www.gem-passion.com
by gagan choudhary
Dioptase from Mozambique
Author: Gagan Choudhary
(This article was first appeared in The Australian Gemmologist, Vol. 24, No.9, pp 215-219)
Introduction
Dioptase, a hydrated silicate of copper, CuSiO2(OH)2, known for its emerald green colour is mainly a collectors’ mineral. Dioptase is rarely used as gem; it is relatively soft with a hardness of 5, lacks toughness and large-sized single crystals are uncommon. The usual occurrence is as grains or druses in massive aggregates. Chemically, dioptase is related to chrysocolla and is found worldwide in copper deposits in association with anglesite (Choudhary and Golecha 2008), quartz, cerussite, fluorite, mimetite, malachite, and hemimorphite amongst others (rruff.geo.arizona.edu/doclib/hom/dioptase.pdf). Some of the known deposits of dioptase include Russia, Kazakhstan, Republic of Congo, Namibia, Zimbabwe, Argentina, Chile, Peru, Czech Republic, Germany, Italy, New Zealand, Australia and USA (www.mindat.org/min-1295.html).
The author had a chance to study one bluish green specimen of dioptase (figure 1) weighing 8.88 carats that displayed its typical crystal form having polished surfaces and said to be from Mozambique. This crystal was interesting due to its crystal shape, relatively large size and transparency.
Figure 1: This bright bluish green prismatic crystal of dioptase was interesting for its size and transparency. Also note that the crystal is terminated by rhombohedral faces, typically associated with minerals belonging to the trigonal system
Materials and Methods
The dioptase specimen submitted to the Gem Testing Laboratory, Jaipur, India was typically reminiscent of a fine quality emerald; it weighed 8.88 carats and measured approximately 16.00 mm x 7.65 mm (Figure 1). The sample had been bought in Mozambique and was presumed to be emerald.
Standard gemmological tests performed included:
- RI measurements (because the sample had polished surfaces).
- Hydrostatic specific gravity (SG), measured using Mettler Toledo CB 1503 electronic balance.
- Exposures to standard long-wave (366 nm) and short-wave (254 nm) UV radiation were used to document fluorescence reactions.
- The spectrum was observed using a desk-model GIA Prism 1000 spectroscope.
- We examined the internal features of the sample with a binocular gemmological microscope with fibre-optic and overhead reflected light and as well as with immersion.
Additionally, the infrared spectrum in the 7000-400 cm−1 range at resolution of 4 cm-1 and 50 scans was recorded using a Shimadzu IR Prestige 21 Fourier transform infrared spectrometer (FTIR) at room temperature with a diffused reflectance and transmission accessories. The spectrum of the powder obtained from one of the corners of the sample was also recorded by KBr technique, as the macro sample did not provide distinct features. The spectra obtained were then converted into absorption spectra using the software. Qualitative energy-dispersive X-ray fluorescence (EDXRF) analysis was performed with PANalytical Minipal 2 instrument using two different settings. Elements with low atomic numbers such as silicon were measured at 4 kV tube voltage and 0.850 mA tube current. Transition or heavier elements were measured at 25 kV tube voltage and 0.025 mA tube current.
RESULTS AND DISCUSSION
Visual Characteristics
The specimen was dark bluish green, a colour typically associated with translucent emeralds exhibited under normal lighting/viewing conditions (Figure 1). However, under transmitted light, it displayed a much brighter green colour and a better degree of diaphaneity (Figure 2), which is quite rare in dioptase. Due to the typical colour shade, the sample was initially presumed to be an emerald; because of the bright vitreous lustre, relatively higher heft and characteristic crystal shape, this presumption was ruled out.
The crystal displayed a distinct six-sided prismatic habit terminated by rhombohedral faces (Figures 1 and 3); a crystal form typically associated with minerals of the trigonal system (Fernandes and Choudhary, 2010). Although not illustrated, such prismatic crystals have been reported for dioptase (Dana and Ford, 1932). In our specimen, many of the crystal faces were polished in a manner that retained their original form. This made our observations much easier.
