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Posted by Rock Currier
Rock Currier May 11, 2012 06:25PMThis article has been prepared for the Mindat Best Minerals project. The aim of this project is to present information on important localities and specimens for each mineral specie. As new finds are made and new knowledge is made available the individual articles will be revised to include this information. Readers are encouraged to contribute by posting a response in this thread. All revisions will be stored, thus ensuring traceability and availability of previously included information. A complete list of articles can be found in the list of finished Best Minerals articles. To cite this version: Currier, R. (2012): Benitoite. revision 1.0. Mindat Best Minerals Project, article "mesg-67-260515". Please be advised that the photos cannot be used without the consent of the copyright holder
Benitoite is a fairly uncommon mineral and Mindat lists about 20 localities, only one of which is of considerable interest to collectors. As you look at the images presented below, it will become clear why this is so. The hand book of minerals lists 6 cm as the largest size for Benitoite crystals, but probably at least one exceeds that size. The best locality for Benitoite by far is the the Gem Mine in San Benito County, California, USA. It has produced tens of thousands of Benitoite specimens over the last 100 years.
Honshu Island, Chubu Region, Niigata Prefecture, Itoigawa City, Hashidate
Honshu Island, Chubu Region, Niigata Prefecture, Itoigawa City, Hashidate, Kinzandani
Honshu Island, Chubu Region, Niigata Prefecture, Itoigawa City, Ohmi, Ohmi river (Ohmi-gawa)
USA,Arkansas, Hot Spring Co., Magnet Cove, Jones Mill Quarry (Martin Marietta Quarry; Highway 51 Quarry; Mid-State Quarry)
California, San Benito Co., Diablo Range, New Idria District, Picacho Peak, Clear Creek area, Hernandez, Clear Creek, Mina Numero Uno
California, San Benito Co., Diablo Range, New Idria District, San Benito River headwaters area, Dallas Gem Mine area, Dallas Gem Mine (Benitoite Mine; Benitoite Gem Mine; Gem Mine)
Here is an article written about the Gem by John Veevaert about the Gem Mine that was published in a recent magazine that he has give Mindat permission to reproduce here.
Benitoite – almost every mineral collector desires to have a specimen of this unique blue mineral in their collection. Aside from a few small nearby deposits only the Benitoite Gem mine has commercially produced benitoite in gem quality and specimens of stunning beauty. Since its discovery in 1907 literally tens of thousands of specimens of benitoite and neptunite have been produced. It is a slow and involved process to remove the encasing natrolite with various chemicals but the effort is worth it to preserve the mineral heritage from one of the most unique mineral deposits in the world. The State of California recognized the significance of this distinctive mineral by declaring it to be the Official State Gemstone of California. On October 1, 1985 in Sacramento, California, the State legislature passed Assembly Bill no 2357. The text was simple but it was everything that everyone who knew anything about benitoite had ever hoped for. Section 425.3 is added to the Government code to read: ”Benitoite is the official state gemstone of California”.
The Benitoite Gem mine, formerly known as the Dallas Gem mine, is situated near the southern tip of San Benito County about 30 km to the north and west of Coalinga, California. The town of King City lies about 45 km to the west of the mine. In a broader sense the mine lies about half way between San Francisco and Los Angeles. When people think about California they tend to think of lots of people and endless beaches. This section of California is considered remote by just about any standard imaginable. Few roads and amenities, that most people are used to, can be found for 30 km in any direction. The only way to get near the mine, other than a privately owned chip sealed road, is via dirt road and most of it is quite bumpy and requires several stream crossings. Travel in the winter time is nearly impossible due to snow or very slick, muddy roads. The mine is set jut upslope of the San Benito River at an altitude of 1,380 meters.
