Mineralogía y génesis de gemas de corales silicificados de Sumatra

Abstract

Mineral characteristics of silicified corals from Barisan Mountains (Miocene) of the Western Sumatra (Indonesia) have been studied by optical microscopy, Raman spectroscopy (spectral curves and hyperspectral images) and scanning electron microscopy with energy dispersive microanalyses. Three different parts were recognized in the silicified coral, as follows: (i) the external shell, the radial septae and inner circular walls and the core central part build with opal with some disperse carbon-rich black grains, showing the G and D Raman bands from residual organic matter; (ii) the inner caverns stuffed with mega-quartz crystals of high crystallinity; (iii) chalcedony fibres occurring among both former zones, including moganite mineral as identified by its 502 cm-1 Raman band. During the subsequent processes of burial and diagenesis the delicate organic structure of corals is substituted and consolidated by silica coming from the alteration of ash-flow and interstratified volcanic rocks. In addition, the presence of AgS and Zn-Cu minerals into the coral fissures suggest later hydrothermal processes probably associated with the regional volcanic activity