Figure 2: Under transmitted light, the dioptase crystal revealed its bright green colour and transparency
Some of the faces displayed hexagonal-shaped cavities, which suggested the presence of associated/attached crystals of hexagonal profile, most probably dioptase itself. These cavities contained impressions of rhombohedral faces with striations (Figure 4). One of the crystal faces also had a twinned daughter crystal attached to it that appeared to be protruding out of the main crystal (Figure 5).
Although the visual characteristics indicated that the specimen was dioptase, a thorough analysis was required to confirm its identity.
Figure 3: Profile view of the crystal shown in figure 1, displaying hexagonal shape and rhombohedral faces. It is not known whether the basal plane is original or was formed during polishing of the faces
Figure 4: Some of the faces displayed hexagonal-shaped cavities, suggesting the presence of associated / attached crystal with hexagonal profile. Also note the -pattern of faces and striations, indicating the presence of rhombohedral faces. Magnified 35x
Figure 5: One of the crystal faces also had twinned daughter crystal attached that appeared to be protruding from the main crystal. Magnified 25x
Gemmological Properties
Gemmological properties as observed in the bluish green dioptase crystal are described below and shown in Table 1.
Table 1. Properties of the 8.88 carats dark bluish green dioptase crystal
Due to the polished surfaces of this crystal specimen, we were able to measure all gemmological properties.
Refractive indices were measured at 1.652 (o-ray) - 1.705 (e-ray) with double refraction of 0.053 and its optical character determined as uniaxial positive. Hydrostatic specific gravity was measured at 3.28. Using a desk model spectroscope a broad absorption band in the yellow-green region, centred around 550 nm was seen in addition to strong absorption in the violet-blue region. Under both long-wave and short-wave ultra violet light, the specimen was inert. No distinct pleochroism was seen. These gemmological properties are consistent with those reported for dioptase (Webster, 1994).
Microscopic Observations
The most prominent and characteristic feature observed in this crystal were the milky zones/planes. When viewed down into the ‘c’ axis or the basal plane, white milky zones are visible oriented in three directions (Figure 6) following the edges of the rhombohedral faces and displaying the trigonal symmetry of the crystal. Observations through the prism faces reveal distinct conical zones/planes (Figure 7). These are phantom-like growth planes comprising aligned fine whitish pinpoints that are oriented along the rhombohedral and prismatic faces of the crystal.
Figure 6. When viewed down into the ‘c’ axis or the basal plane of the crystal, white milky zones were visible oriented in three directions following the edges of the rhombohedral faces and showing the trigonal symmetry. Magnified 64x
Figure 7: Observations through the prism faces show distinct conical zones or phantom-like growth planes emphasized by fine whitish pinpoints. Magnified 48x
Figure 8. Numerous reflecting liquid films and fingerprints were seen in the crystal, some of which also appeared to be aligned along the rhombohedral planes (not shown here). Magnified 64x
Figure 9: Cleavage, one of the most important and characteristic features of dioptase, was visible as intersecting planes in transmitted strong light; these planes follow the rhombohedral directions. Also note fine growth lines along the length of the crystal. Magnified 48x
In addition, the crystal has numerous reflecting liquid films and fingerprints (Figure 8), some of which were also aligned with the rhombohedral faces. Strong doubling of inclusions was also visible due to high double refraction of 0.053. Cleavage, one of the most important and characteristic features of dioptase, was also visible in this specimen. Cleavage planes oriented along rhombohedral directions appear as intersecting planes when strong light is transmitted through the crystal (Figure 9). Growth zoning was also observed oriented along the prism faces while viewed immersed in liquid.
