BENITOITE-NEPTUNITE-JOAQUINITE-NATROLITE
VEIN IN BLUESCHIST
Benitoite is a very rare barium titanium silicate mineral (BaTiSi3O9)
known principally from San Benito County, California, USA. Large,
gem-quality crystals that have been cut, faceted, and polished have the ÒfireÓ
of diamond (~same index of refraction), but have a lovely deep
blue color. Benitoite is a metamorphic mineral. Its principal
occurrence is in California, where multimineralic veins have intruded a
blueschist body between serpentinites and greenstones of the Franciscan
Formation. The Franciscan Fm. is a widespread, heterolithic unit of
Jurassic-Cretaceous age in the Coast Ranges of western California. The
vein mineralization age is late Middle Miocene (12 m.y.).
Four minerals are commonly found in these veins: benitoite
(blue), neptunite (blackish - Na2KLi(Fe,Mn)2Ti2Si8O24),
natrolite (white - Na2Al2Si3O10·2H2O),
and joaquinite (orangish-brown - Ba2NaCe2FeTi2Si8O26(OH,F)·H2O).
The natrolite portion is acid-soluble. Nicely crystalline rock faces like
the one shown below are produced by acid etching the natrolite-dominated
veins. After acid treatment, the other three minerals stand out in
relief.
Locality:
Dallas Gem Mine (a.k.a. Benitoite Gem Mine), near Santa Rita peak,
southeast of New Idria, far-southeastern San Benito County, California, USA
(~36¡ 20Õ 10Ó North, ~120¡ 36Õ 19Ó West).
Blueschist (glaucophane metamorphite) (7.3 cm across) with acid-etched
multimineralic vein having neptunite (= black), benitoite (= 3 bluish crystals
- at far left, at upper right, and ~lower center), natrolite (= white), and
joaquinite (= orangish-brown).