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Figure 1. Two specimens with triply terminated crystals of benitoite.  The crystal on the left is
1.4 cm across and the crystal on the right is 1.1 cm across.  John Veevaert specimens & photos.

Benitoite
by
John Veevaert
All photographs and text are copyrighted by the author.
The text and photos may not be used without permission.
December 5, 1997
(Revised October 10, 2001)
(Revised November 10, 2002)

Introduction

Of all the known minerals few have has a distinguished a place in the world of mineralogy and the hobby of mineral collecting as benitoite (pronounced "ben-ee-toe-ite"). Benitoite is named after San Benito County, California where it was discovered in 1907 (The locality is at the very southern tip of San Benito County.  A few more kilometers to the south and benitoite could have ended up with an entirely different name!). The history of the discovery of the Benitoite deposit is a bit clouded with several accounts but the main characters in the discovery are James M. Couch, L. B. Hawkins, T. E. Sanders and R.W. Dallas.  James Couch is credited as the person who first found the deposit but thereafter who owned it gets a bit convoluted and contentious.

When first discovered it was initially thought to be sapphire. Samples were sent to the University of California, Berkeley for confirmation/identification and soon, after careful analysis by Dr. George Louderback, it was found to be a new mineral. Louderback recognized the mineral as being new to science and began the process of describing it along with two other minerals found that he thought were also new to science.  He formally named the mineral benitoite and called the other two new minerals carlosite and joaquinite.  After further analysis carlosite was found to be neptunite (see Figures 22, 23 and 24), which had been previously described from specimens found in Greenland and the Ural Mountains in Russia.  Even so, the specimens of neptunite from the Gem Mine were far superior to any of the other known occurrence for the species. Joaquinite (see Figures 23 and 28) was a new mineral species but remained incompletely described until the 1970s.

On October 1, 1985 California Governor George Deukmejian signed Assembly Bill No. 2357 formally designating benitoite as the official gemstone of California.  Benitoite of gem quality is found only in California.

Since the discovery of benitoite its popularity and appeal as a gemstone and mineral specimen have continued to expand.  Good specimens of benitoite are in constant demand as new collectors enter the hobby and are exposed to the unique crystal form and striking contrast of both benitoite and neptunite on the same specimen.  It is still only found in California in anything resembling good crystals and gem quality material is only found at the Benitoite Gem Mine.  The mine was sold by Bill Forrest and Elvis “Buzz” Gray in 2001 to Benitoite Mining, Inc. (BMI) a subsidiary of The Collector’s Edge based out of Golden, Colorado.  BMI is currently operating the mine for the production of gem rough and specimens. Trinity Mineral Company (John Veevaert) and Steve Perry Gems (Steve Perry) are working with BMI to market specimens on the Internet through BenitoiteMine.com.
 
 
 

Figures 2 & 3.  View of the landscape in the vicinity of the Benitoite Gem Mine. Note the clearly visible peridotite landscape that is very sparsely vegetated.
Santa Rita Peak is the highest point in the photo.  The photo on the right is of the King City Asbestos Corp. (KCAC) mine located about 1km to the west
of the Benitoite Mine.  It is now a superfund site due to the enormous quantity of exposed asbestos.  (Asbestos is common throughout the serpentinite.)

Locations for Benitoite

Benitoite has been found in several localities in the vicinity of the Dallas, or "Benitoite" Gem Mine (see Figures 4, 5 & 6) as recent owners Bill Forrest and Buzz Gray named it. However, only the Benitoite Gem Mine has produced gem quality benitoite in commercial quantities. The Junnila mine located about 7-8 km to the northwest of the Benitoite Gem mine also has allegedly produced some gem benitoite but it is a very small and insignificant amount. Three other known deposits containing benitoite are located near the Benitoite Gem Mine. They are: the Numero Uno Mine, the Victor Claim and Santa Rita Peak Property. Benitoite from these three locations, all within a 10 km radius of the Benitoite Gem mine is generally very poor in quality but the classic triangular shaped crystals are found at each locality. Verified occurrences of benitoite have documented from the following localities: in Japan, in serpentinite along the Kinzan-dani River; at Broken Hill in New South Wales, Australia in gneiss of granitic composition; in Arkansas at the Diamond Jo quarry near Hot Springs in lithophysae found in syenite (Henry Barwood, personal communication, 1997); and in near Big Creek in the Sierra Nevada foothills of eastern Fresno County where it occurs as minute grains associated with an occurrence of granodiorite (this is located about 200 km to the east of the San Benito localities). Earlier reports of the mineral being found as grains in Texas and Belgium were misreported as benitoite. Bentonite - a clay mineral - was the mineral identified at those locations (Henry Barwood, personal communication, 1997).

