THE AUSTRALIAN GEMMOLOGIST | Calcite – An Unlikely Gemstone with a Legacy
Calcite – An Unlikely Gemstone with a Legacy
Introduction
The mineral calcite (CaCO3) has multiple properties and qualities that make it a material of significance. It has been an essential component in the construction of prisms used in a range of optical instruments, and it is much admired for its many crystal habits and for the rare, faceted gems with their display of complex optical phenomena.
In the late 17th Century, the discovery of rare transparent calcite crystals of optical quality from Iceland and its later arrival into Europe, promoted research into the behaviour and interpretation of light within minerals. By the late 18th Century, Iceland spar was well known among mineralogists and collectors.
By the early19th Century, scientific experimentation and instrument invention progressively established the means and methods of mineral identification based on optical properties.
Gemmology, newly launched as a branch of mineralogy in the early 20th Century, required the use of optical instruments with prisms that once were constructed of optical quality calcite for testing light polarization and pleochroism of gemstones.
Mineralogy
Historically, reference to calcite as a mineral is credited to Pliny the Elder in the first century CE, although its usefulness would have been well known. The name derives from Latin, calx, meaning lime, but it is also known by several terms including calc-spar and calcareous spar.
Calcite, one of the most common minerals on Earth, is the principal constituent of limestone and marble. It is trimorphous with aragonite and vaterite. All are calcium carbonate with the chemical formula CaCO3; calcite is a trigonal system mineral; aragonite and vaterite crystallise in the orthorhombic and hexagonal systems, respectively. Some of the important identifying and characteristic properties of calcite are presented in Table 1.
Calcite crystals exhibit a greater variety of forms than any other mineral. Most common are scalenohedrons and rhombohedrons, and well-developed euhedral crystals of all forms that display a high transparency are aesthetically attractive and collectable (Figure 1a, b, and c).
Table 1. Properties and Occurrence of Calcite (from Calcite CaCo3, 2005)
Figure 1(a). Pyramidal doubly-terminated scalenohedral “dog tooth” habit calcite crystal group displaying stepped growth. Size: 60 x 30 x 30mm. Hunan Province China. Photo courtesy of Craig Bosel.
Figure 1(b). Twinned tabular prismatic calcite crystals on matrix, with faces markedly growth striated. Size: 140 x 105 x 55mm. Hunan Province China.
Photo courtesy of Craig Bosel.
Figure 1(c). Stellate habit bladed calcite crystals on a celadonite matrix.
Size: 82 x 50 x 47mm. Irai Rio Grande so Sul, Brazil. Photo courtesy of Craig Bosel.
Geological Sources
As well as its ubiquitous occurrence in many rocks (Figure 2), calcite occurs in fine granular massive form including stalactites, stalagmites and travertines (Figure 3). Fractures, cavities and vesicles (gas-formed cavities) found in many rock types, but especially basalts, are commonly infilled with calcite and other secondary minerals such as zeolites, quartz group minerals and other siliceous materials (Figures 4 and 5).
Figure 2. Petrographic section of Carrara marble from the Apennine Mountains, Italy. Note the well-formed equigranular calcite crystals displaying high birefringence and twinning lamellae. Crossed polarized view FOV: 2mm. Photo courtesy of the author.
Figure 3. Polished section of a fallen stalactite, displaying layers of contorted sinuous growth patterns composed of microscopic calcite precipitated from calcareous waters over long periods of time. Gibraltar. Size: 30cm across. Photo courtesy of the author.
Figure 4. An open cavity in basalt lined with coarse calcite crystal from Helgustadir quarry, Iceland. Size: approximately 40cm width. Reykjavik, Volcano House, information and visitors centre, Iceland. Photo courtesy of the author.
Figure 5. Fracture-filling of coarse sub-radiating calcite crystals in basalt from Helgustadir quarry, Iceland. Size: approximately 16cm height. Reykjavik, Volcano House information and visitors centre, Iceland. Photo courtesy of the author.
Geology of Icelandic Calcite
Although calcite, as transparent crystals, occurs in many countries, it was the crystals and cleaved crystal fragments that were quarried from the Helgustadir mine in Iceland that have become the reference mineral in the development of crystal physics and crystallography. Iceland spar crystals are also generally specially labelled as such in mineralogical museum collections.
The story of the Icelandic calcite is one of serendipity. Originally, calcite fragments were first noticed weathered from the exposed source by a stream. The source was found outcropping at 90-100m above sea level on the hillside slopes of Helgustadir farm in Eskifjord, southeastern Iceland (Figure 6). Calcite was quarried from this site from the mid-17th Century with most exploitation occurring between 1850 and 1925. The discovery was made sometime in the mid-17th Century, with 1668 as the first documented date when optical quality calcite arrived in Denmark (Kristjánsson, 2012).
