3 Sample Characterisation
3.5 Textural Information in Luminescent Signals
Texture can incorporate both chemical and structural information. Luminescence is an ideal tool to explore this phenomenon.
Figure 3-1 (a) The image shows a growth pattern in a volcanic fluorite crystal from Chemnitz, Germany
Displays strong oscillatory growth zoning that is only visible under CL. The CL image of crystals of fluorite that can be interpreted as encoding the chemical and temperature history of the genesis of the crystals.
(b) The CL spectrum of a distinct green blue zone (as shown by the circle in a) reveals the dominant activation of the luminescence by Sm3+ and Dy3+ ions source (Götze, 2009).
The interpretation of such phenomena present intriguing potential into magma evolution and element enrichment processes with applications for prospecting. The restraining factor is the complexity of luminescence, and the problem of deconvolution and positive identification, characterisation and interpretation of individual
luminescence centres. This is inherently difficult due to the reliance on the correlation between the concentration of the potential lumiphore and the intensity of luminescence that has been implicit in much work completed to date (Dalby et al., 2009).
3.6 Feldspar
Feldspar [(Ca,Na,K)1(Fe,Al,Si)4O8] is a framework silicate similar to quartz (SiO2) which
can be considered as an infinite network of SiO4 tetrahedra combined with AlO4.
Name End member
composition
Space group Tetrahedral order
Sanidine KAlSi3O8 C2/M Disordered
Microcline KAlSi3O8 ̅ Ordered
High Albite NaAlSi3O8 ̅ Disordered
Low Albite NaAlSi3O8 ̅ Ordered
p- Anorthite CaAl2Si2O8 ̅ Ordered
Table 3-2 - Chemistry and summary of the crystal structure of the feldspar group end members.
(Smith and Brown, 1987)
The substitution of Si4+ by Al3+ requires a charge balancing coupled substitution that is accomplished using three elements; K+, Na+, or Ca2+, and these ions reside in the interstitial spaces in the framework. This chemistry determines the creation of a solid solution with three primary end members as illustrated in Figure 3-2. Within the alkali feldspars, the substitution can be considered as a simple solid solution between Na and K, whereas in the plagioclase series the substitution is better modelled as a coupled substitution as described in the formulae below.
Figure 3-2 - Ternary diagram showing chemical and compositional boundaries in the feldspar group.
Source: (Evans and Grove, 2004).
Figure 3-2 illustrates the chemical differentiation that takes place within the solid solution and the further differentiation that takes place in feldspar according to the temperature of formation. This illustrates that to characterise feldspar correctly, more information than simply chemical composition is required, and an understanding of the internal atomic arrangement is necessary. Many feldspars also comprise of intergrowths of more than one composition, sometimes on such a small scale as to make them
undetectable even with a high power optical microscope e.g. cryptoperthite.
Feldspars are defined by both chemistry and structure; feldspars grown at high temperature can be of compositions unstable at lower temperatures and exsolution occurs. Hence, feldspars are frequently intergrowths of two or more feldspar types. The process, which governs the chemistry and structural state of the feldspar formed, are a function of the genesis and geological history of the mineral.
MISI Iron Rich Microcline
3.6.1
The sample MISI is iron rich alkali feldspar that has no provenance, apart from a verbal description that it originated from the USSR. It has the generic formula of KAlSi3O8 with
common trace element impurities of Fe, Ca, Na, Li, Cs, Rb, H2, O, Pb is triclinic, a non-
standard unit cell but by convention classified as ̅, space group ̅ .
Figure 3-3 - Maximum microcline refinement.
Pale blue – Al , Red – O, Large Blue –O, Small dark blue - Si CrystalMaker © data (Brown and Bailey, 1964)
Cleavelandite
3.6.2
Is a variety of albite with a chemical formula of NaAlSi3O8. It is triclinic ̅ with space
group ̅ and is described as a low albite with warped plates parallel to (010). It should be noted that most albite is low albite.