It is highly probable that benitoite occurs elsewhere in the world and it is only a matter of time until new occurrences for the mineral are found. It is also likely that additional localities for benitoite exist in the vicinity of the Benitoite Gem mine.
 

Figure 4. Composite view of the Benitoite Gem Mine as it looked in February, 1990.


Figure 5. Composite view of the Benitoite Gem Mine as it looked in August 2001.
To view an mpg of the mine from this spot click here


Figure 6. Straight on view of the entire mine from across the San Benito River
ravine August 2001. To view an mpg of the mine from this spot click here.
The mpg starts with a view of the KCAC Asbestos Mine to the west.

The following list provide links to other mpg files of the Benitoite Gem mine area:
 
 

View of the mine from the southwest
View of the region to the southwest from the KCAC mine
View from the old cabin on the San Benito River to the Mine
A lower level view from the old cabin to the mine
A walk into the mid workings of the mine

Benitoite is a prized gemstone of extreme rarity. It is in very high demand for both gemstones and as specimens and its popularity is continuing to expand. People all over the world are discovering the vast array of rare and valuable colored gemstones and how exciting and exotic they can be.  Gem benitoite is more rare than other well-known gemstones such as diamond, ruby, emerald and even tanzanite by orders of magnitude!  It is a true rarity among gemstone collectors and the availability of quality stones will likely never be high as the original deposit is in the waning years of its viability. What are left at the mine are a few dissected blocks cross cut by mineralized veins, a fair amount of colluvium and the old mine dump material. The large contiguous blocks that provided the numerous specimens of the past are depleted however. There is a possibility that the Junnila mine may have some very minor reserves of gem benitoite but the deposit is rather small and the overwhelmingly vast majority of the benitoite at that locale is a very low grade (Laurs et al, 1997, references a dubious occurrence of gem benitoite at the Junnila mine.) The Junnila mine was worked recently (1999) for fresnoite and that deposit is more or less exhausted. In all probability, the gem material that exists today will be added to only from future workings of the Benitoite Gem Mine.

I know from personal experience that it is difficult to obtain high quality benitoite specimens and gem rough. I attend many shows around the world and only rarely are quality specimens of benitoite available.  High quality specimens always command a premium, as the supply of these is quite limited

Composition & Formation of Benitoite

The chemical formula for benitoite is BaTiSi3O9. The mineral is thought to have been derived from the surrounding wall rock during a phase of low temperature and high-pressure metamorphism which relates to a blueschist metamorphic facies. 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 veins fracturing the wall rock of serpentine. Van Baalen, 1995, suggests that the benitoite mineralization was subsequent to the formation of the blueschist. His analysis places the age of the blueschist at between 100 and 160 million years of age and the benitoite present in the cross cutting veins at 12 million years. All benitoite found at the Benitoite Gem mine is confined to blueschist rocks.  The last phase of mineralization was confined to natrolite that was injected into the vein system.  It is found to encase most, but not all, of the minerals deposited in the earlier phases.

Surprisingly, there are only a few dozen of the over 4,000 known mineral species which contain both barium and titanium in their chemical composition.  Along with Benitoite there are: Ankangite, Bafertisite, Baotite, Baryolamprophyllite, Batisite, Belkovite, Bornemanite, Byelorussite, Curetonite, Delindeite, Fresnoite (very similar to benitoite chemically), Hawthorneite, Hejtmanite, Ilmajokite, Innelite, Jeppeite, Jinshajiangite, Joaquinite series, Jonesite, Lubuntsovite, Lamprophyllite, Leucosphenite, Lindsleyite, Lourenswalsite, Mannardite, Muirite, Nagashimalite, Pabstite (very similar to benitoite chemically!), Perraultite, Priderite, Redledgeite, Shcherbakovite, Taramellite, Tienshanite, Titantaramellite, Traskite, Verplanckite, Yoshimuraite, and Yuksporite. 38 plus benitoite makes 39 known minerals or only about one-tenth of one percent of all known mineral species!

Crystal Form

Benitoite belongs to the hexagonal class of minerals. 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.  Figures 5 and 6 display the faces seen on benitoite crystals.  Figures 7 through 10 display the rarer crystal faces found on benitoite crystals.