The calcite crystals formed within basalt lava flows around 11 million years ago. Hydrothermal alteration of the basalts in the area resulted in their alteration to a dark green rock rich in chlorite and calcite (Walker, 1958). Magmatic fluids caused dissolution of the original minerals and redistribution at lower temperatures into the various cracks, vesicles and cavities of the altered host lavas as a new generation of secondary minerals. Commonly formed with calcite are other minerals including quartz, various silica materials, and zeolite group minerals, stilbite and heulandite (Deer et al., 1972).
The dimensions of the quarry were described (Russell, 2008) circa 1880 as approximately 22m x 11m. Observations of the floor and sides of the cavern described the calc-spar as occurring in numerous interlocking veins in all directions and of irregular length and width, with the veins opening and pinching out abruptly throughout the basalt. Only a small proportion of the mined calcite was found to be suitable for optical purposes. Most crystals lacked transparency with incipient fractures along cleavage directions showing iridescence. These were known to Icelander workers as light stones, litsteinar or silfurberg (silvery rock) (Kristjánsson, 2012). Other inclusions in the calcite comprised clay material and tube-like cavities, some infilled with fluid and moveable gas bubbles.
Figure 6. Hillside and pathway to the Helgustadir quarry which is now a National Park. Eskifjord southeast Iceland. Photo courtesy of Bruce Groenewald.
Quarrying History from Late 17th Century Until 1925
The original site where a quarry was established is known as the old quarry (Figure 7a). Although not in continuous operation, the quarry supplied calcite for the building industry and optical quality crystals for instrument components over more than two and a half centuries.
Iceland united with Denmark in 1380, and by the early 17th Century was one of the established trading posts of the Danish trading monopoly. Stora-Breidavik, in the Reydarfjordur (the largest fiord along the southeast coast), was only 2km from the quarry site at Helgustadir. It is likely that calcite was first brought to Copenhagen with other commercial commodities via this trading post.
The history of the first calcite quarry in Iceland, from its discovery until its closure in 1975, is well documented (Kristjánsson, 2010), and the following quote summarises its importance to geoscience (Kristjánsson, 2002).
“… it should be evident that few if any other individual geological sites have exerted a comparable influence on the overall advance of knowledge in the physical sciences. However, geoscientists should not forget that sometimes these steps might not have been achieved without contributions from the Earth itself, even from what is literally a “hole in the ground” in a remote region.” (p.425).
Until the mid-19th Century the calcite crystals were transported to Europe by agents using freight carriers and fishing boats that called at trading posts, but travellers also included foreign scientists who visited the quarry site and collected calcite. For these reasons production from the mine are only estimates. One expedition from Europe in the 1830’s claimed to have taken 30-40 tons of crystals, whilst other sources claimed that local merchants recovered 50 tons in the period 1855–1860 and 280 tons in 1863–1872 (Kristjánsson, 2002). Only a small proportion of these amounts were of optical quality, with a stockpile of crystals built up by one operator satisfying the requirements of scientific and educational consumers into the early1880s.
Between 1921 to 1925 government supported prospecting resulted in a number of tunnels excavated into the hillside about 80m below the original old mine and connected to it from which (Figure 7b) several tons of calcite were excavated for export. Operations then stopped as supplies of calcite were being imported into Europe from South Africa (Kristjánsson, 2012).
In 1975, the quarry site and its surrounding area were declared a protected area and listed as a National Monument. It is prohibited to collect minerals and disturb the old machinery and workings (Figures 8 and 9). A few other calcite sites were also discovered; a much less accessible location south of Helgustadir at Hoffell, discovered in 1910, also delivered minor quantities of Iceland spar between 1911 and 1939. Very little material of optical quality was reported from both the old quarry and the site at Hoffell after 1925 (Kristjánsson, 2012).
Figure 7(a), left. Present day site of the old calcite mine with basalt exposed above the old quarry site; (b), right. Present day site of the 1920 tunnel excavated below the old quarry site. This is a national park with no mineral collecting permitted. Photo courtesy of Bruce Groenewald.
Figure 8. Information board at the Helgustadir calcite quarry site which describes the history of the deposit. Photo courtesy of Bruce Groenewald.
Figure 9. Large calcite specimen from the Helgustadir quarry displayed at the Natural History Museum, London. The specimen was acquired in 1876; weight recorded as 4½ cwt (230kg) and size 60cm. Catalogue reference BM. 49516. Photo courtesy of Robin Hansen.
Research
Following its discovery, Iceland spar featured in research by many scientists investigating the behaviour of light in transparent minerals.