The coastal mountains between San Francisco and Los Angeles are expansive and, even today, for the most part, empty of any human development other than ranches and a few small towns and roads which transect them. Imagine the difficulty in accessing this region in 1907, the year benitoite was discovered, when there were no roads and the only means of moving through the area was on foot. The vegetation in the area consists of very thick chaparral that is seemingly impossible to walk through yet that was the task set in front of James Couch as he left Coalinga to prospect the area in southern San Benito County for new mercury deposits in early February.
HISTORY OF THE DEPOSIT
James Marshall Couch had been grub staked by Roderick Dallas and Thomas Sanders with $50 worth of supplies and a horse. It was mid-winter when Couch set out so there was likely some cold weather to contend with as he made his way into the headwaters of the San Benito River looking for traces of minerals eroding out of the adjacent hills. It is reported in several sources that he found a forested glade along the small river and made camp after having been prospecting for several weeks. This offered him a flat place with grass for his horse to feed on and plenty of firewood for himself to keep warm during the long cold nights.
Accounts of the discovery report that on February 22, 1907 Couch climbed the hill across from his camp to look for possible outcrops to investigate. On his way up the slope he stumbled on a small area literally littered with myriad small dark blue crystals. He initially suspected the crystals to be diamonds or sapphires owing to the blue color and set about to collect a small hoard then raced back to Coalinga to announce his discovery. It is possible that the Native Americans who inhabited this region may have seen this deposit at some point in the distant past on a hunting trip but as far as Couch could tell no one had ever been to this spot before. Credit for the discovery was taken by other people, primarily Roderick Dallas and another fellow by the name of Leland Hawkins but sifting through the historical records suggests that Couch was the sole discoverer of the deposit. Imagine this outcrop weathering for millennia with no one knowing of its existence and then imagine being the first person to have ever seen this rich blue mineral laying in abundance on the ground…
It did not take long for the area to be legally claimed and mining to commence. (The claim was given the name of Dallas Gem mine in recognition of the financial backing provided by Dallas for the operation.) Still, none of the characters who were involved knew what it was that they were mining. Speculation centered on the crystals being sapphire or spinel or even some form of volcanic glass. Later in 1907 a small group of specimens found their way to Dr. George Louderback who was professor of mineralogy at the University of California, Berkeley. Louderback quickly recognized that this blue mineral was new to science. He also thought that the accompanying dark black mineral, associated with the blue mineral, was also new to science. He set out to publish a preliminary note in July 1907 on the new species and tentatively called the blue mineral benitoite named after San Benito County and the San Benito River. He suggested that the black mineral be named carlosite after the nearby San Carlos Peak. Later in 1907 Louderback determined that carlosite was in fact neptunite – a mineral which had been discovered in 1893 in Greenland.
After securing more specimens and having a chance to actually go to the mine site Louderback commenced to formerly describe the new mineral benitoite. Many people consider this one of the finest descriptions of a new mineral species ever written. It was published in 1909 with the chemical analyses being completed by Walter Blasedale. All of the hand-colored copies were quickly dispersed to universities and mineralogists around the globe.
Initially all of the recovery of benitoite consisted of breaking open the encasing natrolite by sledge to find the gem nodules. Perhaps hundreds or even thousands of stunning specimens were lost through this method.
It did not take long, however, for the miners to learn that natrolite was soluble in various acids. It must have been quite a scene to see hundreds of kilos of natrolite veins in acid to expose the enclosed benitoite crystals. Today using acids and other chemicals are used to prepare specimens and recover gem rough. However, greater care and effort are taken to clean aesthetic specimens by trimming excess matrix and protecting some of the natrolite which provides a striking contrast between both benitoite and neptunite.
The Dallas operation of the mine went on until 1910. At that time operations were ceased and the mine abandoned. Dallas, however, maintained the assessment work on the claim and was able to patent the ground maintaining permanent control over the deposit. From then until the 1960’s the Dallas family leased to the property to various operators who recovered specimens and gem rough. Perhaps most noteworthy of this group were Pete Bancroft and Ed Swoboda who visited the mine several times between 1935 and 1938 collecting thousands of specimens. In the 1930s it was still no small task to get this remote location. In 1935 they set out to visit the deposit after having been given directions to the locality from their high school teacher, Frank Gulick, who had actively collected at the deposit the year before in 1934.