Twinning of benitoite occurs on the c-axis (0001) with two crystals rotated 180 degrees. Equidimensional twinned crystals are referred to as a "Star of David" owing to their perfect six sided star (See Fig.13).

Most new minerals discovered in this century 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 text-book crystals accompanied by equally striking crystals of neptunite and joaquinite. There have been few occasions in the annuls of mineralogy when a new species was found in such magnificent crystals.
 
 


Figure 7.  The most common faces of benitoite crystals m, pi, mu and c.

Figure 8.  The rare faces on benitoite crystals - d & a.

Four idealized drawings of benitoite crystals taken from Louderback, 1909. The images on the top are of the dominant forms found in the vast majority of benitoite crystals. These are generally present in all benitoite crystals and are: c (0001),  p (10-11,  pi (0-111), m (10-10) and mu (0-110).  The lower crystal drawings represent some of the rarer second order prism and pyramid forms for benitoite.  Particularly the d (22-41) and a (11-20) faces are quite rare.  The stepped faces (alternating c and p faces) between the two pifaces are typically referred to as r faces. The rarest known face on benitoite crystals is the x pyramid face (see Figure 12).  (Note a minus symbol equals a "bar" as it refers to the Miller index system. A "-1" means a bar over the 1.)


 
 
Figures 9 & 10.  The photo on the left displays the secondary prism a face (the bright face is a d face.  The a face emerges
from it to at about the one o'clock position.) The photo on the right displays paired secondary pyramid d faces.   J. Veevaert specimens.


Figures 11 & 12.  The photo on the left displays the stepped oscillatory growth between c and p faces. secondary prism a face.
The photo on the right displays the rarest of all crystal faces on benitoite - a secondary pyramid xface.  J. Veevaert specimens.


Figures 13 & 14. The photo on the left is of a one cm twinned crystal has one crystal much larger than the other.  Note the 180 degree rotation about the
c-axis however. The crystal on the right has a unique hoppered c face 3.5 mm across.  To my knowledge this is the only known specimen with this sort
of crystal face development.  J. Veevaert specimens.


Figure 15. The animated image above helps to provide a three dimensional sense of what an ideal benitoite
crystal would look like.  Many thanks to Albert Hines for his efforts in producing this image!

Coloring of Benitoite

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, or greenish-gray color as well.  The origin of the color of benitoite has not been completely determined out 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, most frequently, as gray to greenish colored crystals with very heavy inclusions of the amphibole crossite (see Figure 16). Some crystals of benitoite appear red to dark maroon due to inclusions of minute neptunite crystals.

Benitoite is also very strongly reactive under a short wave ultraviolet (SW UV) light source. It fluoresces a very bright, opaque, sky blue color (see Fig. 17). The fluorescence seems to be stronger in crystals with crossite inclusions than crystals of gem quality. Some crystals fluoresce a dull reddish 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.

Figure 16.  This 2.8 cm crystal of benitoite is typical of the majority of the benitoite found at the mine.
This specimen is heavily included with crossite fibers coloring the crystal a dull grayish blue.   J. Veevaert specimen.


Figure 17.  These two photos display the fluorescence of benitoite under SW UV light.
The specimen is 6.8 x 5.5 cm in dimension.   J. Veevaert specimen.


Deposits and Associated Minerals

As mentioned earlier, Benitoite is found in several locations around the world but only in San Benito County is it found in anything resembling good crystal form. To date the mineral has been found in southern San Benito County at the Gem Mine, Santa Rita Peak, the Victor Claim, the Mina Numero Uno and the Junnila Mine. All of these deposits are relatively close to each other. It is highly probable that additional deposits exist in the region as all of these deposits are contained in fault-bound blocks of green and blue schist. In all likelihood additional blocks exist in the ultramafic terrane that have not made their way to the surface via erosion. Of the known occurrences the Gem Mine and Junnila Mine have produced the largest crystals while the Mina Numero Uno, Santa Rita Peak, and Victor Claim are known to produce only small, 1cm or less, sized crystals.

