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 “fire” 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° 20’ 10” North, ~120° 36’ 19” 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).