Agate: a mineral that develops with age, water and moganite

Agate is banded or variegated chalcedony and this distinctive appearance allows a ready identification from any source. Many agate thick sections from basic igneous hosts are reminiscent of a series of distorted onion-like rings with the initial bands closely replicating the shape of the supporting gas cavity. However, the banding is frequently distorted and this general pattern is known under various names, for example, fortification or wall lining. A second type is less common and demonstrates apparently gravity-controlled horizontal bands. Agate host rocks are varied but the most abundant agate sources are the gas cavities of basic igneous rocks. This article limits discussion to agates from these basic hosts. However, agates can also be found in some igneous acidic hosts (for example, rhyolite), sedimentary rocks (for example, limestone) and in some fossils.

Agate is greater than 97% silica (SiO2) and shows little variation between different samples. Under normal earth surface conditions, silica occurs in a number of forms. It is most commonly found as alpha-quartz and this is the major component in agate. A second silica constituent is moganite with a concentration at 2 to 25%. Moganite is found in agate that has not been heated by metamorphism or in the laboratory. Together with alpha-quartz, they are usually the only forms of silica identified in agate. However, other forms of silica such as cristobalite and tridymite have been occasionally identified in agate. In agates from basic igneous hosts, calcite is a rarity, as demonstrated by an examination of 180 worldwide agates using powder X ray diffraction. Trace calcite was identified in just 7 agates and 4 came from one source – Las Choyas, Mexico (Moxon unpublished data). The major non-volatile impurities include iron, aluminium, magnesium and sodium but concentrations are very small and measured in parts per million; collectively the total is much less than 1% (Götze, 2001). Surprisingly, water (up to 2%) is the major agate impurity. However, only a fraction of the water is due to actual water molecules (H2O). The bulk of detected water is obtained through water loss from two neighbouring silanol groups (≡Si—OH).

Agate/chalcedony has been found in host rocks as young as 13 millions years (Ma) and as old as 3,720 Ma (Moxon et al., 2006). However, agate has never been made in the laboratory. It develops during ageing and these changes allow valid speculation about agate in its early years. This article considers the essential role that water and moganite play in these age-related changes. Mineral age-related development is rare but it is not unique to agate. One other example showing ageing changes is bone apatite. The change offers a means of approximately dating ancient bones (Bartsiokas and Middleton, 1992).

Relationship between agate and host rock age

Agates could form around the same geological time as the host or hundreds of millions years later. Hence, the relative timing of host and agate formation is a prerequisite before any attempt can be made to link agate and host rock age. Many agate host rocks have been dated using radioactive isotopes. Unfortunately, the agates themselves have not been dated due to either a limited radioactive isotope content and/or costs. Agate/chalcedony from the Yucca Mt., Nevada, USA is an exception and has been radiometrically dated together with the host. Yucca Mt., aged 13 Ma, was under consideration as a potential nuclear waste repository and investigated for more than 30 years. The intended nuclear storage has resulted in many studies examining water flow including its effect on the Yucca Mt. host rock minerals. One of these studies dated the chalcedony that had formed in Yucca Mt. tuffs. Initial chalcedony coatings appeared around 4 Ma after the host formation (Neymark et al, 2002). However, radioactive isotopes are generally limited in agate and published agate/chalcedony dating, as far as I am aware, is limited to this one example.

Alternative methods are required and agate properties do vary from source to source offering the opportunity to test potential links with the host rock age. Once a property has been identified, it needs to be investigated with respect to the known host rock age. A plot of any quantitative data against host rock age will produce either random values and is therefore of no use for dating purposes, or show trends with respect to the age of the host rock. If trends can be identified, then that particular property potentially provides a method of roughly dating the approximate agate age. Over the years, I have looked at a variety of property changes and found some that produce a host age link. Quartz crystallite size, density, moganite and total water content do show either partial or total connections with the host rock age. A number of agates need to be examined from a particular area but at best, the sample mean values show a variation range of 5 to 15% and none match the precision of radiometric dating.

Quartz crystallite size. Most minerals are crystalline: they have a definite form due to the long-range order of component atoms or ions. Even complete crystals in macrocrystalline quartz are composed of many smaller crystal units known as crystallites. Crystallites are measured in nanometres (1 nm = 10-9 m) and the crystallite size can be determined using powder X-ray diffraction (XRD). X-rays are part of the electromagnetic spectrum that has a decreasing wavelength when passing from infrared à visible lightà ultraviolet à X-rays. For XRD, a diffractometer generates and directs X-rays at finely ground crystalline powders and the resulting diffraction and interference effects are recorded electronically. The X-ray angle of incidence is measured in degrees theta but the collected data adds the angle of reflection and is recorded as 2 theta. Each pattern of peak positions is as diagnostic of a particular mineral as are human fingerprints. An amorphous (non-crystalline) substance such as glass or plastic is without peaks and shows as a single broad hump. The collection of peaks (Fig.1) are unique to α-quartz and the agate samples are from (a) Mt. Warning, Australia (23 Ma, host age); (b) Chihuahua, Mexico (38 Ma); (c) Lake Superior, USA (1,100 Ma). All show the same peaks that are given by Brazilian macrocrystalline quartz (d). The largest signal intensity is around 26o 2 theta. This is approximately five times greater than the second largest signal at around 20o 2 theta and 10 to 20 times greater than the remainder. A plot using full intensity would result in these two main peaks minimising the rest. Square root of signal intensity reduces this effect. Points of interest are:

1) Agate shows the same signal positions as the Brazilian macrocrystalline quartz.

