Google Earth© image of area around Milford in southwestern Utah.
There are certain half-dreaming moods of mind in which we naturally steal away from noise and glare, and seek some quiet haunt where we may indulge our reveries and build our air castles undisturbed. Washington Irving
But, times marches on and we needed to get the pipeline route cleared so a call to duty was sounded. But then we had a serendipitous moment for just on the other side of the tree was a large area covered by black obsidian flakes. A spectacular site that confirmed our idea that Native Americans also had enjoyed our view!
Now finding black obsidian in southwestern Utah is not an unusual occurrence; however, these flakes had been worked and were the “leftovers” from the making of projectile points. We found the flakes since loose sand was been removed from the area by shifting winds. We examined a few of the flakes and then left them undisturbed in their natural state. No need to disturb the Gods.
Another event, perhaps as exciting as the chipping area, was noting the power station near Milford generating electricity from geothermal resources. Neither I, nor my crew, had seen such a creature. The Roosevelt Hot Springs Geothermal Area was commercially developed in the early 1900s, mostly as bathhouses, hot pools and adjacent hotels. In 1984 the power plant was brought on-line and uses steam powered turbines created by bringing up hot brines via wells. The area is perched on top of a fractured and faulted subsurface intrusion allowing circulation of the hot brines. At times, hot water followed a fracture and created a surficial hot spring (and filled a swimming pool). At other places, the hot water deposited amorphous silica and opal. One such place we observed was Opal Mound along the Opal Mound Fault. Opal Mound is essentially a venting area for silica-rich hot water; however, the deposit is rather common opal and non-precious.
I had sort of forgotten about Milford and the opal and moved on to various other projects and interests. However, a few years ago while touring the Electric Park venue, one of the Tucson shows, I ran into a couple who were selling Utah Lace Opal (ULO). I am always interested in Utah minerals and therefore had a nice conversation Larry and Joyce Wright, the owners of Aspen Rock and Gem. It seems the couple had been rockhounding and looking for Opal Mound but found it claimed and so headed out to hunt for obsidian. What they discovered on their hike was not obsidian, but a banded “hard foamy rock” that could be stabilized and then cut, polished and used as jewelry in cabs or wire wrapped. The trade name for this rock is Utah Lace Opal.
Utah Lace Opal, non-stabilized. Width ~5 cm.
I purchased a small sample of ULO that was not stabilized but simply cut. Without stabilization, the ULO has numerous vugs that create the “foamy” appearance. So, Utah Lace Opal is a proprietary name but what about the description of the opal?
Much opal in the world can trace their origin of silica to marine or fresh water single-celled organisms called diatoms and radiolarians (both have siliceous skeletons). However, the ULO traces its origins to silica-rich mineralized hot water where the solution captured the silica from rocks in the subsurface. As these high-temperature fluids reached the surface they experienced rapid cooling and an amorphous silica precipitated as opal (sometimes called silica sinter or geyserite).
Although opal is commonly called a mineral, it is a mineraloid since it varies in chemical composition and is microcrystalline and/or amorphous since the atoms do not occur in a regular structure; however, there is an orderly arrangement of closely packed spheres of silica. One distinguishing feature of opal is the amount of water in the atomic structure, perhaps up to 20%: SiO2-nH2O where n represents a variable amount of water.
The Quartz Page (http://www.quartzpage.de/gen_mod.html) noted that opal is classified as:
- Opal-C - microcrystalline opals made of cristobalite [ high temperature polymorph of quartz]. Transitional state in the formation of diatomites [rock formed mostly of diatom tests] and radiolarites [rock formed mostly of radiolarian tests] from opaline skeletons. Found in nodular concretions in sediments,
- Opal-CT - microcrystalline opals made of intergrown cristobalite and tridymite [another high temperature polymorph of quartz]. Found in common opal, as well as a transitional state in the formation of diatomites and radiolarites from opaline skeletons. Found in opaline concretions in sediments
- Opal-AG - amorphous opals with a gel-like structure. This is the structure of potch [common] opal, precious opal and fire opal.
