Agates from Sidi Rahal, in the Atlas Mountains of Morocco: Gemological Characteristics and Proposed Origin

Dumańska-Słowik, Magdalena; Natkaniec‐Nowak, Lucyna; Wesełucha–Birczyńska, Aleksandra; Gaweł, Adam; Lankosz, M.; Wróbel, Paweł

doi:10.5741/gems.49.3.148

Cited by 20 publications

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“…Each has a different shape, size, color, and pattern. As for dominant color, they seem to be similar to Sidi Rahal agates [24]. The agate nodules from Asni are mainly grey-white or red-grey with a characteristic red-orange or pink external rim.…”

Section: Field and Macroscopic Observationsmentioning

confidence: 77%

“…Agates from the Atlas Mts. are hosted by the same type of volcanic rocks (basalt), which were assembled during the Triassic period [24,25]. The main feature, which distinguishes agates from Sidi Rahal compared to agates from the other localities, is the presence of opal CT (disordered αcristobalite, α-tridymite) which was not confirmed in agates from Agouim-Asni-Kerrouchen areas.…”

Section: Discussionmentioning

confidence: 97%

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Agates from Western Atlas (Morocco)—Constraints from Mineralogical and Microtextural Characteristics

et al. 2020

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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.

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Section: Field and Macroscopic Observationsmentioning

confidence: 77%

Section: Discussionmentioning

confidence: 97%

Agates from Western Atlas (Morocco)—Constraints from Mineralogical and Microtextural Characteristics

et al. 2020

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“…Fire agates consist mainly of low quartz, proved by marker bands at 465, 209 and 131 cm –1 and subordinately mogánite with a diagnostic band at 502 cm –1 (Kingma and Hemley, 1994; Götze et al , 1998; Dumańska-Słowik et al , 2013). Mogánite mainly occurs within dark coloured zones (Fig.…”

Section: Resultsmentioning

confidence: 99%

Fire agate from the Deer Creek deposit (Arizona, USA) – new insights into structure and mineralogy

Natkaniec‐Nowak

Dumańska-Słowik

Gaweł

et al. 2020

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Fire agates from Deer Creek are highly appreciated gemstones due to the presence of optical phenomena and rainbow 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 of fire effect in these agates. The multi-layered silica spheroidal forms (‘bubble’-like structure), already noted in hand specimens, could be the centres of reflection and interference of white light. Numerous, microscopic inclusions of Fe and Ti compounds randomly scattered within some silica zones, together with microstructural features of agate, could determine the colour and size of the domains with the optical effect. Deer Creek fire agates form veins within their host volcanic rocks. The silica mineralisation filling the network of fissures in the host rocks was supplied cyclically with aqueous fluids of varying composition, enriched periodically in CO2, Fe, Ti, Mn, Zn and Ca. As a result, the red-brown colour of fire agates was created by scattered pigments of tiny iron oxides (magnetite, maghemite) and titanium oxides (rutile, anatase) within the silica matrix. The precipitation of strongly disordered silica with a characteristic colloform texture is diagnostic for boiling processes in this area.

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“…However, up to now the specific conditions leading to the formation of quartzine are not completely understood. Recent studies have shown that opal-CT, opal-C, cristobalite and moganite are also common in many agates worldwide [29,40,45,54]. Opal-CT represents an intermediate stage between pure amorphous and crystalline silica.…”

Section: Agate Propertiesmentioning

confidence: 99%

Mineralogy, Geochemistry and Genesis of Agate—A Review

2020

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Agate—a spectacular form of SiO2 and a famous gemstone—is commonly characterized as banded chalcedony. In detail, chalcedony layers in agates can be intergrown or intercalated with macrocrystalline quartz, quartzine, opal-A, opal-CT, cristobalite and/or moganite. In addition, agates often contain considerable amounts of mineral inclusions and water as both interstitial molecular H2O and silanol groups. Most agate occurrences worldwide are related to SiO2-rich (rhyolites, rhyodacites) and SiO2-poor (andesites, basalts) volcanic rocks, but can also be formed as hydrothermal vein varieties or as silica accumulation during diagenesis in sedimentary rocks. It is assumed that the supply of silica for agate formation is often associated with late- or post-volcanic alteration of the volcanic host rocks. Evidence can be found in association with typical secondary minerals such as clay minerals, zeolites or iron oxides/hydroxides, frequent pseudomorphs (e.g., after carbonates or sulfates) as well as the chemical composition of the agates. For instance, elements of the volcanic rock matrix (Al, Ca, Fe, Na, K) are enriched, but extraordinary high contents of Ge (>90 ppm), B (>40 ppm) and U (>20 ppm) have also been detected. Calculations based on fluid inclusion and oxygen isotope studies point to a range between 20 and 230 °C for agate formation temperatures. The accumulation and condensation of silicic acid result in the formation of silica sols and proposed amorphous silica as precursors for the development of the typical agate micro-structure. The process of crystallisation often starts with spherulitic growth of chalcedony continuing into chalcedony fibers. High concentrations of lattice defects (oxygen and silicon vacancies, silanol groups) detected by cathodoluminescence (CL) and electron paramagnetic resonance (EPR) spectroscopy indicate a rapid crystallisation via an amorphous silica precursor under non-equilibrium conditions. It is assumed that the formation of the typical agate microstructure is governed by processes of self-organization. The resulting differences in crystallite size, porosity, kind of silica phase and incorporated color pigments finally cause the characteristic agate banding and colors.

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Agates from Sidi Rahal, in the Atlas Mountains of Morocco: Gemological Characteristics and Proposed Origin

Cited by 20 publications

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Agates from Western Atlas (Morocco)—Constraints from Mineralogical and Microtextural Characteristics

Agates from Western Atlas (Morocco)—Constraints from Mineralogical and Microtextural Characteristics

Fire agate from the Deer Creek deposit (Arizona, USA) – new insights into structure and mineralogy

Mineralogy, Geochemistry and Genesis of Agate—A Review

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