A Field Trip to Collect Peridot at Kilbourne Hole, New Mexico
It is a long drive from Spruce Pine, North Carolina, to Tucson, Arizona. Normally I would fly to the annual Tucson gem and mineral shows, but I had been planning a return trip to Kilbourne Hole for several years. It was the perfect time of year to do some mineral collecting in the southwestern desert. Several years ago I left my first career, biomedical research, and started a small internet business selling custom-cut gemstones. I prefer to cut gem material of US origin. Even more satisfying is cutting a piece of gem rough that you have personally collected. Buying facet grade peridot from the Apaches is okay, but nothing beats finding the peridot yourself. Unfortunately, gem peridot is hard to find. There are small peridot deposits in Wyoming and Colorado, and a large deposit on Arizona indian tribal lands. The large deposit at San Carlos is mined on a commercial basis by the Apaches, but it is closed to non-native collectors. That leaves southern New Mexico, specifically the volcanic maars of Dona Ana County, as the most likely place to find a nice piece of american gem peridot. I loaded the pickup with my camping and collecting gear, and two days later I was in El Paso.
My first visit to Kilbourne Hole took place over 20 years ago. My mother, a friend and I had driven over from Phoenix to scoop up all that gem peridot that was bound to be laying all over the ground. Actually, we weren't quite that nieve. We had read Lindberg's 1964 Lapidary Journal article on "the hole," and had studied other rockhound guidebooks that gave fairly good directions on how to get there. On that trip we found the site without trouble, and spent a long, hot, day hiking across the bottom of the hole and around the basalt rim. Our peridot finds were abysmal; a few tiny grains of olivine. Later research, and discussions with other collectors, revealed our mistake, we were looking in the wrong place. In 1997 my mother and I again visited "the hole" in quest of that elusive gem peridot. Better informed, we found lots of olivine, but nothing sufficiently large to cut a gemstone. Subsequent trips have been more successful.
Kilbourne Hole is one of several volcanic maars located in Dona Ana County, New Mexico. Volcanic maars are unusual volcanic features, and Kilbourne is probably the best example to be seen anywhere in the world. Maars look something like a meteor impact crater; but they are formed by an entirely different event. Maars are broad volcanic craters formed by shallow explosive eruptions. It is theorized that the eruptions are caused by the heating and vaporization of groundwater when magma impinges water-saturated rock strata. Pressure builds, and when the overlaying rock is unable to contain the force, an explosive eruption occurs. During maar formation there is relatively little volcanic material emitted, except for gas. Geologically speaking, Kilbourne Hole isn't very old; probably less than 180,000 years. The hole is roughly eliptical in shape; its maximum dimension being about 1.7 miles. It is approximately 400 feet deep. Many maar are filled with water( e.g Zuni Salt Lake, also in N.M.). Kilbourne Hole, located in an arid part of the world, has only a playa in its center. The crater countryrock is Camp Rice formation sediments overlain by a 5 to 10 foot thick layer of Afton basalt. Laying atop the basalt rim of the crater is a surrounding ridge of volcanic ejecta that rises as much as 150 feet above the surrounding plain. The olivine and gem peridot occur as "bombs" or xenoliths that were ejected during crater formation. It is primarily in the basalt explosion breccia that olivine can be found.
February 20th, 2007, about an hour before dawn, I left El Paso and headed for Kilborne Hole. My destination lay only 30 miles west, but the last 20 miles would be dirt road that varied from good to poor condition. My route took me through the small town of La Union, New Mexico. I took Alvarez Street out of town and followed a good dirt road that quickly rose from the Rio Grande Valley up to a plain. Seven miles southwest of La Union I crossed the Southern Pacific Railroad tracks. There the good dirt road terminated at a frequently traveled dirt washboard road that parallels the railroad tracks. Five miles north, at Lanark Siding (just some concrete pillars and railroad electical signal boxes), I turned left (west) onto Dona Ana A-011. The county has signed this intersection, but considering the local vandals propensity to shoot things full of holes, don't count on the condition or presence of any offical roadsigns. For those with GPS capability this intersection will be 31 degrees 58.26 minutes N, 106 degrees 49.16 minutes W. This final leg of the route (Dona Ana A-011) was narrow and equally bumpy. Don't take your motorhome unless you plan on a new paint job. The mesquite bush branches form a natural and very narrow "guard rail." Continue almost due west from Lanark Siding, ignoring a couple poorly improved dirt cross-roads. After traveling another 8.3 miles I arrived at the southeastern rim of Kilborne Hole (31 degrees 57.35 minutes N, 106 degrees 57.38 minutes W). There are dirt roads around the southern, western, and eastern flanks of Kilbourne Hole that provide access to those sections of the crater rim. It you wish to visit this area, be sure to obtain the USGS Kilbourne Hole, New Mexico, 7.5 minute topographic map to help you plan your trip. Although I like to approach Kilbourne Hole from La Union, NM, you can also take a route that winds through the El Paso suburbs. A detailed log for that route is kindly provided here, courtesy of Markus Boenisch, UTEP
[2012 Update: The Southern Pacifice Railroad has built a new siding in the area of Lanark. The country roads have been slightly altered but the above directions are still correct. The county peridoically grades the dirt roads into Kilbourne Hole, but expect the worst.]
