Tag Archives: Magnet Cove

Geo-pic of the Week: Brookite

brookite(FOV approx. 1.5 mm, photo courtesy of Stephen Stuart)

The metallic crystal in the center of the photo above is a mineral known as brookite. It was collected in Magnet Cove, AR. This particular crystal is approximately 0.5 mm in diameter.

Brookite is one of three forms of titanium oxide (TiO2) that naturally occur in Arkansas. These three forms are what are known as “polymorphs”. Polymorphs are minerals that have the same chemical composition but their atoms are arranged differently creating differing crystal structures. It’s the mineral equivalent of being a fraternal twin instead of an identical twin!

The three types of TiO2 crystal found in Arkansas are brookite, anatase, and rutile. When geologists talk about a mineral’s stability, they are talking about how much of a change in temperature and/or pressure (stress) is necessary to change the crystal structure or composition. The more stress required to change it, the more stable the mineral. Brookite is the least stable of the three forms and therefore the rarest. Typically, brookite crystals are yellowish or reddish brown in color, but the variety found in Arkansas is commonly black which is due to the presence of the element niobium (Nb) as an impurity.

This mineral usually occurs around metamorphic rocks or igneous intrusions similar to the intrusion at Magnet Cove.

 

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Geo-pic of the week: Lodestone

lodestone

Above is a picture of lodestone, which was collected from Magnet cove, Arkansas.  Lodestone is composed of iron and oxygen and is just like the mineral magnetite except, lodestone is naturally magnetic. 

Magnetic minerals are rare on earth but, they have been a crucial part of human evolution.  Discovery of magnetic lodestone by ancient people led to the invention of the compass.  The compass, in turn, revolutionized navigation, and lead to the spread of technologically advanced cultures around the world.

Scientists are still uncertain how lodestone became magnetized but, the most accepted theory holds that it forms when magnetite is struck by lightning, which has a strong magnetic field.  The fact that lodestone is primarily found at the earth’s surface supports this theory and scientists have been able to produce rock identical to lodestone by exposing magnetite to lightning. 

Statemap Field Blog—Nov 12-14, 2013

Hello all!

This week we took a break from our regular field area, and were treated to a three days of field trips relating to the sparse though important igneous rocks of Arkansas led by our own Mike Howard.  Mike, who is regarded as the preeminent expert on the mineralogy of Arkansas, will be retiring in a few weeks after a 39 year career at our agency,

We stopped at numerous locations in Pope, Conway, Pulaski, Saline, Hot Spring, and Pike counties.  The first day, we headed up to the northernmost igneous intrusion in the state which is north of Dover just off Highway 7 on Dare Knob.  From there we stopped at several carbonatite sills (igneous intrusions that follow sedimentary bedding planes) in the Arkansas River Valley near Oppelo.

2013-11-13 0052013-11-13 0012013-11-13 0132013-11-13 014The next day we started out with a visit to the 3M quarry on Granite Mountain.  Granite Mountain is actually a relatively small exposure of an igneous batholith which extends to an area of over 250 square miles in the subsurface.  Also, it is not actually granite at all, but nepheline syenite.    It is being processed mainly into roofing granules.  The other large active quarry in the area, Granite Mountain Quarry, produces mostly aggregate materials.

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From there we went to Bauxite where we saw a large olistolith in the syenite.  A xenolith is a piece of the country rock that is incorporated into the melt of an igneous intrusion.  In this case the country rock was probably Jackfork Sandstone as evidenced by tight folding in a quartzitic rock slightly metamorphosed by contact with the igneous body.   From there we made several stops in the Magnet Cove area, where we saw broken phenocrysts (large crystals in a finer matrix) of pseudoleucite, carbonatite, garnet, rutile and pyrite.2013-11-13 0372013-11-13 040

We also saw the largest barite pit in the U.S. (now abandoned), and a nearby vein of smoky quartz that was easily accessible.

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The next day we drove to the diamond mine at Murfreesboro, with which Mike has had an association since even before it was made a state park.  I found a good barite crystal, but unfortunately, no diamonds.

These trips were attended by almost the entire staff, and we much appreciated getting to see these rare rocks and minerals with knowledgeable commentary by the state’s most expert resident.  We will miss having him at the survey to answer our many questions, but hope that he enjoys many years of well-deserved leisure activities.  Thanks, Mike!!

Until next week, I’ll see you on the outcrop!

AGS Magnet Cove Field Trip with the Texas A&M Geology and Geophysics Society

AGS Magnet Cove Field Trip with the Texas A&M Geology and Geophysics Society
By Lea Nondorf

On Saturday, January 18, 2014, Bill Prior and I met up with sixteen students, ranging in age from freshman to seniors, from the Texas A&M Geology and Geophysics Society. We started the morning at 9 a.m. with a brief introduction on the geology of Magnet Cove at the Sinclair gas station along Highway 51 in Magnet Cove. Everyone was freezing, but you could tell they were super excited to start the day.

Magnet Cove is an area of unusual petrologic and mineralogic interest that derives its name from the presence of lodestone in the soil and from its basin-like shape. It is located in northern Hot Spring County, Arkansas, about 12 miles east of the city of Hot Springs. The diameter of Magnet Cove is about 3 miles (running northwest to southeast) with an overall area of less than 5 mi2 (Howard, 2007).