Figure 3-4 - Albite cleavelandite refinement. Yellow – Na, Red – O Dark blue – Si, Light Blue - Al CrystalMaker© data (Ribbe et al., 1969)
Moonstone
3.6.3
Moonstone is an alkali feldspar (NaK)AlSi3O8 best described as a microcline, which has a
typical composition Or25- Or35. It is a cryptoperthite with the perthite lamellar
comparable in thickness to the wavelengths of light as shown by (Fleet and Ribbe, 1963) and Bollmann and Nissen (1968) using an electron microscope. They also showed the typical adularescence (blue sheen) seen in moonstone is due to the irregular nature of the alternating sheets of orthoclase and low albite that were approximately parallel to
̅ ] to ̅ [. The subsequent interference effects occurred because of reflections
from the irregular planes. Further descriptions are contained in (Bollmann and Nissen, 1968, Akizuki and Sugawara, 1970, BACHINSKI and MÜLLER, 1971). Moonstone was selected as a sample to explore if the cryptoperthite lamellar structure of moonstone would be a significant influence on XEOL and TR XEOL emissions.
Copper Bearing Feldspar
3.6.4
Plush Oregon sunstone is plagioclase feldspar, compositionally a labradorite in the range of An 50-70. It shows a moderate to high order and is found in a highly fractionated
porphyritic basaltic flow as large phenocrysts where green, red, and yellow to colourless varieties are found, some displaying a ‘schiller’ ( from the German ‘twinkle’, an
iridescence from within the body of the stone) .
Figure 3-5 - Labradorite refinement.
Small dark blue –Si, Small light blue - Al, Large light blue – Ca, Red - O CrystalMaker © data (Angel et al., 1990)
This copper bearing feldspar is a relatively rare gemstone, found in the Plush region of the state of Oregon in the USA. This gem quality feldspar (commonly known as ‘Plush’ feldspar) has a copper content in the range 50-500 ppm and is found in a number of varieties including colourless, red, green, and an included material displaying a schiller
from metallic platelets (Hofmeister and Rossman, 1985). In the last decade, significant amounts of copper diffused andesine have entered the commercial gem market imitating the natural ‘Plush’ material and there is an urgent need to discriminate
between the natural and treated material. The commercial treatment/diffusion process used has not been disclosed. The samples investigated include known commercially treated samples and known natural material. I also tested samples I prepared in the laboratories at St Andrews these treated samples included samples of both plagioclase and alkali feldspars. I replicated the treatment process described by Emmett et al (2009) that they considered likely to be comparable to the commercial technique. The process is completed at relatively high temperatures ~10000c with the samples to be treated packed into a refractory medium of powdered ZnO doped with between 1-5 weight % CuO for periods of 24 -168 hours heating taking place in an oxidising atmosphere.
The first description of the natural material was probably Aitkens (1931) who noted the occurrence of a gem quality labradorite, with a similar description to the material currently recovered, the original description was from an ‘unspecified locality’ in southern Oregon. The geology of the areas currently exploited for ‘Plush feldspar’ was described by Peterson (1972) that drew upon work completed by Fuller (1917).
Hofmeister and Rossman (1985) produced a comprehensive characterisation of the natural gem material. They concluded that the colouration of the red material was due to the presence of copper, but they were less convinced regarding the source of the green colour, although one suggestion considered was that copper was responsible for both colours and the size of nano inclusions of copper metal determined the colour. A
(2010). The current accepted cause of colour in the red plush feldspar is that it is due to either copper substitution or the inclusion of metal nano particles (Hofmeister and Rossman, (1985). The origin of the green colour has not been satisfactorily determined. My research explores both natural copper bearing samples ‘plush feldspar’, and
compares them with samples with samples having an artificially induced copper content following diffusion by heat treatment. I completed heat treatment experiments on a suite of feldspar samples representing both alkali and plagioclase members included in
Table 3-3.
Sample ref Description
RT 1-5, 21, 22, 23, 54, 55, 56, 58, 29, 30, 35, 36 45
Commercially treated Andesine
RT
6,7,15,16,40,42,43,44
Natural occurring Plush Feldspar
RT46 Albite natural colourless
RT47 Bytownite natural colourless
RT48 Microcline natural colourless
RT49 Andesine natural colourless
RT50 Labradorite natural colourless
RT51 Anorthite natural colourless
RT52 Oligloclase natural colourless
RT53 Andesine natural colourless
RT57 Andesine natural colourless
AF1 Cleavelandite natural colourless