Abstract
Agate samples collected from the vicinity of Asni and Agouim (Western Atlas, Morocco) were investigated using microscopic observations supported by Raman micro-spectroscopy. The agates are marked by the presence of various microtextures typical of epithermal vein deposits, including jigsaw-puzzle, feathery, and lattice-bladed. The first two indicate that the formation of agates was likely marked by recrystallization of metastable silica phases (i.e., opaline silica or massive chalcedony). The presence of lattice-bladed (after barite and calcite) quartz may be, in turn, ascribed to the boiling-related conditions that could have triggered the formation of abundant copper and iron sulfides found within silica matrix. Additionally, the local occurrence of growth lines (so-called Bambauer quartz) and intergrowth of length-slow and length-fast chalcedony are linked to the variations of physico-chemical conditions during rock formation (alkaline-acidic). According to Raman spectroscopy, silica matrix of the agates is made of α-quartz with a local admixture of moganite (from 0.0 up to 78 wt.%), but also contains numerous solid inclusions of hematite, celadonite, as well as poorly-organized carbonaceous material and rutile. These phases were likely emplaced during low-temperature hydrothermal activity of SiO2-bearing fluids that originated from post-magmatic hydrothermal activity developed within host rocks and/or meteoric waters.
Highlights
Agates are defined as banded and colorful chalcedony with admixture of other silica polymorph varieties and paragenetic minerals [1,2,3]
These specimens are mainly found in Asni, where agates were collected in the vicinity of hematite veins crosscutting basaltic rocks
The ν2 bands at ca. 465–456 cm−1 and ν4 band cm overestimated as compared with the results provided by X-ray powder diffraction
Summary
Agates are defined as banded and colorful chalcedony with admixture of other silica polymorph varieties and paragenetic minerals [1,2,3]. They are mainly found within fine-grained volcanic rocks and their tuffs [4], but are known from sedimentary and hydrothermal environments [2,3]. Other features of agates are strongly varied and exceptional (e.g., moss or botryoidal appearance) They show significant diversity in wealth of colors, structural and textual features, shape, and patterns, which make each specimen a unique product of inanimate nature both under micro and macroscale. The mineralogical/petrographical approach assumes that agate can be considered as a rock that consists mainly of low-quartz, with admixture of other mineral phases
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