From the 1940s until 1967 the deposit was leased by several people with various levels of production. Most notably Miller Hotchkiss in 1952, Clarence Cole from 1953 to 1967, and Josephine Scripps, sub-leased from Cole, in 1966. Their efforts were modest in terms of overall production.
In 1967 Elvis “Buzz” Gray and Bill Forrest leased the property. They were experienced operators and set about to rehab the mine site and develop a logical plan for the deposit. Their efforts produced countless thousands of good specimens and considerable gem rough. It was also their effort to promote benitoite as a gemstone that raised the awareness such that California designated the mineral as the official gemstone of California in 1985. They bought the mine in 1987 from the Dallas family. They continued to operate the mine from 1967 until 2000. Frequently seen at the mine was Fresno County Sheriff’s Deputy Bob Kahl. Bob was there to lend some security to Bill and Buzz and also to help out as needed.
In 2001 Gray and Forrest sold the mine to Collector’s Edge of Denver, Colorado. Bryan Lees, the principal owner of the company, oversaw a complete and thorough mining of the remaining deposit. He exposed the original vein system and found that it had been completely worked out. His operation also ran the entire dump material, through a custom jig system to separate gem rough. He then set out to process the colluvial and eluvial material that had eroded from the original deposit over the millennia. A conveyor belt also carried larger rocks off and each, while wet, were visually inspected for specimen potential. His operation produced many tons of specimen grade material and thousands of carats of gem rough. The mine site was completely rehabilitated according to Surface Mining and Reclamation Act standards in 2004.
Bryan Lees sold the mine to Dave Schreiner of Coalinga in 2004. Dave was determined to create a fee dig operation there but has been hindered with environmental regulations set by the Environmental Protection Agency and Bureau of Land Management due to the vast amount of naturally occurring asbestos. Reckless All-Terrain Vehicles and motorcycles over the last 50 years have created enormous erosional problems as well so the agencies established a closure of the area to all vehicular traffic allowing only private property owners and valid claim holders access. For the past several years Dave was trucking material from the mine to the Los Gatos Community park just outside of Coalinga and allowing people to screen the material for gem rough and specimen material. The opportunity to find specimen material or gem rough does not currently exist unless groups access the mine via a private chip sealed road. Even then the previous mining efforts by Collector’s Edge left very little material behind.
In 2008 John Veevaert of Weaverville, California (author) and Steve Perry of Davis, California negotiated the purchased the entire nine plus tons of inventory of benitoite specimens, partially processed and unprocessed mine run material from Collector’s Edge. We have made some of this material available for collectors to purchase and try their hand at cleaning their own specimens. We are also processing a lot of the material for specimens and minor gem rough.
The vein systems at the Benitoite Gem mine have been exhausted. Collector’s Edge removed all overlying colluvium and bedrock to expose what was left of the vein system. Any mineralized segments of original vein remaining at that time were removed. All of the colluvial material was processed twice to look for gem rough and specimen potential. After completion of that activity the mine site was reclaimed and recontoured.
Despite the effective mining techniques used by Collector’s Edge a minor amount of productive material still remains. The mud adhering to much of the material is tenacious and nearly impossible to remove without repeated wetting and abrasion. This masking mud allowed considerable material to pass through the washing system undetected and was eventually used in the reclamation work. Hence, some of this mineralized material will continue to be found through the action of rain and intense field collector interest. The mine, however, will never again produce at a commercial scale.