2) The signals become narrower from (a) to (b) to (c) to (d). The narrower signals demonstrate an increasing crystallite size.

3) These XRD scans have used a fairly rapid time scan. A slower scan would reveal moganite in all the agate samples. However, the moganite signal (m) still shows its presence in the younger agates.

Moganite is the most recently discovered form of silica having been identified in 1984 and finally accepted as a new mineral in 1999. Detection of moganite in agate is usually done using powder XRD. However, moganite quantification can be difficult at low concentrations as it is widely distributed and makes a limited contribution to the moganite XRD signals. Raman spectroscopy is more sensitive and readily identifies trace moganite that has been detected in agate from hosts as old as 1,100 Ma. Visual evidence of this identified development will be considered next.

Scanning electron microscope (SEM)

Optical microscopes are limited to a magnification of about 1,000 x and a thin section examination of agates from hosts aged between 30 Ma and 1,100 Ma does not produce any observable age-related changes. Electron microscopes use accelerated electrons in place of light and the SEM has an increased magnification up to around 500,000 times. The SEM can be used to examine a fractured agate surface that has been coated in a conducting material such as gold or carbon. Early SEM work on fractured agate identified globules in the non-white areas (Lange et al., 1984). Further work distinguished differences between the white bands and the non-white area. The white bands in the older agates were described as showing a stacked plate-edge like structure while the globular structure showed a general age-related increase in size (Moxon, 2002). There is a common trend with an increasing age-related globular growth but differences are not readily quantifiable because there are variations in globular size within the same agate.

 

Deposits Fig 1ed_1
Fig. 1. X-ray diffraction signals produced by agates from: a) Mt. Warning, Australia (23 Ma host rock age); (b) Chihuahua, Mexico (38 Ma); (c) Lake Superior, USA (1,100 Ma). All show the same signals that are given by the Brazilian macrocrystalline quartz d). A weak moganite signal (m) is shown in two of the agate samples (a) and (b). The square root of reflection intensity has been used in all cases; silicon (Si) is added as an internal standard.

Relationships between different features are lost at high SEM magnifications and one examination routine is shown by the link between increasing size and detail in the Lake Superior agate (Fig. 2, I à IV). Here, the globular nature of the clear area (a) contrasts with a fine white band (b). This band is very unusual, the normal plate-like structure is not observed and it appears to be a collective infill of broken fragments. Further structural detail is demonstrated by taking enlargements around the centre producing the Fig. 2 micrographs. Much of the observed surface debris is caused by the preparation. However, the higher magnification shown in Fig. 2 (IV) does allow an observation of genuine globular growths that have also developed on the white band at (e). The micrograph in Fig. 3 shows a white band that is well formed and typical of that found in agates older than about 60 Ma. The bottom edge of the micrograph (c) shows the repeated vertical stacking of the plate-like edges. Occasional twists of the “plates” have produced flat surfaces (d).

The white band differences between young and old agates can often be demonstrated without the use of the SEM. In my experience, agates from younger hosts (for example, Mt Warning Australia (23 Ma); Isle of Rum, Scotland (60 Ma); Woolshed Creek, New Zealand (89 Ma); Rio Grande do Sul, Brazil (135 Ma) show white bands that are less intense and often more diffuse than those from older agates such as those from Botswana (180 Ma) or Lake Superior (1,100 Ma). If a lapidary diamond saw is available, then an age-related judgement can be made. The agate slabs in Fig. 4 I and II, both 2mm thick, are respectively from Botswana (180 Ma host age) and New Zealand (89 Ma). When viewed against strong white light, the white bands in the older agates reveal a distinct orange/brown colour (white band “a” in Fig. 4 I and III). Younger agates either transmit the full visible light spectrum or produce a weaker brown colour (Fig. 4, IV). The difference is caused by the loss of the blue end of the visible light spectrum. Blue has a smaller wavelength and the transmission of white light through the well-developed plate edge-like structures results in the smaller wavelengths being bounced and scattered. Hence, the final transmitted light in the older agates is mainly from the larger wavelengths: orange/red giving this brownish colour. The poorly formed white bands in younger agates transmit more, or all, of the smaller visible wavelengths.


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Terry Moxon (UK)

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