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Microcrystalline sphalerite in resin globules suspended in Lake Kivu, East Africa

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Abstract

The origin and chemical nature of micron-sized spheres found as suspended particles in Lake Kivu are examined. It can be shown that the hollow spheres, with a wall thickness of 500 Å, consist of a complex polymeric resinous material which has little functionality, except for hydroxyl groups. The spheres arise in the process of degassing of water samples at depth. Tiny gas bubbles, about 1 micron in size, act as scavengers of dissolved resinous material. The newly created resinous membrane promotes the selective coordination of zinc dissolved in the water column. In the prevailing H2S regime, formation of sphalerite crystals in induced. The size range of the crystals, 5 to 50 Å, corresponds to 1 to 10 unit cells and suggests that the resinous membrane also acts as a template in sphalerite growth processes. The sources of the zinc and dissolved gases (CO2, CH4, H2S) are hydrothermal springs seeping from the lake bottom into the basin. Water discharge is substantial; about 100 years are required to fill the lake to its present level (ca. 550 km3 water). The average Kivu water contains 2 ppm zinc. Thus, 1 million tons of zinc are contained in Lake Kivu in the form of sphalerite.

Zusammenfassung

Harzkügelchen von etwa 1μ Durchmesser treten in suspendierter Form im Kivusee auf. Die Kügelchen sind hohl und besitzen eine äußere Membrane von rund 500 Å Dicke. Sie sind das Ergebnis von Entgasungsvorgängen die sich in einer Wassertiefe von mehreren hundert Metern abgespielt haben. Die Oberfläche kleiner Gasblasen diente als „Kristallisationszentrum“ für die im Wasser gelösten Harze, die ihrerseits eine selektive Koordinierung von gelöstem Zink bewirkten. Die Anwesenheit von H2S führte zur Bildung von Zinkblende mit Kristallgrößen zwischen 5 und 50 Å. Gase (CO2, CH4, H2S) und Zink entstammen salinaren hydrothermalen Lösungen, die im Gefolge vulkanischer Tätigkeit dem Seeboden auch heute noch entweichen. Der Durchschnittsgehalt von Zink im Kivusee beträgt 2 ppm, was mehr als einer Million Tonnen an Zink für den Gesamtsee entspricht.

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Degens, E.T., Okada, H., Honjo, S. et al. Microcrystalline sphalerite in resin globules suspended in Lake Kivu, East Africa. Mineral. Deposita 7, 1–12 (1972). https://doi.org/10.1007/BF00206891

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