Now for a few words of caution about collecting in this area. You will be in a desert. Don't expect to find water, shade, telephones, toilets, mini-marts, soda machines, or any human ammenities in the area (my cell phone did receive a strong signal and functioned well from the rim of the crater). Because of intense summer heat, daytime summer temperatures are normally above 100 degrees F. Plan your trip for the winter months; late November through February. During those cooler months you are likely to be joined at "the hole" by other collectors, ATV enthusiasts, hikers, gun lovers (about 20 Texans were enjoying target shooting when I was there), etc. At other times you may be entirely alone. If you break a leg, or your car breaks down, be prepared to deal with that possibility on your own. The area is sparsely vegetated with yucca and bush mesquite. Although I encountered none, several varieties of poisonous rattlesnakes inhabit the region. Watch where you step. Don't stick your hands in any rock crevices and the snakes won't bother you. Hiking around the crater rim, above the basalt lip, is relatively safe. The slopes vary from firm compacted tuff to loose sand. Although portions of the prime collecting area are quite steep, safe footing is usually available. Just watch your step, be careful, and you will be okay. If young children are on your trip, they should be carefully watched, and should probably not be taken far from your vehicle.
Kilbourne Hole is designated a National Natural Landmark. To the best of my knowledge it is open to public access. The crater rim lies on federally owned land. The New Mexico/El Paso BLM surface management map (1999 edition) shows that the bottom of the crater is largely private property. The entire crater area is used for cattle grazing; a barbed wire fence transcects the northern third of the hole. Be certain to respect property rights; never damage a fence, and leave all gates as you find them. Because land ownership and status continually change, it is the visitor's responsibility to assure that access, mineral collecting, gun shooting, ATV driving, beer drinking, etc., is still allowed (in other words, DON'T ASK ME, CALL THE LAS CRUCES BLM OFFICE to confirm that you are still allowed to engage in whatever activity you plan for your visit).
[If you have the Google Earth Plugin installed on your computer, and a fast internet connection, you may enjoy visiting this web page to see a 3D topographic tour of Kilbourne Hole]
Okay, so what did I find, and where did I find it? As you know, I was looking for facet grade peridot, the name given to gem quality olivine. Chemically, peridot is a magnesium-iron orthosilicate with the general formula (Mg,Fe)2SiO4. In nature one can find peridot that varies in composition from 100% Mg to 100% Fe content. Even elements such as Cr can get involved in the mix. The result of all this natural blending is peridot that varies tremendously in color; from a rather ugly brown to a beautiful slightly yellowish-green. As previously stated, Kilbourne Hole peridot was carried to the surface as globular shaped masses included in a fluid basalt magma. The explosion that created the crater threw masses of olivine, basalt, ash, and country rock high into the air. Most of the ejecta fell back into the crater and piled up around the rim. After the formation of the crater, sediments were transported into and onto the crater floor and now cover anything of much interest to the mineral collector. It is the crater rim where olivine xenolith "bombs" can most easily be found. Much of the crater is rimmed by a 5 to 10 feet thick layer of black basalt. Above this basalt is a grayish layer of explosion breccia (broken rock fragments) that consists of angular basalt fragments of various sizes, olivine xenoliths, and assorted fragments of sedimentary rocks, all loosely consolidated in a grayish volcanic ash. Weathering out of this hodgepodge of volcanic ejecta are the nodules of olivine that are the object of our search. Unfortunately, most of the olivine in these nodules is shattered into tiny grains not much larger than rock salt. Hike around the rim and you will see thousands of nodules that have been cracked open by me and numerous other mineral collectors. Beautiful little piles of green sand litter the rim and slopes. With a little close observation you will begin to spot nodules that have recently eroded from the breccia, or have somehow been missed by others. The olivine xenoliths vary in size from as small as one inch to over two feet in diameter. Their surfaces will be grey to a rusty brown color. Occasionally, you will find a xenolith that still has a large mass of basalt clinging to its surface, or even a huge block of basalt that has a small included nodule of olivine.