There are several theories which have been advanced regarding the formation of these interesting deposits. Coleman, 1957, suggests that the deposits occurred as a result of titanium and sodium enriched fluids, derived from the intruding serpentinite, mixing with barium and manganese rich metamorphosed Franciscan Formation blocks within the larger ultramafic body. Van Baalen, 1995, suggests that since titanium is relatively insoluble that the minerals were formed by the metamorphism of the contact between the hanging wall greenstone and the footwall schist in the presence of basic mineralizing fluids rich in sodium and somewhat deficient in silica - such as a syenite is. He suggests that all of the elements needed, barium, titanium, lithium, REE, etc., were present in the immediate environment and were not derived from distant sources. The closest syenite to the Benitoite Gem Mine is about 1 km away.  Laurs, et al (1997) suggest that benitoite formed when barium and titanium were mobilized during the alteration of blueschist, and possibly greenstone, in the presence of fluids rich in calcium and magnesium while the region was experiencing large scale metamorphism.

There are a number of other minerals that have been found at the Benitoite Mine.  A surprising number of these are considered rare species.   This article will not try to cover all of the minerals as there are plenty of references available which detail the various species found at the mine.  The comprehensive list of specie found at the four major localities are presented in Table 1.

There are a couple of interesting mineral associations related to benitoite. Of particular interest is the mineral fresnoite.  It has been found at all four main localities for benitoite and in very close proximity to benitoite at the Junnila Mine.  Fresnoite (see Figure 18) has the formula Ba2TiSi2O8 while benitoite is BaTiSi3O9. Clearly these two minerals are close chemically but are distinctly different species in terms of appearance. The question begs to be asked and answered: What chemical, pressure and heat conditions were needed for the formation of these two chemically similar minerals so close to each other? Mark Kast, personal communication 1997, has developed a paragenetic sequence for the minerals at the Junnila mine and believes that fresnoite was one the last minerals to form which may indicate a further deficiency in available silica and a preference for the formation of fresnoite vs. benitoite.

Another chemically similar mineral to benitoite is jonesite (see Figures 25, 26 and 27).  Jonesite is known only from the Benitoite Gem mine and is considered extremely rare.  It may be more plentiful than thought however since it is easily overlooked during the cleaning process due to its small crystal size and sparse distribution. Jonesite has the chemical formula of Ba4(K,Na)2Ti4Al2Si10O36·6(H2O). Wise et al (1977) state the benitoite and jonesite were not found together and they speculated that jonesite formed from solutions rich in aluminum - conditions that would not generally allow for contemporaneous mineralization of benitoite.  I have close to a dozen specimens that have both benitoite and jonesite present growing on the wall rock, which contradicts this hypothesis.  It is not readily apparent if both occurred at the same time or not however their close proximity to one another 1 mm or less on several of my specimens) suggests that they were crystalized from the same solutions which also produced neptunite and joaquinite.  It may be that last phase aluminum rich mineralizing solutions that deposited the vast amount natrolite are responsible for mobilizing barium and titanium from existing benitoite to allow for the formation of jonesite.  Jonesite is typically encased by natrolite also but it may represent the first mineral to be deposited from the natrolite solution.  The vast majority of benitoite found at the Benitoite Gem mine has been lightly to moderately etched.  This alone would provide more than enough barium and titanium needed to account for the relatively very small amount of jonesite observed at the Benitoite mine.  The presence of both fresnoite and jonesite imply that the paragenesis of these interesting deposits is a lot more complex than previously thought.

Table 1 below lists all of the mineral species that have been found in the four most significant occurrences of benitoite in the vicinity of the Benitoite Gem Mine. An "X" indicates a known occurrence. Source: Mark Kast, personal communication, 1997.
 
 
 

Figure 18.  Specimen of Fresnoite collected  6/8/97 by the Author. Location: Junnila Mine.
The crystal measures 1.4 cm in its longest direction. V. Rhoads specimen - J. Veevaert photograph.

Specimens

The following are images of benitoite and other minerals found at the Benitoite Gem Mine.

Additional specimen images from the collection of the author can be seen here.
 
 
 

Figures 19-21.  The photo on the left displays fine triply terminated crystal of benitoite 1.7 cm across. The specimen
in the center has crystals to 2.2 cm across.  It also has a 1.8 cm gem crystal.   the specimen on the right has crystals
to 1.7 cm across including a fine 1.3 cm triply terminated crystal on the right side.   J. Veevaert specimens.
Figures 22- 24.  The photo on the left displays two fine crystals of neptunite to 2.1 cm in length.   The center photo is
of a specimen with crystals of neptunite to 2.4 cm in length.  It also has gem crystals of benitoite to 1.4 cm across.  The
photo on the right displays a twinned crystal of neptunite on  the 301 miller indices face.   J. Veevaert specimens.


