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Journal Article
Published: 01 May 2025
American Mineralogist (2025) 110 (5): 721–730.
...Seungyeol Lee; Huifang Xu; Hongwu Xu; Wenqian Xu Abstract Nanomineral opal-CT is a natural precursor to quartz formed by various geological processes, including weathering, biological precipitation, hydrothermal alteration, and shock metamorphism. These processes play a crucial role...
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Journal Article
Published: 01 December 2024
American Mineralogist (2024) 109 (12): 2037–2051.
... minerals in different proportions typical of soils and amended them with 10–70 wt% iXAMs in the form of a 1:1 weight mixture of ferrihydrite and opal-A. We quantified these iXAMs in mineral mixtures by analyzing powder X-ray diffraction (PXRD) data using the Rietveld method and compared the results...
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Journal Article
Published: 15 November 2024
American Mineralogist (2024)
... koktaite, magnesiovoltaite, mascagnite, opal, pyracmonite, salammoniac, selenium (S-rich), 75 sulfur, tamarugite, and tschermigite. Hoperanchite is most closely associated with sulfur, 76 mascagnite, and koktaite. 77 Combustion metamorphism (CM) is a special case of pyrometamorphism where organic78 rich...
Journal Article
Published: 01 November 2024
American Mineralogist (2024) 109 (11): 1871–1887.
...Janice L. Bishop; Peter Schiffman; Enver Murad; Randal J. Southard; Lukas Gruendler; M. Darby Dyar; Melissa D. Lane Abstract Solfataric alteration at the South Sulfur Bank of the former Kilauea caldera produced opal, Mg- and Fe-rich smectites, gypsum, and jarosite through silica replacement...
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Series: Geological Society, London, Special Publications
Published: 15 March 2024
DOI: 10.1144/SP525-2023-25
EISBN: 9781786209573
... stratigraphy of the study area ( Dalland 1988 ; Eidvin et al. 2007 ). These ooze sediments are characterized by small-scale polygonal faulting, interpreted to be caused by compaction and water escape ( Cartwright et al. 2003 ; Wrona et al. 2017 ). A fossilized Opal A/CT boundary (diagenetic boundary...
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Journal Article
Published: 01 February 2024
American Mineralogist (2024) 109 (2): 392–396.
... the structural position of point dipolar oxygen atoms. The elucidation of liquid and amorphous solid structures had to wait for the advent of radial distribution analysis. Here I shall start by discussing the merits and demerits of such analysis, followed by geometrical constraints on opals put on by theoretical...
Journal Article
Published: 01 February 2024
American Mineralogist (2024) 109 (2): 397–398.
... on the structure of opal-A using synchrotron X-ray diffraction (XRD) and pair distribution function (PDF) analysis ( Lee et al. 2022 ). Several comments were made by de Jong (2024) on the strengths and weaknesses of PDF analysis, the existence of 4- and 8-membered silica rings in the opal-A structure...
Journal Article
Published: 01 November 2023
American Mineralogist (2023) 108 (11): 1997–2003.
...William H. Peck; Dianne Keller; Victoria S. Arnold; Faith McDonald; Lily C. Kuentz; Paul M. Nugent Abstract Crushed ore in Adirondack wollastonite mines (New York) shows textural evidence for wollastonite dissolution and cementation by calcite and opal. The reaction CaSiO 3 + CO 2 = CaCO 3 + SiO 2...
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Journal Article
Published: 03 July 2023
American Mineralogist (2023) 108 (7): 1275–1288.
... characteristics of (1) low-Cs-bearing ore (amorphous silica opal-A and opal-CT type) with low Cs (average of ~0.2 wt%), Na, K, Al, and Ca contents, and (2) high-Cs-bearing ore (clay type) with high-Cs (average of ~1.40 wt%), -Na, -K, -Al, and -Ca contents. It is reported for the first time that Cs primarily...
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Journal Article
Journal: Geology
Published: 03 January 2023
Geology (2023) 51 (2): 189–192.
... of crystal growth within the gel. Over time, the gel hardened to form opal-A G . This silica phase is thermodynamically unstable and recrystallized to quartz that has a distinct mosaic texture. 7 2 2022 20 4 2022 9 10 2022 © 2023 Geological Society of America 2023 Mineral...