Fifty years ago a collector could simply walk around the crater and find nodules that had naturally weathered and fractured to reveal their contents, be it green sand or a 10 carat gemmy peridot fragment. Today, lacking the X-ray vision of superman, I find it necessary to give each nodule a sharp whack with my rock hammer. Usually one sharp blow is sufficient to reduce a nodule to a pile of green sand. After breaking a couple hundred nodules you will begin to recognize those which have a chance of containing peridot fragments of larger size. If you can't find any large fragments of peridot, don't get discouraged; keep walking, keep wacking those nodules, and pay careful attention to the appearance of the peridot as the nodules fall apart under the impact of your hammer. Perhaps one in a thousand nodules will be more coarsely granular than the others. If you come across one of these rare peridot xenoliths, take your time and more carefully "dissect" it with your hammer. Sometimes a six inch nodule might contain only one small area that yields a nice 5-10 carat fragment of facet grade olivine. Other times you might hit the "mother lode" and find a nodule that contains coarsely fragmented peridot throughout, and many clean fragments large enough to cut gemstones. It doesn't happen often, but if you persevere, you can find one or more of these rare nodules. Believe me, it is thrilling to spend six hours finding nothing but highly shattered olivine, then strike a xenolith and have it cleave into a pile of facet grade peridot shards. A real adrenaline "high" for a gem collector.
There seems to be no rhyme or reason as to which xenoliths contain the more coarsely fragmented peridot. The "good ones" can be large or small, on this or that section of the crater rim, dark brown or fine green in color. Collecting here is much like playing the lottery. Your investment is sweat labor (and an aching back). In this game, if your number hits, it is usually a very nice prize. On this trip I spent three eight-hour days hiking the crater rim, subjecting perhaps one or two thousand olivine nodules to the "Estwing test." The result was a total of four nodules that contained peridot fragments large enough to cut one to two carat gemstones. Two nodules were a less desirable olive-green color, two nodules were the more desirable yellowish green. One of the four gemmy nodules I found had apparently been split by natural weathering. I picked it up and immediatly noticed that it was very coasrsely fragmented. It was brought home, more or less intact, for photography and more careful "dissection."
Since my trip out to the hole, I have had time to facet two pieces of peridot collected during this trip. One piece finished a 2.13c dark olive green triangular brilliant gemstone with a very unusual vessicular gas bubble inclusion. The second piece finished a gorgeous 1.56c yellowish-green cushion cut. You can imagine how highly I prize these two hard-earned gems.
So, if you truck out to "The Hole," will YOU find any facet grade peridot? Maybe. It all depends on how long you stay, how many olivine nodules you can find and examine, and how hard you work. It can be very discouraging spending hours hiking around the crater finding nothing but green sand. It is equally exhilerating when that last olivine nodule splits under your rock hammer and reveals bright yellow-green glassy fragments of peridot a quarter to half inch in size! Do I think my collecting trip to Kilbourne Hole was worthwhile? You bet! Just wish the place was closer to North Carolina. Post Script: I was going to write an article on this collecting trip for Lapidary Journal or Rock & Gem Magazine, but why kill a tree? I have plenty of internet server storage capacity to load this report on my commercial web site. Besides, you are probably a fairly dedicated mineral collector if you have managed to find these web pages, and have read this far. If you have found this article informative, and wish to thank me, you might consider buying your wife, daughter, or girlfriend, one of my custom cut peridot gemstones. The pricing is very reasonable and the cutting will be exquisite. Pardon my lack of modesty, and crass commercialism, but I do have many satisfied customers. If you don't have any need for a peridot specimen, or a faceted gemstone: happy collecting at "The Hole!"
References: "The Peridot Deposits of Kilbourne Hole, New Mexico," by James D. Lindberg, October, 1964, Lapidary Journal. "Kilbourne Hole Peridot," by John R. Fuhrbach, Spring, 1992, Gems & Gemology. Volcanoes of North America by Wood and Kienle, 1990, Cambridge Univ. Press