Black Agates from Paleoproterozoic Pillow Lavas (Onega Basin, Karelian Craton, NW Russia): Mineralogy and Proposed Origin

Abstract

The present study provides the first detailed investigation of black agates occurring in volcanic rocks of the Zaonega Formation within the Onega Basin (Karelian Craton, Fennoscandian Shield). Three characteristic texture types of black agates were identified: monocentric concentrically zoning agates, polycentric spherulitic agates, and moss agates. The silica matrix of black agates is only composed of length-fast and zebraic chalcedony, micro- and macro-crystalline quartz, and quartzine. In addition to silica minerals, calcite, chlorite, feldspar, sulphides, and carbonaceous matter were also recognised. The black colour of agates is related to the presence of disseminated carbonaceous matter (CM) with a bulk content of less than 1 wt.%. Raman spectroscopy revealed that CM from black agates might be attributed to poorly ordered CM. The metamorphic temperature for CM from moss and spherulitic agates was determined to be close to 330 °C, whereas CM from concentrically zoning agates is characterised by a lower temperature, 264 °C. The potential source of CM in moss and spherulitic agates is associated with the hydrothermal fluids enriched in CM incorporated from underlaying carbon-bearing shungite rocks. The concentrically zoning agates contained heterogeneous CM originated both from the inter-pillow matrix and/or hydrothermal fluids.

Chemically Oscillating Reactions as a New Model for the Formation of Mineral Patterns in Agate Geodes and Concretions

“…It can then be understood that the favoured phase to form from a process resulting in crystallisation has been explained based on the irreversible thermodynamics of the crystal structure. In Palaeoproterozoic agates from the Ludikovi Group in northwest Russia, the mineralisation of quartz was demonstrated to be strongly influenced by the abundance of organic matter [14]. The crystallisation of quartz has also been proposed to proceed from inside towards the outside edge, along with co-diffusing substances that eventually reach saturation and remain preserved as bands [1].…”

“…Concretions are known to include fossil biomass or organic matter, and, therefore, it is natural to hypothesize that organic matter decay might be involved in their formation [8]. This is distinct from agate geodes, however, which often contain organic matter [14,15], but no fossils, to the knowledge of the author, and which usually occur in volcanic rocks instead of sedimentary rocks for concretions. Yet, the variable occurrence of organic matter within agate geodes and concretions requires an adequate explanation with predictive capability.…”

Mineralogical and Geochemical Characteristics of Banded Agates from Placer Deposits: Implications for Agate Genesis

“…Agate generally refers to the silica aggregate with a banded texture, and outputs as crust and amygdala in the basalt, andesite. − Agate is mainly composed of fibrous chalcedony, crystalline quartz, and a minor amount of pseudo-granular silica. − The mineral phases existing in these silicas are mainly α-quartz and moganite. Moganite is the monoclinic silica and usually coexists widely with α-quartz in the pseudo-granular silica and fibrous chalcedony. − The presence of moganite in agate indicates a lower crystallinity of silicas at the early stages of diagenesis. , Because of the various formation conditions, there is a significant content difference of moganite ( I 501 / I 463 ) in different bands measured by Raman spectra.…”

Editorial for Special Issue “Agates: Types, Mineralogy, Deposits, Host Rocks, Ages and Genesis”

“…The Special Issue "Agates: Types, Mineralogy, Deposits, Host Rocks, Ages and Genesis" contains two reviews [1,2] and six research articles devoted to detailed studies of agates from Morocco [3], China [4], Russia [5][6][7], and Cuba [8].…”

“…Svetova et al [6] provide the first detailed investigations of black agates hosted by volcanic rocks of the Zaonega Formation within the Onega Basin (Karelian Craton, Fennoscandian Shield, Russia). Black agates are rare in nature.…”