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).

 

 

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