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JEWELRY-QUALITY MODIROVAL AMMONITE, MADAGASCAR



The world market enjoys an expansive range of the interior- and jewelry-quality ammonite extracted at the Modiroval deposit in Madagascar. Mining is carried out through pit-holes, with primitive tools and without preliminary exploration. The productive stratum occurs at a depth down to 20 m and belongs to the Albian stage. It is traced as a strip of 500 m for the space of 15 km. The productive stratum is dense marl composed of calcite, siderite and aragonite. Small quantities of apatite, kaolinite, quartz, goethite, chlorite, hydromica and pyrite are present. Out of accessory minerals, ilmenite, zircon and organic matter are identified. Marl is rich with fine mineralized pockets 0.05–1.5 mm in size. The fossil shells preserve mostly initial aragonite composition. Formation of the productive stratum is connected with the massive mortality of fossil shells and their concentration deep under the sea. Mineralization of fossil shells during diagenesis of marine sediments took place under low temperatures and high content of iron. Madagascar experience of ammonite extraction and processing can be of use in Russia which holds considerable reserves of this jewelry- and ornamental-quality material with the unique decorative and process characteristics.



: 7
2018
УДК: 622.793.5
DOI: 10.25018/0236-1493-2018-7-0-160-168
Authors: Petrochenkov D. A.

Authors' Information:
Petrochenkov D.A., Candidate of Geological and Mineralogical Sciences,
Assistant Professor, e-mail: p-d-a@mail.ru,
Russian State Geological Prospecting University named after Sergo Ordzhonikidzе
(MGRI-RSGPU), 117997, Moscow, Russia.

Key words:
Jewelry- and ornamental-quality raw material, ammonite, nautilus, Cretaceous deposits, marl, aragonite, Madagascar.

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