FIGURES
Journal Article
Journal: Geology
Published: 02 December 2022
Geology (2023) 51 (1): 85–90.
... increase in acoustic impedance; Figs. 1B and 2C ). X-ray diffraction measurements from ODP Site 762 also show that sediments above H1 have high concentrations of opal-A, whereas below H1, the sediment has high concentrations of opal-CT ( Fig. 1B ). H1 therefore corresponds to the opal-A to opal-CT...
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Journal Article
Published: 21 November 2022
Mineralogical Magazine (2023) 87 (2): 186–193.
..., KZnCl 3 ⋅2H 2 O, halite, sylvite, opal and gypsum. Kalithallite forms lamellar to tabular crystals up to 5 × 30 × 40 μm combined in open-work aggregates up to 1 mm across. It is transparent, colourless in individuals and white to pale cream coloured or pale beige in aggregates, with vitreous lustre. D...
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Series: GSA Special Papers
Published: 26 September 2022
DOI: 10.1130/2022.2556(04)
EISBN: 9780813795560
... ABSTRACT The Monterey Formation and related formations in California have long been the subject of field and laboratory studies on silica diagenesis. Biogenic or amorphous silica (opal-A) alters to a more-ordered opal-CT and eventually to the crystalline end member, quartz, with increasing...
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Journal Article
Journal: Lithosphere
Publisher: GSW
Published: 16 August 2022
Lithosphere (2022) 2022 (Special 9): 8587468.
... and the biogenic content. Based on six VPCA components of first derivative reflectance spectrum measurements and laboratory analyzed biogenic contents of core MD972148, a set of empirical equations for estimating CaCO 3 , TOC, and opal contents have been established. The equations were tested using data from core...
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Journal Article
Published: 01 July 2022
American Mineralogist (2022) 107 (7): 1353–1360.
...Seungyeol Lee; Huifang Xu; Hongwu Xu Abstract The structure of opal-A was not fully understood due to its poorly crystalline nature. To better understand its structural characteristics, we have analyzed opal-AN (amorphous-network) and opal-AG (amorphous-gel) using synchrotron X-ray diffraction (XRD...
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Journal Article
Published: 01 March 2022
The Journal of Geology (2022) 130 (2): 77–110.
...Jürgen Herrmann; Roland Maas Abstract Formation of sedimentary opal-A G in opal fields of eastern Australia has been linked to intensive weathering of their Cretaceous host rocks—the “deep weathering” model. Here we examine possible links between weathering history as recorded in mining exposures...
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Journal Article
Journal: GSA Bulletin
Published: 11 May 2021
GSA Bulletin (2022) 134 (1-2): 348–370.
... similar to those in the diatom-rich sediments forming RSS3-Ia where they underwent opal A/CT transformation. As noted above, diatom deposition is likely to be highly focused into the depocenters of each basin, thereby forming isolated chert layers across the Ross Sea basins. Directly overlying the chert...
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Journal Article
Journal: AAPG Bulletin
Published: 15 February 2021
AAPG Bulletin (2021) 105 (2): 391–436.
...David K. Larue; Christian Hager; Thomas Merrifield; Gena M. Evola; David Crane; Phillip Yorgensen ABSTRACT Unconventional opal-A and opal-CT diatomaceous reservoirs in the Monterey Formation of the San Joaquin Valley in California have produced for more than 30 yr. These opal-A reservoirs...
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Journal Article
Published: 28 October 2020
Journal of Sedimentary Research (2020) 90 (9): 1037–1067.
...Shahab Varkouhi; Nicholas J. Tosca; Joseph A. Cartwright ABSTRACT Silica diagenesis leads to dramatic petrophysical variations in the host sediment across the depth of an opal-A to opal-CT transition zone. Predicting the present-day diagenetic status of opal-A to opal-CT transition zones, i.e...
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Journal Article
Published: 08 April 2020
The Canadian Mineralogist (2020) 58 (2): 231–246.
...Dereje Ayalew; Raphael Pik; Sally Gibson; Gezahegn Yirgu; Seid Ali; Dereje Assefa ABSTRACT Opals are widespread within Miocene volcanic sequences in the North Shoa province of Central Ethiopia. The opal occurs as cavity fillings in a 5 m thick seam of glassy rhyolitic ignimbrite that is sandwiched...
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