English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Tools annual

Global: Zendy Free Plan

Global: Zendy Tools monthly

Global: Zendy Plus monthly

Preliminary X-ray, SEM, TEM, and optical mineralogical results are reported to discuss the paragenesis, age distribution, and origin of clay minerals, silica modifications and authigenic silicates found in nine drilling sites of Leg 14 of the Deep Sea Drilling Project, located off northwestern Africa and northeastern South America. Most noncarbonate minerals are diagenetic alteration products of volcanic ashes. Continent-derived clay minerals, such as mica-illite, kaolinite, and chlorite (rare), are frequent only in terrigenous sediments, especially off the Amazon River and off Cape Blanc. Smectites, mainly normal montmorillonite (+greenish ?nontronite, and hectorite-saponite), are the dominant clay minerals in sediments of all ages and sites. Montmorillonite, ultimately derived from the decomposition of volcanic glass, is especially abundant in volcanogenic clays and ashes, nannofossil marl-oozes, hemipelagic siliceous oozes, and brown pelagic clays. A mixed-layer mineral, consisting of regularly interstratified chlorite and trioctahedral montmorillonite, occurs mainly in pelagic clays. Four types of amorphous or microcrystalline silica were observed in siliceous oozes and silicified mudstones and bedded cherts: (a) unaltered, amorphous opal (in siliceous organisms); (b) unidimensionally disordered low-cristobalite (-tridymite), either cryptocrystalline ("opal-cristobalite") or as fibrous "lussatite" needles, slowly crystallizing from closely-packed silica gel spheres (in cherts of intermediate maturity); (c) macrocrystalline fibrous chalcedony; and (d) crystocrystalHne quartz, commonly filling recrystallized radiolarian tests and forming the matrix of homogenized, mature, quartz-rich cherts. Palygorskite, commonly associated with small amounts of sepiolite, is an abundant constituent of volcanogenic white (and brown) clays, carbonaceous clays, some siliceous muds, "white" lussatite cherts, and pelagic clays, all of Latest Cretaceous to Paleogene age. A diagenetic origin from degraded bentonitic clays by precipitation from magnesium-rich solutions with excess silica, derived from the devitrification of silicic ash and/or the dissolution of opaline organisms is suggested. The maximum occurrence of sepiolite is farther from shore than that of palygorskite. Authigenic clinoptilolite, the only zefolite identified in all sites, has a similar age range (minimum age: Middle Miocene; minimum depth of burial: 120 meters) and paragenesis as palygorskite. It has also been formed diagenetically from the devitrification of volcanic glass and palagonite, especially in pelagic and volcanogenic brown clays and black carbonaceous shales, and at least 20 million years after their deposition. High-temperature sanidine, possibly also of authigenic origin by hydrothermal solutions, was found in Late Cretaceous pyroclastic clays and altered vitreous ashes, associated with volcanic glass, palagonite, and montmorillonite.

Mineralogy and Origin of Clay Minerals, Silica and Authigenic Silicates in Leg 14 Sediments