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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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Journal Article
Journal: AAPG Bulletin
Published: 15 November 2019
AAPG Bulletin (2019) 103 (11): 2665–2695.
...% of the bulk mineralogy of the samples, most of which (50%–90%) is mixed-layer illite–smectite (I/S). The main other constituent is SiO 2 in the form of opal-CT (α-cristobalite and α-tridymite) and quartz. The fraction of expandable I/S in mixed-layer I/S ranges from 100% in some of the shallowest samples...
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Journal Article
Journal: AAPG Bulletin
Published: 15 September 2019
AAPG Bulletin (2019) 103 (9): 2117–2137.
... from rapid dissolution and precipitation processes of opal-A (amorphous silica), opal-CT (cristobalite-tridymite), chalcedony or cryptocrystalline, and microcrystalline quartz. The microcrystalline quartz crystals inherit the crystallographic orientation of the length-fast chalcedony and grow parallel...
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Journal Article
Published: 01 September 2019
American Mineralogist (2019) 104 (9): 1323–1335.
...Peng Yuan; Dong Liu; Junming Zhou; Qian Tian; Yaran Song; Huihuang Wei; Shun Wang; Jieyu Zhou; Liangliang Deng; Peixin Du Abstract The occurrence of minor elements in the structure of biogenic diatomaceous opal-A is an important issue because it is closely related to biogeochemical processes driven...
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Journal Article
Published: 24 July 2019
Petroleum Geoscience (2019) petgeo2018-122.
... by the transformation of opal-A to opal-CT, and opal-CT to quartz. These changes manifest as flat spots on 2D seismic profiles. These flat spots were mistaken as hydrocarbon–water contacts, which led to the drilling of well 7117/9-1. Rock physics analyses on this well combined with a second well (7117/9-2) drilled...
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Journal Article
Journal: Economic Geology
Published: 01 February 2019
Economic Geology (2019) 114 (1): 67–92.
.... Therefore, the SXFM data of sample 852-2A are best interpreted by their element distributions and associations, and the elemental abundances are not considered accurate. Zinc-rich chimneys are dominated by sphalerite, barite, and pyrite/marcasite ± opal A, and detailed mineralogical descriptions...
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Journal Article
Published: 01 January 2019
The Canadian Mineralogist (2019) 57 (1): 81–90.
... in the Czech Republic. The associated minerals are calcite, siderite, whewellite, quartz, opal, baryte, and clay minerals (at Libouš) and marcasite, pyrite, baryte, sphalerite, and gypsum (at Bílína). Ferrierite-NH 4 forms white radiating aggregates up to 2 mm across consisting of thin flattened prismatic...
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Journal Article
Published: 01 May 2018
American Mineralogist (2018) 103 (5): 803–811.
...Erwan Martin; Eloïse Gaillou Abstract At Lake Tecopa, in California, white play-of-color opals are found in vesicles of a volcanic ash layer from the Huckleberry Ridge Tuff super-eruption (2.1 Ma). They show characteristic traits of opal-AG by X-ray diffraction and scanning electron microscopy...
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Series: Geological Society, London, Special Publications
Published: 01 January 2018
DOI: 10.1144/SP464.2
EISBN: 9781786203724
... on a Perkin-Elmer 3030 atom-absorption-spectrophotometer. Biogenic opal percentages were calculated (=biogenic silica × 2.4: Pavicevic & Amthauser 2000 ). For organic geochemical analyses, representative portions of selected samples were extracted for approximately 1 h using dichloromethane in a Dionex...
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Journal Article
Published: 01 July 2017
The Canadian Mineralogist (2017) 55 (4): 701–723.
...Boris ChauvirÉ; Benjamin Rondeau; Francesco Mazzero; Dereje Ayalew Abstract Since its discovery in 2008, the opal deposit in the Wegel Tena area of Ethiopia, located within an Oligocene ignimbritic trap, has become a major provider of precious opal. This study provides new field evidence...
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Journal Article
Published: 01 June 2017
American Mineralogist (2017) 102 (6): 1154–1164.
...Michael Schindler; Mostafa Fayek; Britanney Courchesne; Kurt Kyser; Frank C. Hawthorne Abstract Understanding the retention mechanism of U by amorphous silica (i.e., opal) in the environment is of great importance to nuclear-waste disposal because opals can retain U for millions of years...
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