FTIR analysis
Since dioptase is a hydrous material, IR spectra taken in transmission and diffused reflectance modes in the range 7000 – 400 cm-1 exhibited broad absorption bands at around 7000 - 6250 and 5250 – 4500 cm-1 and complete absorption of wavelengths from 4350 to 400 cm-1 (Figure 10). The spectrum ranging from 5250 to 4500 cm-1 appeared to consist of two bands closely spaced at around 5100 and 4750cm-1. These bands are related to OH and Si-OH combination modes (Frost et al., 2007). This spectral pattern is similar to hydrous materials such as serpentine (Choudhary, 2009), opal (Choudhary and Bhandari, 2008) and others. This pattern matched with that present in our database for the single crystal of dioptase obtained in transmission mode.
Figure 10: IR spectra taken in transmission and diffused reflectance modes in the range 7000 - 400cm-1 exhibited broad absorption bands at around 7000 - 6250 and 5250 - 4500 cm-1 and complete absorption of wavelengths from 4350 to 400 cm-1. This pattern is similar to those observed in other hydrous minerals.
Figure 11: For better clarity, IR spectra were also measured by KBr method. The spectra in the region 4000 - 400 cm-1 displayed strikingly clear and distinct peaks related to O-H vibrations, Si-O stretching and Si-O bending modes. This was similar to that reported in RRUFF database for dioptase.
EDXRF analysis
Qualitative elemental analysis revealed the presence of Si and Cu, the elements expected for dioptase. No other element was detected.
Conclusion
Because dioptase typically occurs as small crystals it is considered a collectors’ gem and used rarely as fashioned gemstone. However, it can occur in larger crystals as detailed in this report and can provide a fine quality material for faceting. This crystal is significant due to its size and transparency although cleavage would make cutting and polishing and use in jewellery more difficult.
The identification of dioptase should not be a problem for practising gemmologists because of its typical crystal shape, gemmological properties and infrared spectra especially using the KBr technique.
The origin of this dioptase was reported as Mozambique where it was purchased, however its provenance from other African localities cannot be ruled out.
References
Choudhary G. (2009) Serpentine cat’s eye. Gems & Gemology, 45(2), pp. 151-152.
Choudhary G. and Golecha C. (2008) Note from the laboratory – Anglesite, an unusual collector’s gemstone, The Australian Gemmologist, 23(7), pp. 314-315.
Choudhary G. and Bhandari R. (2008) A new type of synthetic fire opal: Mexifire, Gems & Gemology, 44(3), pp. 228-233
Dana E.S. and Ford W.E. (1932) Textbook of Mineralogy, 4th ed., CBS Publishers & Distributors, New Delhi, India, pp. 128 and 603.
Fernandes S. and Choudhary G. (2010) Understanding Rough Gemstones, Indian Institute of Jewellery, India, pp. 100, 111.
Frost R.L., Palmer S.J. and Reddy B.J. (2007) Near-infrared and mid-IR spectroscopy of selected humite minerals. Vibrational Spectroscopy, 44(1), pp. 154-161.
Hamelmann F., Heinzmann U., Szekeres A., Kirov N. and Nikolova T. (2005) Deposition of silicon oxide thin films in TEOS with addition of oxygen to the plasma ambient: IR spectra analysis. Journal of Optoelectronics and Advanced Materials, 7(1), pp. 389-392.
Madejova J. and Komadel P. (2001) Baseline studies of the clay minerals society source clays: infrared methods. Clays and Clay Minerals, 49(5), pp. 410-432.
Putnis A. (1992) Introduction to Mineral Sciences, Cambridge University Press, UK, pp. 95-96.
Webster R. (1994) Gems, 5th ed. Revised by P. G. Read, Butterworth-Heinemann, Oxford, UK, pp. 331-332.
Internet references
http://www.mindat.org/min-1295.html
http://rruff.geo.arizona.edu/doclib/hom/dioptase.pdf
http://rruff.info/Dioptase/R040028
All photographs and photomicrographs by Gagan Choudhary