Composition: Barium titanium silicate
Crystal system: hexagonal; bar 6 m 2
Hardness: 6 - 6.5
Specific Gravity: 3.6
Refractive Index: 1.75-1.8
Color: usually blue, but can be yellowish
Transparency: Crystals are transparent to translucent
Associated Minerals: albite, fresnoite,
joaquinite, neptunite, natrolite, sanbornite, serpentine, and taramellite
Barium titanium silicate (36.3% BaO, 20.2% Ti02,
Benitoite (pronounced "ben-ee-toe-ite") is named after
San Benito County, California where it was discovered in 1907. It has
a sapphire blue color and was first thought to be a variety of sapphire.
But it is one of only a few minerals to crystallize in the bar 6 m 2
class called the ditrigonal-dipyramidal symmetry class. While this class
is techically hexagonal, it produces trigonal (triangular) looking crystals.
Additionally benitoite is also a fluorescent mineral.
Benitoite from San Benito, County is associated with the minerals
neptunite, natrolite and joaquinite. This combination of minerals was
formed from hydrothermal solutions in a natrolite dike in the green
schist of the serpentine parent rock. These highly saturated solutions
contained a number of unusual elements such as barium, cesium, fluorine,
iron, lithium, manganese, niobium and titanium.
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. Until this mineral was found in 1907, there was no
known naturally occurring representative of this crystal class. Benitoite
is the first species known to crystallize in the ditrigonal dipyramidal
class of the hexagonal crystal system.
Crystals are so distinctive that no tests are necessary.
Only the mines of San Benito County, California
have good crystals some of which are good enough to cut stones from.
SW Texas produces tiny grains in Eocene sands. There are also some other
California localities that produce small crystals.
As a gemstone and as a mineral specimen
FACTS & HISTORY:
When first discovered Benitoite was initially thought to be sapphire.
Samples were sent to the University of California, Berkeley for identification.
Dr. George Louderback found it to be a new mineral. Two other minerals
were also found. He thought they too, were new to science. He formally
named Benitoite and called the other two new minerals carlosite and
After further analysis carlosite was found to be neptunite
which had been previously described from specimens found in Greenland
and the Ural Mountains in Russia. Even so, the specimens of neptunite
from the San Benito County were far superior to any of the other known
localities of this species. Joaquinite was also a new mineral species
but remained incompletely described until the 1970s.