166-8 Turquoise Terminology: Reviewing Major Element Chemistry of Global Turquoise Sources

Session: Mineralogical Characterization of Economic Resources: From Critical Minerals to Gemstones

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Turquoise is a copper-aluminum phosphate mineral and the naming member of the turquoise mineral group. The isostructural turquoise group consists of turquoise, planerite, faustite, aheylite, chalcosiderite, and an iron analogue (currently unnamed). X-ray diffraction and Raman spectroscopy are not always able to differentiate between turquoise group minerals because of their isostructural, micro- to cryptocrystalline, and heterogeneous nature. Most turquoise group analyses represent intermediate members of one or multiple solid-solution relationships; strict compositional boundaries for the use of the name “turquoise” have not been established.  In this study, 30 turquoise samples from 16 localities were analyzed by LA-ICP-MS for major element concentrations to investigate the distribution of copper, iron, zinc, and other cations. CuO concentrations showed a broad range, from 1.71 – 9.64 wt.% (ideal turquoise = 9.78 wt.%, determined stoichiometrically). Fe₂O₃ was present in major element amounts in most samples (up to 7.61 wt.%), while only two natural samples had ZnO contents greater than 1.0 wt.%. The concentrations of divalent cations potentially filling the A-site were lower than expected, indicating vacancies characteristic of planerite. Iron commonly substituted for aluminum, though only two samples were iron-dominant at that site. Samples from two mines (Evans, Mexico, and Mona Lisa, Arkansas, USA) had intermediate compositions with average CuO below 5.0 wt.%. Boundary cases such as these demonstrate the practicality of the transitional term “planerite-turquoise” in a mineralogical setting, suggested for use when wt.% CuO is between 3.5 and 6.0 wt.%. In gemological and archaeological contexts, a broader use of “turquoise” is appropriate.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-7232

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