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CORROSION MINERALOGY OF AN 1800 SPANISH PIECE OF EIGHT

James R. Craig^1,, John E. Callahan², Joe T. Kimbell³ and Todd N. Solberg⁴

¹ Department of Geological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, U.S.A.
² Department of Geology Appalachian State University, Boone, North Carolina 28608, U.S.A.
³ 152 Orange Blossom Road, Tavernier, Florida 33070, U.S.A.
⁴ Department of Geological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, U.S.A.

E-mail address: jrcraig{at}mail.vt.edu

Spanish colonial eight real silver coins, commonly called Pieces of Eight, were used throughout the Spanish-speaking world for hundreds of years. We undertook a detailed mineralogical, textural, and chemical investigation of an 1800 Carlos IIII eight real coin recovered from the wreck of the Spanish frigate Santa Leocadia, which sank on the rocky shore of Ecuador on November 16, 1800, with a loss of more than 140 lives and of 2,100,000 pesos of gold and silver coins. The coin is a typical eight real piece, composed of about 90% Ag and 10% Cu. It was buried in clastic sediments beneath the oxidized zone, such that it reacted with sulfur released by sulfate-reducing bacteria. Consequently, the coin has been totally encapsulated in a mixture of sand, gravel, and shell fragments cemented by metal sulfides. The residual coin consists of silver with small interspersed micrometric grains of copper. Reaction of the dissolved metal with the bacterially generated diagenetic sulfur in the intergranular fluids resulted in extensive cementation of the sediment particles by Ag and Cu sulfides. In the 1-mm zone immediately adjacent to the coin, Cu-bearing acanthite occurs as concentric layers with intervening zones of sand and clay. Beyond this zone, acanthite formed from Ag dissolved from the coin occurs as a more or less continuous interstitial cement with local small islands of covellite, CuS. Copper also occurs as films of Cu carbonate on quartz grains, as isolated grains of jalpaite Ag₃CuS₂, stromeyerite AgCuS, mckinstryite (Ag,Cu)₂S, and as atacamite Cu₂Cl(OH)₃, which rim and replace detrital carbonate grains.

Keywords: piece-of-eight, coin, silver, copper, acanthite, atacamite, corrosion, environmental mineralogy, archeology.