THE GEOLOGY OF THE AREA
The Benitoite Gem mine is located in the New Idria district situated in the southern end of the Diablo Range. The district has been prospected since the early 1850s for gold, mercury, chromium, asbestos and mineral specimens. The district is found within a large body of serpentinite which was tectonically emplaced into surrounding sedimentary and metamorphic rocks in the Jurassic. A subsequent plate collision put downward pressure on the serpentinite. However, being of a lighter density portions separated and migrated upward the overlying layers of rock. This established the foundation of a setting where these portions of serpentinite would experience low temperature and high pressure metamorphism – that of a blue schist facies. During the mid Miocene the region was intruded by small igneous bodies – primarily of syentic composition. This produced numerous calc-silicate vein systems throughout the district. It was one of these systems, altering the blue schist rock, that deposited the mineral resources found at the Benitoite Gem mine.
ORIGIN AND STYLE OF MINERALIZATION
The elements that comprise benitoite are thought to have been mobilized from the host rock and deposited in a late stage cooling of hydrothermal fluids in calc-silicate veins fracturing the wall rock of blue schist blocks. Other nearby benitoite deposits have a similar setting so this bears out the hypothesis. Previous geologic investigations have suggested that the benitoite mineralization was subsequent to the formation of the blueschist. Their analysis place the age of the blueschist at between 100 and 160 million years of age and the benitoite present in the cross cutting calc silicate veins at approximately 12 million years. All benitoite in situ at the Benitoite Gem mine is confined to blueschist blocks altered by hydrothermal veins. The last phase of mineralization was confined to a pulse of natrolite that set into the vein system. The vein filling natrolite is found to encase most, but not all, of the minerals deposited in the earlier phases. The upper zone of the Benitoite Gem mine has a large concentration of albite veins and is void of natrolite. It is in this upper zone that rich, deep blue and lustrous crystals of benitoite were found commingled with milky white albite.
Most of the natrolite veins are less than 2 cm thick. The minerals of interest at the mine are confined to the vein systems and frequently are attached to both vein walls. This creates an obvious problem during specimen preparation as one side of the vein has to be mechanically removed to offer a chance at producing a specimen.
Though the lower level portions of the vein system were typically filled with natrolite some pockets were left intact. These pockets typically display blocky natrolite crystals and in some cases neptunite and benitoite crystals are still exposed. The size of these pockets is generally quite small rarely exceeding 2 cm in size. As mentioned earlier, the upper zone is void of natrolite. All minerals found in that zone tend to be only hidden by clay minerals which can be easily removed with water. Pocket zones were still not large and mineralization was much more scattered but the crystal quality of benitoite and joaquinite is much higher not having been subjected to the natrolite bearing solutions.
MINERALS FROM THE BENITOITE GEM MINE.
Benitoite is the primary mineral of interest from this deposit. The chemical formula for benitoite is BaTiSi3O9. It is a ring silicate and crystallizes in the hexagonal crystal system. Early in the theoretical development of crystallography it was hypothesized that there was a class of the hexagonal system that would produce trigonally shaped crystals. The discovery of benitoite provided the mineral world with the first species known to crystallize in the ditrigonal-dipyramidal class of the hexagonal crystal system. This class is referred to as the “Benitoite Type” in Dana. Twinning of benitoite occurs on the c-axis (0001) with two crystals rotated 60 degrees. Equidimensional twinned crystals are referred to as a "Star of David" owing to their perfect six sided star. Twinning is only found in benitoite crystals included with crossite. Complete crystals of this form are very rare.
Benitoite is found in two classes at the mine – a euhedral crystals heavily included with amphibole minerals such as crossite or actinolite and as crystals attached to the vein walls. It is in the latter areas that gem benitoite has been recovered. The crystal faces of benitoite were selectively etched during the natrolite phase of mineralization. Rare examples of crystals exists with all brilliantly lustrous faces occurring but in most benitoite the c face and prism faces retained their luster while the pyramidal faces became duller due to etching during the natrolite phase.
Other principal minerals found in the vein system at the mine include neptunite, the joaquinite series, frensoite, silica pseudomorphs after serandite, apatite, jonesite and various sulfides of copper including chalcocite, djurleite and covellite. The sequence of minerals has been determined to be: albite, apatite, silica pseudomorphs, neptunite, copper sulfides, benitoite, joaquinite series, jonesite and natrolite which filled most of the vein system.
