Six samples of vein quartz were provided by Mr Killian Driscoll in June 2009. The samples are representative of larger collections of experimentally knapped quartz derived from samples collected from outcrop at three localities at Belderrig, Co. Mayo. The six samples studied derive from three blocks of vein quartz, comprising of three unburnt samples and three that had been burnt in a fire. The six samples studied comprise three unburnt samples and three from the same localities that had been burnt in a fire. These two sets of samples are referred to hereafter as unburnt and burnt, respectively. The samples have been numbered for reference in this report as shown in Table :
sample weight (g)
Table 1 List of samples
Geologically, all samples are from hydrothermal veins, that is to say they crystallized from hot brine in fractures opened up in the rocks which now enclose the veins.
The burnt samples were heated rapidly by placing in a pine wood fire started one hour previously. They were left in the fire for about two hours as the fire burnt away and were then removed by hand, having become cool enough to handle. The maximum temperature reached by the quartz samples will have been at the beginning of the two hours spent in the fire. This temperature was not measured but can be estimated to be at least 500 °C (Urbas and Parker, 1993).
Samples were examined with the naked eye and with a hand lens. All unburnt samples are translucent and consist of massive crystals with grain sizes of a few millimetres (up to a few centimetres in sample 5). The burnt samples in every case are more opaque than the unburnt equivalents. There is a higher density of visible fractures in the burnt samples compared to the equivalent unburnt samples. Where traces of the rock enclosing the quartz are included with the sample (psammite and metadolerite localities), the burnt host rock has a more reddish brown colour compared to the more greenish brown of unburnt host rocks. There are no obvious textural differences between the quartz in the burnt samples compared to their unburnt equivalents.
Standard petrological thin sections 0.03 mm thick with cover slips were made of each sample. Thin sections were examined using a transmitted light petrological microscope, both in plane polarised light and between crossed polars, at total magnifications of 20 to 500.
All samples consisted of >99% quartz. Other minerals present have not been identified but in all cases are present only as isolated grains <0.05 mm diameter. The edges of the veins contain occasional fragments of the surrounding rocks, but these regions have not been studied. Some of the more significant textural features observed in thin section are summarised in Table 2. The three types of difference observed between burnt and unburnt samples are described in sections 3.1 – 3.3.
The most consistent difference concerns fluid inclusions (Roedder 1984). These range from tens of mm in size down to the limit of resolution of the microscope (about 1 mm). Fluid inclusions form when the quartz crystals grow from hydrothermal fluid and represent small samples of that fluid. On cooling they usually separate into a liquid and a bubble of vapour, as is seen in all three samples studied. In the unburnt samples, many fluid inclusions are intact and can be recognised by the presence of a darker, rounded vapour bubble within an otherwise liquid inclusion (Figs. 1 a, b). In the burnt samples, no fluid inclusions above about 5 mm have retained their fluid contents and all are gas filled. In some cases, empty fluid inclusions are surrounded by fluid escape structures (Figs. 1 c, d). The loss of fluid contents, called decrepitation, is a predictable consequence of burning in a wood fire. This is because the pressure generated by the fluid will have considerably exceeded that prevailing at the lower temperature conditions of hydrothermal quartz growth. Decrepitation will also be favoured by fractures formed during heating. Decrepitation of fluid inclusions is likely to be a general feature of hydothermal quartz exposed to a wood fires because almost all hydrothermal quartz crystallizes at temperatures below 500 °C.
|Number & treatment||Grain size
|Fluid inclusions||Fractures||Sub-grain textures|
|1 Unburnt||2 - 10||Two phase (liquid + vapour bubble) and empty||No macroscopic fractures; few microfractures||Sub-grain boundaries; undulose, and sometimes lamellar, extinction|
|2 Burnt||2 - 10||Empty only||Few macroscopic fractures; abundant microfractures in most crystals, with orientations varying between crystals||Sub-grain boundaries; undulose extinction|
|3 Unburnt||1 - 8||Two phase (liquid + vapour bubble) and empty||No macroscopic fractures; abundant microfractures in many crystals||Sub-grain boundaries; undulose, and sometimes lamellar, extinction|
|4 Burnt||1 - 8||Empty only, with fluid escape structures||Few macroscopic fractures; abundant microfractures in most crystals, with orientations varying between crystals||Sub-grain boundaries; undulose extinction|
|5 Unburnt||4 - >30||Two phase (liquid + vapour bubble) and empty||No macroscopic fractures; few microfractures||Sub-grain boundaries; undulose, and common lamellar, extinction|
|6 Burnt||4 - >30||Empty only, with fluid escape structures||No macroscopic fractures; abundant microfractures in most crystals, with identical orientations across sub-grain boundaries||Sub-grain boundaries; undulose extinction|
Table 2 Features observed by thin section examination
Figure 1. Appearance of fluid inclusions before (a, b) and after (c, d) exposure to fire. All photographs taken in plane polarized light with a horizontal field of view of 145 µm.
