relief) formed by natural chemical etching.

Feb 172017
 

Dr. Cílek, the Director of the Czech Republic’s Academy of Sciences Institute of Geology delivers a fascinating talk about the Bohemian Karst region of the Czech Republic, around Beroun, that weaves the human historical, mystical, and mythological elements with the underlying geology and speleology.

(00:36:32, stereo audio, 70.1 mb)

Bohemian karst (Český kras) landscape formed in a limestone of Silurian and mainly Devonian age. The area hosts several international stratotype and parastratotype sections, including the main Silurian/Devonian Global Boundary Stratotype Section at Suchomasty. Photo credit: Milos Sejn.

Bohemian karst (Český kras) landscape formed in a limestone of Silurian and mainly Devonian age. The area hosts several international stratotype and parastratotype sections, including the main Silurian/Devonian Global Boundary Stratotype Section at Suchomasty. Photo credit: Milos Sejn.

Feb 142017
 

Uranium is a widespread and ubiquitous element. It has a crustal abundance of 2.8 parts per million, slightly more than tin. Primary deposits of uranium tend to concentrate in granitic or alkalic volcanic rocks, hydrothermal veins, marine black shales, and early Precambrian age placer deposits. Secondary (or epigenetic) deposits of uranium are formed later than the surrounding rocks that host the mineral deposit. Uranium is soluble in oxidizing aqueous solutions, especially the U+6 valence state, and can be redistributed from primary source rocks into porous sedimentary rocks and structures by groundwater and form secondary (epigenetic) uranium mineral deposits.

Epigenetic deposits of uranium in sedimentary rocks form the bulk of uranium deposits in Colorado. These include the many mines of the Uravan, Cochetopa, Maybe, and Rifle districts, and other scattered places including the Front Range and Denver Basin. Primary uranium deposits in Colorado occur in hydrothermal veins, especially in the Front Range. Continue reading »

Jan 232017
 

The CGS’s Matt Morgan and Jon White were two of the co-authors on one of the top-ten Geological Society of America (GSA) 2016 book chapters and journal articles, this out of 600 papers. The article describes a comprehensive forensic analysis of the massive West Salt Creek rock avalanche that occurred in late May 2014 in western Colorado (USA). The analysis relied on large-scale (1:1000) structural mapping accomplished via high-resolution unmanned aircraft system imagery along with seismic data generated by more than twenty stations within approximately 500 miles (800 km) of the event. The avalanche was the largest mass-movement slope failure in the historical record of Colorado: it killed three people and narrowly avoided destroying a gas wellhead.

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Citation: Coe, Jeffrey A., Rex L. Baum, Kate E. Allstadt, Bernard F. Kochevar, Robert G. Schmitt, Matthew L. Morgan, Jonathan L. White, Benjamin T. Stratton, Timothy A. Hayashi, and Jason W. Kean. 2016. “Rock-Avalanche Dynamics Revealed by Large-Scale Field Mapping and Seismic Signals at a Highly Mobile Avalanche in the West Salt Creek Valley, Western Colorado.” Geosphere 12 (2): 607–31. doi:10.1130/GES01265.1.