The Cove is an intrusive igneous body created by mantle-derived magma that pierced through existing Paleozoic sedimentary rocks (Figure 1, listed as Sm, MDa, and Ms) of the Ouachita Mountains approximately 100 million years ago (Late Cretaceous).

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Figure 1. Generalized geologic map of Magnet Cove.

Stop 1

Our first stop was along HWY 270 (Figure 2) looking at the Mississippian Stanley Shale (359-323 million years old). The Stanley Shale is composed primarily of sandstones and shales deposited in a deep ocean basin. These deposits were later faulted, folded, and uplifted during the Ouachita Orogeny (Ouachita Mountains) approximately 323-307 million years ago (early to middle Pennsylvanian Period).

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Figure 2. Looking at the Mississippian Stanley Shale. Everyone was clearly excited.

Stop 2

Our second stop was along Ross Cuttoff Road where we looked at the Garnet Pseudoleucite Nepheline Syenite (GPNS). Now say that mouth full five times fast. Although there were just a few boulders in the ditch, the phenocrysts of pseudoleucite were very prominent.

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Figure 3. A sample of the Garnet Pseudoleucite Nepheline Syenite.

While learning all about GPNS, one particular nice gentleman stopped by to show us his great garnet, rutile, magnetite, and possibly brookite crystals that he collected from his yard. These crystals were absolutely amazing. As we were looking at his collection, another enthusiastic gentleman pulled up telling us all about his beautiful property and all of the different rocks for us to collect. Well of course we were excited, so we jumped in our vehicles and took off. As we were driving, Bill and I noticed how we were traversing over the syenite rim and out of the basin-Oops. When we arrived at his property, we were amazed by the beautiful view, particularly the view of the Magnet Cove basin. And, he did have a variety of rock, but mostly of red, white, and gray Arkansas Novaculite.

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Figure 4. All smiles and thumbs up for Magnet Cove (background).

Stop 3

Here we stopped at the old American Titanium Pit where rutile (chemical composition of TiO2, or titanium oxide) was mined during WWII. We collected pyrite cubes and rutile from this pit. It was pretty muddy, but everyone managed. What troopers!

Stop 4

After lunch, we headed to the old Kimzey Magnetite Pit along Highway 51 where we collected lodestone, rutile, and garnet. A few also found pieces of possible Kidney-Ore hematite and phlogopite. This spot seemed to have it all. Plus, everyone enjoyed using the compass to determine whether or not they had lodestone. A stronger lodestone will really spin the compass needle. Also, the highly magnetic soils in this area make it almost impossible for anyone to use a compass in Magnet Cove.

Stop 5

About a quarter-of-a-mile west of stop 4 along Highway 51, we stopped to collect coarse-grained garnet, biotite ijolite. Ijolite is a feldspar-free, nepheline-rich igneous rock. At this location, we were able to pick up smaller biotite booklets that had weathered out of the boulders present in the ditch.

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Figure 5. Checking out the garnet, biotite ijolite. Unfortunately, we needed a pick and sledge to break off pieces of the boulder for collecting, which no one had.

Stop 6

Adjacent to the old Kimzey Calcite Pit about one half mile west from Stop 5, a great carbonatite outcrop is exposed here just west of the bridge. This particular carbonatite looks identical to calcite, but is different in the fact that this carbonatite is an igneous rock derived from the mantle. Carbonatite even fizzes like calcite. Some other minerals that can be found with the carbonatite include carbonate-fluorapatite (light yellow-green), monticellite (brown), biotite, magnetite, pyrite and perovskite. Also, this is the type locality for kimzeyite, a dark brown zirconium-rich garnet (Howard, 2007).

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Figure 6. Getting up close and personal with the carbonatite. Check out those rhombs.

Stop 7 and 8

Our last and probably most exciting stop was located IN Cove Creek. Although the day had warmed up quite nicely, the creek was still very frigid (and deep). After Bill showed off his awesome pyrite cubes with molybdenite that he had found in the creek 30 years ago, there was no turning back, we were finding pyrite cubes with molybdenite. Only Bill and I were lucky enough to have waders and boots, but that didn’t stop some of the others from jumping in. We trudged up the creek with no luck and quickly came back to where we started only to find nice pyrite pieces in the creek (of course). A few in the group pulled out nice hand-sized specimens with the molybdenite coating. How awesome!

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Figure 7. Leaving the creek with gold in hand, fool’s gold that is.

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Figure 8. Success in the cove with nice samples of pyrite.

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Figure 9. Close-ups of the pyrite found in Cove Creek.

Our final stop was just a few hundred feet past the pyrite area. Here we were able to venture to Cove Creek once again to look at jacupirangite, a dark-colored igneous rock composed primarily of pyroxene and magnetite (yes, jacupirangite does slightly attract a magnet). Some brave souls crossed the creek once again to get a closer look at the igneous rock and the lighter-colored syenite dikes. Just FYI, jacupirangite can become very slippery when wet. 😉

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Figure 10. Jacupirangite in the creek.

References

Howard, J.M., 2007. Magnet Cove, A Synopsis of its geology, lithology, and mineralogy. Arkansas Geological Survey. AGES, Brochure Series 004. 11 p.