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Fire agate from the Deer Creek deposit (Arizona, USA) – new insights into structure and mineralogy

Published online by Cambridge University Press:  05 February 2020

Lucyna Natkaniec-Nowak
Affiliation:
Faculty of Geology, Geophysics, and Environmental Protection, AGH University of Science and Technology, Krakow30-059, 30 Mickiewicz Av., Poland
Magdalena Dumańska-Słowik*
Affiliation:
Faculty of Geology, Geophysics, and Environmental Protection, AGH University of Science and Technology, Krakow30-059, 30 Mickiewicz Av., Poland
Adam Gaweł
Affiliation:
Faculty of Geology, Geophysics, and Environmental Protection, AGH University of Science and Technology, Krakow30-059, 30 Mickiewicz Av., Poland
Anna Łatkiewicz
Affiliation:
Institute of Geological Sciences, Jagiellonian University, Krakow30-387, 3a Gronostajowa str., Poland
Joanna Kowalczyk-Szpyt
Affiliation:
Faculty of Geology, Geophysics, and Environmental Protection, AGH University of Science and Technology, Krakow30-059, 30 Mickiewicz Av., Poland
Anna Wolska
Affiliation:
Faculty of Geography and Biology, Pedagogical University, Krakow30-084, 2 Podchorążych str., Poland
Stanislava Milovská
Affiliation:
Earth Science Institute, Slovak Academy of Sciences, 1 Ďumbierska str., 974 11 Banská Bystrica, Slovakia.
Jarmila Luptáková
Affiliation:
Earth Science Institute, Slovak Academy of Sciences, 1 Ďumbierska str., 974 11 Banská Bystrica, Slovakia.
Karolina Ładoń
Affiliation:
Faculty of Geology, Geophysics, and Environmental Protection, AGH University of Science and Technology, Krakow30-059, 30 Mickiewicz Av., Poland
*
*Author for correspondence: Magdalena Dumańska-Słowik, Email: dumanska@agh.edu.pl

Abstract

Fire agates from Deer Creek are highly appreciated gemstones due to the presence of optical phenomena and rainbow colours that cause fiery effects to be observed on their characteristic brown base. The specific morphology of poorly ordered chalcedony (crystallinity index = 0.1–1.5) with an admixture of mogánite (av. 6.6%), micro-quartz and opal-C forming a colloform texture seems to be responsible for the presence of fire effect in these agates. The multi-layered silica spheroidal forms (‘bubble’-like structure), already noted in hand specimens, could be the centres of reflection and interference of white light. Numerous, microscopic inclusions of Fe and Ti compounds randomly scattered within some silica zones, together with microstructural features of agate, could determine the colour and size of the domains with the optical effect. Deer Creek fire agates form veins within their host volcanic rocks. The silica mineralisation filling the network of fissures in the host rocks was supplied cyclically with aqueous fluids of varying composition, enriched periodically in CO2, Fe, Ti, Mn, Zn and Ca. As a result, the red-brown colour of fire agates was created by scattered pigments of tiny iron oxides (magnetite, maghemite) and titanium oxides (rutile, anatase) within the silica matrix. The precipitation of strongly disordered silica with a characteristic colloform texture is diagnostic for boiling processes in this area.

Type
Article
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland, 2020

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Footnotes

Associate Editor: Martin Lee

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