All three samples show an increase in either macroscopic (visible to the naked eye), or microscopic fractures or both. In burnt samples, microfractures have developed in one or more orientations within a given crystal and they differ in abundance and in orientation from one crystal to another. They tend to have a characteristic length of 50-100 mm and appear not to be interconnected. Macroscopic fractures in burnt samples have developed along quartz grain boundaries and/or within crystals. They have a spacing and length of millimetres to centimetres.
Between crossed polars, all three unburnt samples exhibit sub-grain boundaries, common patchy, undulose extinction; on a much smaller scale a repeating lamellar undulose extinction pattern is exhibited by some crystals in all three samples (Fig. 3). These features indicate discontinuities (sub-grain boundaries) and continuous spatial variation (undulose extinction) in the atomic structure of the crystal. They all probably reflect geological stress to which the crystals have been subjected since they formed.
Comparison of the burnt with the corresponding unburnt samples reveals that undulose extinction and most sub-grain boundaries are little affected, if at all, by heating. However the lamellar structures are absent in the burnt samples. The reason for the loss of the lamellar structures is unknown but might reflect the release of strain in the crystals during heating.
Both fluid inclusion decrepitation and fracture development may affect knapping behaviour and hand specimen transparency. The removal of lamellar undulose extinction is unlikely to affect either property because no new weaknesses are created in crystals.
Macroscopic fractures, with a spacing of millimetres to centimetres, run entirely through samples or through much of the sample. Burnt samples will therefore break readily along macrofractures when the quartz is knapped.
Microfractures are unlikely to affect the relative resistance to knapping in different directions because whilst fractures develop in preferred orientations in each crystal, their orientations differ from crystal to crystal. The overall strength reduction may be modest because microfractures are not interconnected. However sample 6 may be an exception because it includes crystals large enough to be individually knapped. Individual burnt crystals from sample 6 may break more easily during knapping than in its unburnt equivalent (sample 5) along the preferred microfracture directions.
The reduced transparency of quartz in hand specimen after heating in a fire may be due to two factors: decrepitation of fluid inclusions and formation of microfractures. After decrepitation, fluid inclusions contain residual vapour from the inclusion and/or air, but no liquid. The loss of liquid increases the refraction of light passing between quartz crystal and the inclusion and so decrepitated inclusions block more light than those which are mainly liquid filled. The difference in light transmission can be appreciated by comparing intact inclusions (Fig. 1a,b) with decrepitated inclusions (Fig. 1c,d). Microfractures can also reduce the transmission of light in a similar way to inclusions, by refraction of light when it crosses a fracture. Whilst light can also travel along fractures so that they appear bright in thin section (Fig. 2b), the overall effect of many microfractures in many orientations in a hand sample should be to reduce light transmission. Whether microfacturing or fluid inclusion decrepitation is the main cause of increasing opacity in the samples studied remains uncertain.
Fig. 2. Fractures in unburnt and burnt quartz samples. Both are photographed between crossed polars and have a horizontal field of view of 1220 µm.
Fig. 3. Types of undulose extinction in unheated quartz, sample 5, crossed polars.
Roedder, E. (1984) Fluid Inclusions. Reviews in Mineralogy v. 12, Mineralogical Society of America, 646 pp..
Urbas, J. and Parker, W.J. (1993) Surface Temperature Measurements on Burning Wood Specimens in the Cone Calorimeter and the Effect of Grain Orientation. Fire and Materials v. 17, pp. 205-208.
30th July 2009