Feature Gems & Gemology, Spring 1992, Vol. 28, Issue 1

Opal from Querétaro, Mexico: Fluid Inclusion Study

Ronald J. Spencer, Alfred A. Levinson, and John I. Koivula

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The distinctive orange-to-red “fire" opals from Querétaro have not been found in significant quantities at any other locality. Through modern analytical methods, it has been determined that these opals formed at about 160ºC, from hydrous silica gels contained in fluids with small (up to 10%) variable amounts of  NaCl and CO2. These data, combined with geologic data, help explain the rare occurrence of this type of rhyolitic opal deposit.