Pervious literature suggested that jonesite was not found in situ with benitoite. The author has several specimens and knows of others that show a commingling of benitoite and jonesite together. So their emplacement in the vein system may have been concurrent.
Not all benitoite is created equally. While the more popular color is a rich sapphire blue with a hint of violet, benitoite comes in clear, white, pink, reddish brown or greenish-gray color as well. The origin of the color of benitoite has not been completely determined as of yet. It does contain traces of iron (Fe), hence it has been proposed that color may be due to the Fe2+ - Ti4+ or the Fe2+ - Fe3+ intervalance charge transfer. Joan Mamarella (personal communication 1997) suggests in her thesis that the blue color is derived from the titanium in the blue portions of the crystals being paramagnetic while the titanium in the white portions are diamagnetic.
Benitoite is very strongly dichroic such that when shifted in the proper orientation it looks either colorless, blue, or rich violet-blue in color. Benitoite has a high birefringence - higher than that of diamond. Hence, cut stones come alive with fire from refracted light. The blue color of benitoite is not affected by any treatment such as heat or irradiation. The colorless sections of a benitoite crystal have been changed to orange when heated (Bill Forrest personal communication 2002).
Benitoite is also found, frequently, as gray to greenish colored crystals with very heavy inclusions of the amphibole crossite. Some crystals of benitoite appear red to dark maroon due to inclusions of minute neptunite crystals. The vast majority of crossite included are floaters with complete terminations on both ends.
Benitoite is strongly reactive under a short wave ultraviolet (SW UV) light source. It fluoresces a very bright, opaque, sky blue color. The fluorescence seems to be stronger in crystals with crossite inclusions than crystals of gem quality. Some crystals fluoresce a dull reddish orange color under Long Wave Ultraviolet light. The strong reaction under SW UV light is a very useful tool for locating specimens at the mine and for prospecting.
The primary use of benitoite is for mineral collectors and use as an ornamental gemstone. Though not suitable for use in rings due to its hardness of 6.5 it is dazzling in pendants, earrings and necklaces. The vast majority of finished gems are under ½ carat weight. Gems over 1 carat are rare and gems over 2 carats are considered very rare. The largest known piece of rough recovered was a 34 carat piece found by the current owner Dave Schreiner shortly after he acquired the mine from Collector’s Edge, Inc. It produced a finished stone of over 8 carats and several smaller stones. The largest known faceted benitoite is owned by Mike Scott and is a 15.42 ct stone.
The prices of finished gemstones have skyrocketed in recent years. As a gem benitoite is orders of magnitude scarcer than diamonds, emeralds and rubies. High quality finished stones of one carat with good clarity and color will fetch $3,000 or more. Stones of two carats can bring $8,000-$10,000. It will continue to appreciate in value in future as the supply is limited and future commercial production will not happen.
Most new minerals discovered in the last 100 years have been as a result of some unusual diffraction pattern in an X-Ray analysis or as some microscopic crystals that could only be appreciated under a scanning electron microscope. Benitoite caused an immediate sensation with its large well-formed crystals accompanied by equally striking crystals of neptunite and joaquinite. There have been few occasions in the history of mineral discoveries when a new species was found in such magnificent crystals.
The Benitoite Gem mine has produced the world’s finest known crystals of neptunite. It’s chemical formula is: KNa2Li(Fe2+, Mn2+)2Ti2Si8O24. The principle habit is prismatic though some stubby crystals are known. At a macro scale neptunite appears to be jet black in color. It is actually a deep red color which is seen at certain angles in large crystals or in micro crystals where light can pass through it. Neptunite two types of twins. The most common is a twin on (301). The rarest twin is that found on Miller Indices (001). To my knowledge there are no more than eight or nine of these are known.
