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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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Journal Article
Published: 05 February 2020
Mineralogical Magazine (2020) 84 (2): 343–354.
... 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...
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Journal Article
Journal: AAPG Bulletin
Published: 15 November 2019
AAPG Bulletin (2019) 103 (11): 2665–2695.
... 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 to 40% in the deepest. The mixed-layer I/S fraction was partitioned into illite and smectite for the analyses of smectite...
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Journal Article
Published: 30 October 2019
Geological Magazine (2020) 157 (7): 1023–1030.
... of Victoria. We expand this record to the Cenomanian of northern New South Wales, where opalized bio-gastroliths (temporary calcium storage bodies found in the foregut of pre-moult crayfish) form a significant proportion of the fauna of the Griman Creek Formation. Crayfish bio-gastroliths are exceedingly rare...
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Journal Article
Journal: AAPG Bulletin
Published: 15 September 2019
AAPG Bulletin (2019) 103 (9): 2117–2137.
... 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 to the surface of the detrital grains, therefore...
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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 (2020) 26 (3): 373–385.
... 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