Figures 25-27.  These are three photos of jonesite.  Jonesite is the rarest mineral found at the Benitoite Gem Mine.  The photo on the left is of a 2.5 mm spray on neptunite. The  photo in the center  is of a small 1.5 mm spray on joaquinite crystals and the photo on the right is of a 3.5 mm single crystal - rather enormous for the species.   J. Veevaert specimens.
 
 
 
Figures 28 & 29.  These are other interesting species found at the mine. They include from left to right:
silica pseudomorphs of serandite (?), apatite and joaquinite.  The pseudomorphs are 3-4 mm in length.
The apatite crystals are 6 mm and the joaquinite on the right is 5 mm from top to bottom.  J. Veevaert specimens.


Figures 30,  31 & 32.  This specimen on the left is of a 4 mm crystal of djurleite exposed from natrolite matrix.
The specimen in the center of a joaquinite crystal cluster that is 6 mm from top to bottom.  The photo on the right is of the only
known specimen of native silver associated with djurleite and benitoite. The wire is about 1.5 mm in length.   J. Veevaert specimens.

The table below lists all of the minerals which have been found in the four most significant occurrences of benitoite in the vicinity of the Benitoite Gem Mine. An "X" indicates a known occurrence. Source: Mark Kast, personal communication, 1997.

TABLE 1.

Mineral Name 
Gem Mine
Mina Numero Uno
Victor Claim
Junnila Mine
Actinolite/
Tremolite
X
 
X
X
Analcime
X
   
X
Anilite
   
X
 
Andradite
X
 
X
X
Anthophyllite
     
X
Apatite
X
X
X
 
Baoite
X
     
Bario-Orthojoaquinite
     
Benitoite
Calcite
 
Chrysocolla
   
Native
Copper
     
Covellite
 
Crossite/
Glaucophane
Digenite
     
Djurleite
   
Feldspar
Fresnoite
Galena
     
Joaquinite
Jonesite
     
Malachite
   
 
Montmorillonite
     
Natrolite
 
Neptunite
Parawollastonite
 
 
 
Pyrite *
X
     
Pyrrhotite
 
   
Quartz
X
 
Serandite
X
     
Silver **
X
Stevensite
   
Stilpnomelane
 
 
Strontio-
Joaquinite
 
* - Found on specimen as micro (<.5mm across) crystals with jonesite.
** - Known on only one specimen - a small wire with djurleite and benitoite.

Summary

The discovery of benitoite is remembered as one of the most exciting new finds in the mineral kingdom of the past century.  The current mining operation of the property is working to ensure an increased supply of gem rough and specimens.  Specimens of benitoite are continuing to enjoy a growing increase in popularity but the supply of exceptional specimens remains, as in the past, limited.  For the foreseeable future there will be a steady supply low-end to moderately priced specimens.  The mine officially closed from a commercial perspective in July 2006.  It is now owned by Dave Schreiner of Coalinga who has a fee dig/collection plan in operation.  The mine will never again produce on a commercial scale.
 

References

Coleman, R.G. (1957) Mineralogy and Petrology of the New Idria District, California. Ph.D. dissertation, Stanford University. UMI Dissertation Services.

Laurs, B.M, Rohtert, W.R., Gray, M. (1997) Benitoite from the New Idria District, San Benito County, California.  Gems & Gemology, vol. 33 no.3

Louderback, G.D. (1907) Benitoite, a New California Gem Mineral. University of California Publication, Bull. Dept. Geol., 5:149-153.

Louderback, G.D. (1909) Benitoite, its Paragenesis and Mode of Occurrence. University of California Publication, Bull. Dept. Geol., 5:331-380.

Sinkankas, J. (1976) Gemstones of North America. Van Nostrand Reinhold Company: New York, pp. 280-288.

Wise, W.S. and R.H. Gill (1977) Minerals of the Benitoite Gem Mine. Mineralogical Record, 8(6):442-452.

Wise, W.S. and A. Pabst, J. R. Hinthorne (1977) Jonesite - A New Mineral from the Benitoite Gem Mine, San Benito County, California. Mineralogical Record, 8(6):453-456.

Van Baalen, M.R. (1995) The New Idria Serpentinite. PhD. dissertation, Harvard University. UMI Dissertation Services.

Other links to pages with information about Benitoite

UC Berkeley Exhibit on Benitoite - My Favorite!!!
Google Search of Benitoite

The best search engine on the Internet!
 
 


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