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depositsmag.com/2018/03/27/agate-a-mineral-that-develops-with-age-water-and-moganite/
Agate: A mineral that develops with age, water and moganite
27/03/2018
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Terry Moxon (UK)
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.
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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.
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.
- - -
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.
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.
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).
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).
High temperature dehydration of agate
Agate enthusiasts will be aware of the Brazilian enhydro(s) agates where bulk water has become trapped at the centre of the agate. Less well known is the fact that practically all agate contains water molecules that are free to enter and leave through structural pathways (Moxon, 2017). The ease of movement depends on the age of the agate and the surrounding water vapour pressure. In addition to free H2O, there is the less well-known silanol water (≡Si—OH). These silanol groups are found on the surface and within the agate structure. Strictly, silanol water is not water but the hydroxyl group. Agate is rich in these groups and, over the geological time scale, two neighbouring Si—OH groups combine to release water and form the Si—O—Si bond; this silanol water is often referred to as structural defect water.
Agate enthusiasts will be aware of the Brazilian enhydro(s) agates where bulk water has become trapped at the centre of the agate. Less well known is the fact that practically all agate contains water molecules that are free to enter and leave through structural pathways (Moxon, 2017). The ease of movement depends on the age of the agate and the surrounding water vapour pressure. In addition to free H2O, there is the less well-known silanol water (≡Si—OH). These silanol groups are found on the surface and within the agate structure. Strictly, silanol water is not water but the hydroxyl group. Agate is rich in these groups and, over the geological time scale, two neighbouring Si—OH groups combine to release water and form the Si—O—Si bond; this silanol water is often referred to as structural defect water.
The plot exhibits a four-stage agate development pattern for the first 450 Ma. Initially, there is 60 Ma of linear growth followed by growth cessation for the next approximate 200 Ma. Growth restarts for around 30 Ma followed by little change for the next 150 Ma. There is no further change with agates from hosts aged 1,100 Ma. Agates from Brazil (135 Ma host age) #10 and New Zealand (89 Ma) # 9 are clearly off trend: suggesting agate formation ages of 25 and 30 Ma respectively (data from Moxon and Carpenter, 2009).
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