Agate and chalcedony from igneous and sedimentary hosts aged from 13 to 3480 Ma: a cathodoluminescence study

Abstract

Chalcedony and agates from a variety of world-wide hosts have been examined using cathodoluminescence (CL). Gaussian fitting of the experimental data shows that there are two dominant spectral emissions at ∼400 and ∼660 nm. A third subordinate peak is also found at ∼470, ∼560 or ∼620 nm. An age-related link is shown between the respective decreasing and increasing relative intensities of the 660 and 620 nm emissions. It is proposed that this change is due to a condensation reaction between neighbouring Si–OH groups eliminating water and forming a strained Si-O-Si bond.Agates from a variety of hosts and regions produced no clear demonstrable CL distinctions. However, a set of Western Australian agates was examined from host rocks that had been subjected to burial metamorphism. Cathodoluminescence produced different spectral emissions in the petrographic fibrous and granular regions of these agates. One agate shows a partial transformation of the petrographic fibrosity into granularity. This conversion is characterized by emission bands at 570 nm and 460 nm. Similar emission-band changes were produced by heating Brazilian agates for 35 days at 300°C. The identification of these changes in agate could serve as an indicator of palaeoheating within the parent rock.

Knappable Materials in the Criș Valley, Romania

“…Banded chalcedonies (whether parallel or concentric) are called agates (Flörke et al 1982;Graetsch et al 1985;Heaney & Davis 1995;Moxon & Reed 2006;Moxon et al 2007). Chalcedony, although technically a form of microcrystalline quartz, has a somewhat different micro-structure and may form differently.…”

Mineral chemistry of some agates from Gurasada (Mures Valley, Romania)

“…According to Moxon and Reed (2006), most of agates consists of a mixture of two silica polymorphs: α-quartz and moganite. Among the quartz structural types, the length-fast chalcedony (chalcedony s.s.), quartzine (also called lengthslow chalcedony), pseudochalcedony and microquartz are the most common.…”

Chemical Treatment Effects of Blue, Yellow and Green Colors with Heating Methods on Agates of Cheshme Shoor Area, Qom, Iran

“…Agates forms as an infilling within some basaltic or rhyolithic gas vesicles but the mechanisms that produce agate in an igneous environment remain an enigma, also the silica sources, temperature, method of deposition, transportation and final mechanism of crystallization are all unknowns that have added to the difficulties in determining the origin of agate [3]. The majority of recent workers would accept that agates form at temperatures <100˚C [4] and consist of a variety of quartz. These minerals contain varying amount of water (H 2 O and Si-OH groups) which can be used to determine the mineral species present [5].…”