Agate Genesis: A Continuing Enigma

Abstract

This review covers the last 250 years of major scientific contributions on the genesis of agates found in basic igneous host rocks. From 1770 to 1955, the genesis question was frequently limited to discussions based on observations on host rock and agate thick sections. Over the next 25 years, experimental investigations examined phase transformations when silica glass and various forms of amorphous silica were heated to high temperatures. This work demonstrated that the change from the amorphous state into chalcedony was likely to be a multi-stage process. The last 40 years has seen modern scientific instrumentation play a key role in identifying the physical and chemical properties of agate. The outcome of this work has allowed limited evidence-based comment on the conditions of agate formation. There is a general consensus that agates in these basic igneous hosts form at <100 °C. However, the silica source and the nature of the initial deposit remain to be proven.

Occurrence and Distribution of Moganite and Opal-CT in Agates from Paleocene/Eocene Tuffs, El Picado (Cuba)

“…Mineralogical and geochemical investigations were aimed to characterize the silica minerals in the agates and to provide data for the reconstruction of agate formation processes in Moa. On the other hand, the results of the study should provide additional general arguments for the ongoing discussion concerning the enigma of agate genesis [3,4]. Emphasis was placed on the detection and distribution of the monoclinic silica polymorph moganite and opal-CT, which both occur in considerable amounts in the agates from the surroundings of Moa.…”

“…Remarkable are chalcedony from Palmira near Cienfuegos, opal from Loma de los Ópalos and Pontezuela (near Camagüey) as well as silicites from Corralillo on the northern coast of Cuba (Figure 1). Silicified wood occurs in Upper Cretaceous volcanic sequences of the Sierra de Najasa SE of Camagüey (Sibanicu) and above ophiolites in the Sierra de Cubitas north of Camagüey (Miocene) [2,3]. Agates were sporadically found in Upper Cretaceous rocks overlaying ophiolites of the Camagüey fault zone NE of Camagüey.…”

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

“…Agate and chalcedony are varieties of silica that are mostly composed of minute crystals of α-quartz. Chalcedony is band-free and characterised by uniform colouration, whereas agate is defined as banded or patterned chalcedony [1]. Agates usually contain significant amounts of other silica phases (opal, cristobalite, tridymite, quartzine, moganite, macro-crystalline quartz) and paragenetic minerals (carbonates, clay minerals, zeolites, iron compounds, etc.)…”

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].…”

“…This Special Issue was prepared during the 2020 and 2021 COVID-19 pandemic. Terry Moxon [2] died on 5 January 2021, from COVID-19. He was one of the leading scientists in the research of agates, an excellent reviewer, and a responsible coauthor.…”