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by gagan choudhary
Aventurine K-feldspar
Author: Gagan Choudhary
(This article was first appeared in Midlands Focus, 14th issue, pp 13 – 17)
Introduction
Feldspar is one of the most common minerals found on the earth crust however, gem quality are somewhat rare. When found they display a wide range of mesmerizing optical effects or ‘phenomenas’; adularescence, chatoyancy, asterism, play of colour or labradorescence and aventurescence are quite commonly encountered in gems of the feldspar group.
The feldspar group is divided into two main sections depending on the compostion- potassium feldspars (including orthoclase, microcline and sanidine) and the plagioclase feldspars (including albite, oligoclase, andesine, labradorite, bytownite and anorthite). Out of the various species in the plagioclase series, oligoclase and labradorite are the most important gem-quality species. Oligoclase (sunstone) is characterized by its optical phenomena known as ‘aventurescence’ or this variety also known as ‘aventurine feldspar’.
Figure 1: These three specimens of feldspar showing aventurescence and asterism were proved to be K-feldspar rather than sunstone (oligoclase) feldspar
Three specimens- orange brown, pinkish brown and a dark brown were submitted for identification at the Gem Testing Laboratory of Jaipur, India weighing 6.32, 7.29 and 1.57 carats respectively. Two specimens (orange brown and pinkish brown) were faceted (figure 1, a & b) while one was fashioned as a cabochon and displayed four-rayed asterism (figure 1.c). All these specimens were proved to be potassium feldspars rather than oligoclase (sunstone).
Visual observations: The 6.32 carat faceted orange brown sample (again, figure 1.a) displayed a distinct aventurescence which appeared to be caused by elongated inclusions while the 7.29 carat pinkish brown stone (figure 1.b) had somewhat weaker effect caused by scattered brownish inclusions; in both specimens inclusions were seen with unaided eyes. The 1.57 carat cabochon displayed four-rayed asterism as well as aventurescence beneath the star rays (figure 1.c); one of the rays was stronger as compared to the other. All the three samples were translucent in diaphaneity due to presence of numerous ‘phenomena’ causing inclusions. The visual appearance of these three specimen indicated feldspar but the species was still to be determined. However, aventurescence is very well known in sunstone oligoclase species.
Gemmological Properties: The gemmological properties of three specimens are summarized in table 1 and described below.
Refractive Index: The RI values of the faceted specimens varied between 1.520 and 1.530 with negative in sign and birefringence of approximately 0.007. These values were with in the range with those reported for orthoclase feldspar and slightly lower than the oligoclase (sunstone) feldspar. The 1.57 carat cabochon displayed a spot RI at around 1.530.
Specific gravity: SG value of the 7.29 carat specimen was measured at 2.58 while the other two at 2.59.
Fluorescence: All the three specimens displayed a weak red glow under short wave UV while inert under long-wave.
Absorption spectrum: No absorption in the visible region was observed under desk model spectroscope in any of the samples.
Table 1: Gemmological properties of three specimens of aventurine K-feldspar
Microscopic Features: Studied samples display mainly the mineral inclusions which were oriented as per the growth patterns of the feldspar group, i.e. lamellar (or in layers). Specimen weighing 6.32 carats was consisted of fine long ‘lath’ like inclusions oriented in planes in one direction; these inclusions appeared dark brown to black in transmitted light (figure 2.top) while bright iridescent in reflected light (figure 2.bottom). These lath-like inclusions have previously been identified as hematite that causes this optical phenomenon. In addition, this specimen also exhibited fine whitish platelets oriented along the direction of ‘lath’ like inclusions (figure 3, left) while some scattered and reflecting (figure 3, right). Further, some wavy growth features (figure 4) were also observed in this specimen. Similar lath-like inclusions were also observed in the asteriated specimen.
Figure 2: These ‘lath’ like inclusions were responsible for aventurescence in 6.32 carat specimen which appear black and /or brown in transmitted light (top) while display strong iridescent colours under reflected light using fibre optic (bottom)
Figure 3: Some whitish flaky inclusions were also observed oriented in planes (left) along ‘lath’ like inclusions as well as scattered throughout (right)
Figure 4: Wavy growth features were also observed in the specimen weighing 6.32 carat oriented perpendicular to the ‘lath’ like inclusions
Figure 5: These scattered reddish brown platelets which appeared to be pseudo-hexagonal and rhomb-shaped are typically associated with hematite were observed in 7.29 carat specimen. Also note the skeleton pattern in few platelets
EDXRF analysis: Qualitative EDXRF analysis revealed the presence of Al, Si, and K with traces of Ca, Ba and Fe. These properties are consistent with the alkali (potassium) feldspar rather than plagioclase feldspar (or oligoclase feldspar).
FTIR analysis: FTIR spectra in the region 400 to 6000 cm-1, displayed a complete absorption till approximately 2200 cm-1 and absorption in 3000 and 3800 cm-1 region. The pattern of absorption varied from a small hump to strong absorption band depending on the transmission of wavelengths. Sample (cabochon) with a better transmission displayed a small hump while the samples (faceted) with a lower transmission exhibited a strong absorption band. However, IR spectra did not indicate the species at all.
Conclusions: The samples studied did appeared to be sunstone feldspars as they readily displayed aventurescence, however physical, optical and chemical properties did not match to those of sunstone but to those of potassium feldspar. The overall features of these specimens were consistent to those of orthoclase feldspar. However, we do not know the exact locality of these stones; K-feldspars with similar properties originate from Tanzania. These stones display the visual appearance of sunstone feldspar but the properties are slightly lower which may not prove to be strongly conclusive. In such cases, EDXRF analysis proved to be the identifying criteria of these feldspars and without this analysis, they might have been misidentified.
References
- Fritz E. & Laurs B.M. (2007). Gem News International. Cat’s eye K-feldspar and other chatoyant gems from Tanzania. Gems & Gemology, Vol 43, No.2, pp 170-171
- Henn U. (2004). Aventurescent oligoclase feldspar from Oregon, USA. The Journal of Gemmology, Vol 29 No.2, pp 72-74
- Laurs B.M. (2002). Gem News International. Sunstone feldspar from Tanzania. Gems & Gemology, Vol 38, No. 2, pp 177-178
- Webster R. (2002). Gems – Their sources, descriptions and identification, fifth edition. Butterworth-Hainemann, Oxford.
All photographs and photomicrographs by Gagan Choudhary