Rare and attractive micromounts are found at Granite Mountain

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Granite Mountain

Current Status: working mines, generally not open to public.

WHEN I WAS nearly out of college, even though had I collected minerals for over 10 years, I discovered there were several active quarries in the Little Rock area where I might recover some minerals. I was on a professional field trip led by one of the state geologists in 1973 for the South-Central Geological Society of America when I made my first visit to the Granite Mountain area in Pulaski County. While riding in the quarry to the first stop, aegerinemy mineralogy professor Kern Jackson shouted out "Stop the car, there's one!" In the middle of the quarry floor was a large solitary boulder. In that boulder was a 6 inch cavity filled with black aegirine needles. Dr. Jackson took a small sledge hammer, extracted a specimen and told me, "Have at it!" I got a 2 inch by 2 inch plate covered with a bristling coating of the sodium pyroxene. I was hooked on this locale!

Since then, I have been lucky enough to visit the active quarries many times, though they are closed to regular rockhounds. I have escorted a number of mineralogists, isotope specialists, petrologists, and other people interested in the igneous rocks of central Arkansas and the unusual minerals associated with these rocks.

The list of minerals from this area is continually growing, primarily due to the work of several mineralogists - Charles Milton, now deceased; Henry Barwood; and researchers at the University of New Orleans, along with some of the specialists with the USGS and US National Museum.
Most of the truly rare and attractive minerals are microscopic, but some are larger and a lucky collector may be rewarded with interesting larger specimens of various zeolites. More than 75 minerals have been identified from Granite Mountain, and the best available written summary is by H. Barwood in the Arkansas issue of Rocks and Minerals (1989, July-August issue). Some of the minerals are the result of the growth of crystals from the wall rock into gas cavities, while others are the result of various fluid phases and degassing of the intrusion.

natrolite and analcimeGas cavity minerals
Many species have been reported, mostly micros, but several minerals make attractive specimens when the cavities were large enough. Translucent natrolite match-stick crystals project above white analcime.

zeoliteIf lucky, the collector may get a single specimen with several different minerals, like translucent fluorapophyllite and pectolite spheres with microcrystals of pyroxene and titanite.

fluorapophylliteSeveral habits of fluorapophyllite are seen here, tabular or equidimensional being most common.

natroliteSometimes one comes across an unusually large example, like these combined crystals of natrolite, over 3 inches in length, capped by some hyaline opal. This specimen came from a large pocket, all the individuals being broken from the matrix by a quarry blast. Literally hundreds of natrolite crystals were pulled out onto a newspaper. Covered with dirt, my collecting partner thought they were all broken on both ends so he took a couple and gave the rest to me. Imagine his surprise when they were washed and he discovered they weren't broken on the terminations, but coated with fluorescent hyaline opal!

stilbitePegmatite minerals
Possibly the most colorful mineral discovered in fair abundance in the past few years is orange stilbite, which occurs on cream to white bladed orthoclase crystals.

analcimeSparse reddish analcime crystals are most interesting because of their color. Red, orange, and yellowish analcime are rare, white to gray is the typical color. Specimens with only analcime are rather unusual because there are typically several other zeolites and wall-rock minerals present.

titaniteOne pegmatite contained peculiar elongated needles of titanite. Needles of titanite up to 1/2 inch in length were present in the cavities between the feldspar crystals. Several different colors of titanite occur here, but most are microscopic crystals. A pinkish tan is most abundant, followed by honey-colored, and a rare submetallic gray (due to niobium).

gas pocketConsiderable effort is sometimes involved in recovering a specimen from this site. Here is a nice mineralized gas pocket that came out intact after about 20 minutes work with a sledge hammer on a boulder. Judicious trimming with a crack hammer brought it down to a small cabinet-size piece. Most of the minerals are microscopic, but it is still interesting to study under a microscope or with a hand lens.

The Granite Mountain intrusion is misnamed, as it is syenite, not granite, and contains a different assemblage of minerals than granitic rocks. Rarely does anyone see any quartz in these rocks, but instead there are zones with considerable nepheline. At first glance the syenite appears to be a monotonous gray, medium-grained igneous rock, but first appearances can be deceiving. There are many features which yield mineral specimens, like coarse pegmatitic zones, altered xenoliths, thin dikelets, and gas pockets. Mining is continuous at several of the quarries, which is good for this is necessary if new exposures and minerals are to be discovered. Only a few square miles of the igneous mass in Pulaski and Saline Counties are exposed, and natural exposures yield few interesting minerals due to weathering. Because the mineralization is relatively sparse, quarrying is essential to the recovery of specimen material from this location. The syenite actually consists of two main masses, the earlier intrusion came in about 95 million years ago, and the later mass about 86 million years ago. The intrusive was some 250 square miles in extent, but later Tertiary and Quaternary sediments cover all but a few square miles. In Pulaski County, the majority of the rock exposed at the quarries is of a type termed pulaskite (it was first named from exposures here in Pulaski County, AR). Pulaskite is cut by dike-form bodies of the younger nepheline syenite which is coarser grained and may even be pegmatitic in texture.

Even though the Granite Mountain location is generally restricted to most collectors, sometimes college classes, professional field trips, and other groups, like Friends of Mineralogy, are allowed in by the operators. Access to sites like Granite Mountain is an example of the benefits of membership in an organized club or association.