Tag Archives: Geology pictures

Geo-pic of the week: Dogtooth Calcite

dogtooth2E

(FOV approx. 10 cm, photo by Corbin Cannon)

Even though they might look like it, those crystals in the picture above didn’t come out of a dog’s mouth. They are crystals of dogtooth calcite. Calcite (CaCO3) is the primary mineral that makes up limestone. It occurs in several crystal shapes. The two most commonly found in Arkansas are 6 sided rhombohedrons and the scalenohedral shape you see above. When it forms in this scalenohedral crystal structure it is called “dogtooth spar”.

Calcite is a very common mineral, but this particular crystal form of the mineral is typically only found in Arkansas in conjunction with the minerals sphalerite (zinc ore) and galena (lead ore) in the lead and zinc districts. Calcite is also a polymorph, like the mineral brookite from a previous geo-pic. This means calcite has “sister” minerals with the same chemical composition, but differing crystal structures. The three polymorphs of CaCO3 are: calcite, aragonite, and vaterite.

Advertisements

Geo-pic of the week: Iridescent Iron Oxide on Novaculite

iridescent iron edited(photo courtesy of John David McFarland)

The photo above illustrates surface iridescence.  Iridescence is a play of colors caused by the interference of light waves.  This phenomenon is also called thin film interference. You have probably seen this effect on soap bubbles and oil sheens.  Light reflecting from a thin coating of iron oxide on the piece of novaculite above is producing the play of colors.  Light waves are reflected from the top of the iron coating and the base of the iron coating producing multiple waves.  A color is seen when the waves interfere constructively.  The resultant color is dependent on the thickness of the coating and consequently, streaks and bands of differing color develop since the thickness of the iron oxide coating varies. The colors also change when the angle of reflection is changed.

Geo-pic of the week: “The Great Little Rock Silver Rush”

 

Argentiferous galena enhanced

In September of 1982, this 800 lb. boulder was excavated by a backhoe operator during construction of the La Quinta Inn on Fairpark Boulevard (currently Day’s Inn), Little Rock, AR. Another worker on site recognized it as galena (ore of lead) and, wanting to score some quick cash, the finder began contacting local geologists, hoping to sell. Eventually, then State Geologist, Bill Williams, heard about it and sent another geologist from the Arkansas Geological Survey (AGS), Ben Clardy, to investigate. Clardy bought the boulder for $100 and the backhoe operator loaded it onto Clardy’s truck for transport back to the AGS office.

At the office, an engine lift was rented to remove it. The agency’s chemist, Gaston Bell, assayed a piece for silver, determining it contained 1 – 2 %, making it high-grade silver ore. Feeling he had cheated the seller, Clardy contacted him with the results but the seller was happy with the $100 deal. The State Geologist reimbursed Clardy and placed the specimen on display in the lobby of the AGS office.

News of the find spread quickly, as the story was picked up by local newspapers. Someone claiming to be the hotel property owner announced that the backhoe operator had stolen the rock and came to the AGS office demanding it back. It was now property of the state, but Bill Williams told him he could have it, as long as he could bring some large guys to carry it off; he didn’t want heavy equipment in the office lobby. The man left and never raised his claim to the rock again. A couple days later another piece was found on the property in the same mineralized pocket which was at the intersection of two quartz veins. The property owner took possession of that piece and sent it to Colorado where it was smelted and produced a substantial silver bar.

Around the same time, as the public became aware of the find, some midnight rock poachers began sneaking onto the property, after hours. Small chunks of galena appeared around town for sale, being marketed as “Little Rock Silver Ore”.

At least one silver company took an interest in the find, conducting a series of soil tests over several blocks surrounding the La Quinta property. They soon abandoned the effort due to the difficulty of mining in such an urbanized area. Results of their tests were never disclosed. Eventually, construction of La Quinta was completed, the lot was paved over, and thus ended the “Little Rock Silver Rush”.

The original 800 – pound chunk is still on display in the lobby of the AGS office in Little Rock. Part of the other piece, which was not melted down, was displayed in the lobby of the La Quinta Inn on Fairpark Boulevard before the property changed hands.

Based on written correspondence with Michael J. Howard