- Opal-AN - amorphous opals with a network-like structure. A common example is hyalite or glass opal.
As best that I can determine, the geothermal solutions rising along faults zones near Milford deposited both common opal and hyalite; however, Aspen Rock and Gem noted “it [ULO] went from one silica phase to another forming Opal-C to Opal-CT to quartz and is seen in various stages and is still maturing. Vugs with Botryoidal Hyalite Opal clusters form stalactites and stalagmites giving lacey layers to the formation.” Since neither cristobalite nor tridymite may be identified with ordinary microscopy, I was unable to note Opal-C or Opal-CT in my specimen. It appears that I have common opal (Opal-AG) and glassy hyalite (Opal-AN).
The vibrant, and distinguishing, bands of ULO are created by common and hyalite opal with various impurities of aluminum (blue-green), magenta (manganese), orange (iron), gray (magnesium), yellow (titanium) (Aspen Rock and Gem).
Photomicrograph common opal. Width FOV ~1.2 cm.
Photomicrograph glassy and colorful hyalite opal. Width FOV ~1.2 cm.
Polished specimen of Picasso Marble so named due to the abstract nature of the veins. Width ~ 7 cm.
Another popular Utah rock with even less provenance information (at least that I can locate) is Zebra Stone or White Tiger Stone---or you can substitute rock or marble in place of stone! There seem to be several localities in the world that produce a “zebra stone”; however, most pieces quarried from Utah are labeled as such. It seems that any rock with black and white stripes from limestone to quartzite to sandstone to granite to gneiss etc. may be termed zebra rock. I have noticed zebra rock for sale at numerous rock shows and usually it is simply identified as being collected in Utah.
Photomicrograph dolomite "stripe" in Zebra Rock. Width FOV ~1.6 cm.
The piece in my collection was purchased several years ago at a rock shop in southern Utah. When asked about locality data all the proprietor would part with was a claim and quarry west of Sevier Lake in Millard County. Per the geologic map of Utah, the rocks exposed along the west side of the Lake are Cambrian and Ordovician in age. However, a stop at the BLM office in Fillmore indicated that zebra stone can be found in Lawson Cove, also in Ordovician rocks but southwest of Sevier Lake in the Wah Wah Mountains. My specimen is composed of both white and dark (blackish) crystalline dolomite. Zebra Stone or White Tiger Stone is a rock with an interesting pattern and certainly catches the attention of collectors and rockhounds.
My last striped rock from Utah has very good locality data since it collected several decades ago near Vernon, Utah, in the West Desert (Tooele County). In fact, Vernon Hills Wonderstone is one of Utah’s best known collectable rocks. It is a welded, glassy, volcanic tuff (air fall ash, rhyolitic in composition) in which the glassy particles were “welded” together by heat and compaction. In fact, mid-Tertiary welded tuffs are very common in western Utah; however, the Vernon Wonderstone is different from most in that it displays a variety of colorful concentric or layered patterns. These vibrant colors are due to post-depositional, circulating hydrothermal waters depositing pyrite (mostly) that was later oxidized by rainwater creating hematite, goethite and other iron oxides. Lapidaries love Vernon Hills rocks since the non-porous nature of the rock, along with a high percentage of silica, creates a suitable hardness for polishing.
Polished Vernon Hills Wonderstone. Width of polished surface ~5 cm.
As far as I know, the Vernon Hills location is on BLM land and is still available for collecting. In fact, during my last trip the area was covered by literally tons of pieces. There may be mining claims; however, they are well marked.
I am not a big collector of striped rocks or any of the “picture types;” however, since they were collected in Utah, home of most of my professional work, I picked them up at shows or on the ground.
Stibbett, B.S. and D.L. Nielson, 1980. Geology of the Central Mineral Mountains, Beaver County, Utah: Department of Energy Division of Geothermal Energy Contract DE-AC07-78ET28392, DOE/ET/28392-40.