QUARTZ: RED AND AUTHIGENIC

Length of crystal ~1.7 cm

At a small venue in Tucson I stumbled upon an interesting specimen labeled “red quartz.”  I immediately shelled out a couple of bucks since it was a doubly terminated, nice floater crystal, and it was red.  It seemed obvious the crystal was authigenic quartz since it had sharp hexagonal crystal faces with simple terminations.  It “looked” essentially identical to the authigenic quartz known as Pecos Diamonds (see Blog posting July 11, 2013) except for the red color.  The red specimen came from a well-known collecting locality at Domeño, Chelva, Valencia, Valencian Community, Spain (www.mindat.org).  Here the Triassic Keuper Formation produces floater red quartz crystals from gypsum bearing marls. 

The Triassic Period (~252-201 Ma) was named for rocks exposed in Germany where there is a tripartite division:  the Bunter is mostly early Triassic with an arbitrary boundary with the Late Permian (mostly terrestrial), the Muschelkalk is Middle Triassic (inland marine with some restricted circulation basins) while the Keuper is Late Triassic (lots of deposition in evaporitic basins with minor marine incursions).  Much of the Keuper deposition is related to basins created by the initial breakup of supercontinent Pangaea---the future North America rifting apart from the future Europe (Laurasia).  Rocks of the Keuper (readers will notice that Keuper is used as a formation/group name as well as a generic name for the Late Triassic) are exposed over much of Europe including the British Isles. The rocks also were extensively deformed by numerous Mesozoic and Tertiary orogenies.  The terrestrial sediments of the Keuper are well-known for producing dinosaurs (and numerous other fossils).

The red colored Spanish quartz is known as ferruginous quartz, Eisenkiesel, or locally Jacintos de Compostela.  The latter name is often used by agencies selling specimens.  The red color is due to inclusions of hematite scattered evenly throughout the crystals and is different from other red-colored crystals created by surficial hematite.  There are very few localities around the world that have produced Eisenkiesel and the Spanish locality is certainly the best known.

So, if readers are interested in collecting interesting and different specimens of quartz look for Jacintos de Compostelas at the next rock show.