AMMONITES AND SEPTARIES OF ATATAO DEPOSIT, MADAGASCAR: FEATURES OF GEMOLOGY AND PRODUCTION

The world market presents ammonites and septaries for interior design and jewelery from the Atatao deposit, Madagascar. Mining is carried out by shallow open pits dug by primitive tools without preliminary exploration. The pay zone 2–5 m thick occurs at a depth 10 m from the surface and is connected with the Upper Jurassic sediments represented by glauconitic, plastered marls and clays. The marketable quality ammonites occur in concretions with septaria. The size of concretions is from 5 to 2 m. Ammonites have mostly calcite composition, up to 85 % by mass. Quartz, kaolinite, hydromica, aragonite, plagioclase, gypsum, hematite, apatite and ilmenite are also present. Aragonite forms ammonite shells. Ammonites are not used as a jewelry material due to low ornamentality and poor polishing capacity. Of commercial interest are samples suitable for interior design, both in the form of whole shells and longitudinal polished cuts. The product of the primary commercial interest is septaries. Septaries consist of marl and variously colored streaks of calcite. There are two commercial grades: with mostly light yellow and with mostly dark brown calcite. The color of calcite is determined by the content of admixture-elements of Mg, Mn and Fe. Marl of septaries consists mainly of calcite with inclusions of quartz, aluminosilicates, gypsum, goethite, hematite, ilmenite, leucoxene, apatite, represented by hydroxyapatite; aragonite is traced in fragments of fossil shells. Septaria of the Atatao deposit are highly decorative and feature good process characteristics, which allows them to be used for manufacture of wide range souvenirs, as well as in jewelry. The experience of production of ammonites and septaries in Madagascar can be useful in the Russian Federation possessing significant reserves of these minerals suitable for interior design and jewelry manufacturing.

For citation: Petrochenkov D. A. Ammonites and septaries of Atatao deposit, Madagascar: features of gemology and production. Gornyy informatsionno-analiticheskiy byulleten'. 2019;3:142-150. [In Russ]. DOI: 10.25018/0236-1493-2019-03-0-142-150.

Keywords

Jewelry and ornamental raw materials, ammonite, septaries, Upper Jurassic sediments, calcite, aragonite, gypsum, Madagascar.

Issue number: 3
Year: 2019
ISBN: 0236-1493
UDK: 553.5 (691)
DOI: 10.25018/0236-1493-2019-03-0-142-150
Authors: Petrochenkov D. A.

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

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