JOAQUINITE – Ce (TL)
Joaquinite was first discovered at the Benitoite Gem mine. It’s chemical formula ls: NaFeBa2Ce2(Ti,Nb)2
Jonesite was discovered in 1957 by Francis Jones. It was later described in 1977 as a new species. It is by far the rarest mineral found at the deposit. It’s formula is: Ba4(K,Na)2
Natrolite is comprised of: Na2Al2Si3O10 • 2H2O. It is ubiquitous throughout much of the deposit. It was deposited in the last pulse of mineralization in the vein system and is only rarely encountered in crystals. Where the veins did not entirely fill with natrolite stubby, prismatic crystals are found. The crystals are invariably milky white and opaque. Much of the well crystallized natrolite in collections labeled as coming from the Benitoite Gem mine is in fact from several other nearby deposits.
In addition to the above species the most commonly encountered include the copper sulfides djurleite and chalcocite. Other noteable species include: apatite, albite, manganese oxides, silica pseudomorphs presumed to be after serandite and analcime. Several oddities are also known including boatite and a unique specimen of wire silver exsolved from a djurleite crystal.
Most of the early efforts to recover benitoite was done mechanically and was focused primarily on the recovery of gem rough. This resulted in countless thousands of specimens that were battered and not suitable for collections. Once the use of acids was employed to remove the natrolite the quality of specimens increased exponentially. It is hard to rank the finest specimens but two specimens in the LA County Museum are considered to be among the best – a large 3 cm triply terminated crystal and a fabulous ring of adjoined benitoite crystals. A smaller specimen in the author’s personal collection is considered one of the finest with the three species – benitoite, neptunite and joaquinite arranged in an aesthetic position. It has been dubbed the “sushi plate”.
So, what does it take to get a cleaned specimen of benitoite and/or neptunite? Attractive specimens of this beautiful mineral assemblage just don't happen - they are literally crafted! The process involves several steps which are all time consuming and require a bit of artistic aptitude. One thing to keep in mind during the process is that removing matrix is a one way procedure. Once it is removed it cannot be put back. Too much removal is as detrimental to the finished specimen as leaving too much.
Several steps applying an etch inhibitor, soaking in acid and rinsing are necessary to remove enough of the natrolite so as to end up with an aesthetic specimen. More information on how to clean specimens from this deposit can be found at benitoite.com.
To be done right an average specimen requires no less than 10-20 days of attention. Many require a great deal more. In the past I have personally spent over a month’s time on one rock! Doing the work with batches of specimens is the only way this becomes feasible and economical. It is no wonder that prices for the top levels specimens are what they are for this rare mineral given the amount of time it takes to make these specimens look the way they do. The labor costs of cleaning specimens from the Benitoite Gem mine correctly are rarely recovered...
A final word here on benitoite from the Benitoite Gem Mine. I am guessing that 98% of all benitoite crystals have a matted luster. The pinacoid and prism faces will be lustrous 10 times out of 10. The pyramidal faces, however, will not. That is just the way Mother Nature created these things. Some crystals will be lustrous on all faces and those specimens are rare. The other thing to remember is when something has sat around for millions of years some fracturing is likely to occur. The vast majority benitoite crystals were fractured though eons of natural processes. When you are finished etching the specimens with benitoite you will likely see the fractures as the specimen dries. There is nothing you can do to change this fact, so accept it. I etch hundreds of specimens a year and find the severe fracturing probably 80% of the time and put most of those specimens in the wholesale flats. That is why the high quality benitoite specimens command high prices. They are, unfortunately, the exception to the rule.
The original article was published by Minerals, Issue 3, 2011: http://www.mindat.org/article.php/1333/Minerals+Newspaper+-+Issue+3
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Crystals not pistols.
Edited 52 time(s). Last edit at 12/20/2015 04:45PM by Olav Revheim.
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Copyright © mindat.org and the Hudson Institute of Mineralogy 1993-2017, except where stated. Mindat.org relies on the contributions of thousands of members and supporters.