gratonite

gratonite

[′grat·ən‚īt]
(mineralogy)
Pb9 As4S15 A mineral composed of lead arsenic sulfide, occurring in rhombohedral crystals.
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(See the last page of the July-August 2007 issue for some historical background, or the July online installment of "What's new in the mineral world.") If you had the means, you could take home from Denver a simply smashing 19th-century Philly specimen of such things as Utah cuprotungstite, Peruvian gratonite, German kermesite, pre-Bolshevik Russian gem crystals and other historical treasures.
The best gratonite specimen we ever saw came from this collection.
It is characterized by sphalerite and galena, along with pyrite, chalcopyrite, tetrahedrite-tennantite, marcasite, arsenopyrite and gratonite. Wurtzite occurs near the boundary with the Transition Zone.
The galena in the Lead-Zinc Zone is occasionally found partly replaced by gratonite. The galena occurs frequently on pyrite, associated with tetrahedrite, sphalerite and chalcopyrite.
Alabandite (*)Apatite Arsenic (*)Pb-As-S glass (*)Barite (*)Baumhauerite-2a (*)Bournonite (*)Calcite (*)Chalcopyrite Chatkalite Clinozoisite Geocronite Gratonite (*)Hutchinsonite Jamesonite Manganaxinite Marcas ire (*)Orpiment Realgar Robinsonite (*)Scheelite Seligmannite Stannite (*)Wurtzite
Assemblages include: pyrite-hematite-realgar associated with high-silver mineralization consisting of a complex association which includes gratonite, baumhauerite, aramayoite, "waxy" sphalerite, and galena (Ward, 1961); and alunite-native sulfur rug fillings in enargite-pyrite pipes (Cerro de Pasco Corporation, 1950).
Cerro de Pasco is the type locality for gratonite (Palache and Fisher, 1940), and has produced the world's best specimens.
Crystals of gratonite on the type specimens are dark lead-gray and up to 1 x 1.5 cm in size, in stout to elongated hexagonal prisms with trigonal pyramid terminations, and in some cases with small modifications.
Fine native copper, large barite crystals, gratonite crystals, spectacular formations of post-mining sulfates, and the world's finest voltaite are among the features of this historic deposit.
The deeper, unaltered portions of the orebodies are occasionally crossed by thin veins containing sulfides and sulfosalts including sphalerite, galena, tetrahedrite and gratonite.
Gratonite, which collectors usually associate with the mines at Cerro de Pasco, Peru, was found at Rio Tinto in the 1980's.