Copenhagen, as the first port to receive export goods from Iceland, provided Danish scientists with the earliest opportunity to study the properties of transparent quality calcite. In 1669, Danish physicist Erasmus Bartholin (1625–1698) (Encyclopædia Britannica, 2024) published an accurate description of his observations using calcite. He noted that images seen through “Icelandic feldspar” (calcite) were doubled and that, when the crystal was rotated, one image remained stationary while the other rotated with the crystal. Perceiving that light passing through calcite was split into two rays, he called the stationary image the “ordinary beam” and the moving image the “extraordinary beam.”(Kristjánsson, 2012; Figures 10 and 11). Since the physical nature of light was poorly understood at the time, Bartholin was unable to explain the cause and it was only after Thomas Young (1773–1829) proposed the wave theory of light, that an explanation was possible.
French mineralogist René Just Haüy, (1743–1822) demonstrated that larger crystals of various shapes of calcite can be cleaved into progressively smaller rhombs, down to grains of minute dimensions which he termed “molécules intégrantes” (Wood, 1977; Figure 12). In 1784, Haüy proposed that the regular external form of a crystal is the consequence of the inner regularities of arrangement. His observations formed the concept of the repeating units that make up a crystal. It is also reported that Haüy had initially broken the calcite by accident (Kristjánsson, 2010).
Figure 10. Rhombohedral habit of calcite displaying characteristic forms and stepped terraces of cleavage layers. Size: 110 x 85 x 75mm. Helgustadir, Eskifjord, Iceland. Private collection. Photo courtesy of Craig Bosel.
Figure 11(a). Images of a paper label demonstrating strong double refraction when viewed through a rhomb-shaped calcite crystal. Private collection.
Photo courtesy of Craig Bosel.
Figure 11(b). Facets and an ink line appear as double images when viewed through the table of a faceted calcite. Size: 5.19ct. Photo courtesy of Peter Groenenboom.
Nicol Prisms
From the early 19th Century, the main application of Iceland spar was in composite polarizing prisms. Scottish scientist William Nicol (1777–1851) (Scottish Geology Trust, n.d.) is credited with the development of specially constructed calcite prisms (Nicol, 1829) and for many subsequent improved versions. Nicol was lecturer in Natural Philosophy (as science was then called) in Edinburgh and in 1829 published a report describing his “Nicol’s prism” that utilises the double refraction of Iceland spar to produce plane polarized light (Nicol, 1829; Nicol, 1839).
Until 1829, polarized light had been produced by plates of dichroic materials (mainly tourmaline) which preferentially absorbed light vibrating in one direction in the crystal. Nicol divided a rhombohedral cleaved crystal of Iceland spar along its shorter diagonal then reattached the resulting portions using resin. In this way he developed a method of producing singularly polarized light by eliminating one of the polarized rays (the ordinary ray) by total internal reflection at the inclined interface between the crystal and the resin, a material with a lower refractive index (RI. 1.533) than the ω-ray of the calcite (Figure 13). These “Nicol prisms” were widely known by 1840, and many modifications of Nicol’s original design were introduced in the following decades.
The Nicol prism and other constructions of prisms made from optical quality calcite were used in polarized light microscopes, termed petrological microscopes, until being substituted by artificially made PolaroidTM polarizers in the mid-20th Century.
Figure 12. Renowned French scientist René Just Haüy (1743–1822) pictured measuring a crystal which probably represents Iceland spar. Haüy’s interest in the subject was initially aroused when he found that calcite crystals of various shapes could be cleaved to reveal the “molécules intégrantes”. Image courtesy of Wikipedia (2024a).
Figure 13. Drawing of a calcite rhomb showing the construction of a Nicol prism.
Many instruments used in industry required calcite prisms and these included polarimeters, photometers and, in the early 20th Century, X-ray diffractometers. The demand for optical-quality Iceland spar surprisingly peaked during World War II (1939–1945) as it was required in the form of thin plates used in optical ring gun sights (Cairncross, 2022; Wood, 1977). Calcite crystals continue to be used in certain types of dichroscopes.
Another important development for geoscience made by Nicol was the production of microscope thin sections (Falcon-Lang, 2012) which are used by petrologists to study and identify rock minerals by their optical properties. He developed the technique firstly using material from a fossil tree from the Carboniferous age that had been discovered near Edinburgh around 1828. In collaboration with an Edinburgh lapidary, George Sanderson, Nicol experimented with samples to make the first translucent thin sections by fixing ground slices of the fossil wood onto glass plates using the resin Canada balsam (Falcon-Lang, 2012). This resin was still in common use for this purpose in the late 20th Century. Thin sections, whether prepared manually or by mechanised methods, continue to be used for petrological and petrographic studies.
Application of Nicol Prisms
By the early to mid-19th Century many instruments were manufactured using Iceland spar prism components for applications in industry and scientific study. Integral to the construction of the petrological microscope and allowing mineralogical examination are two prisms: the polarizer and the analyser. These prisms restrict light vibration to one plane of the direction of travel. This is known as plane polarized light. The polarizer is situated within the condenser system below the rotatable stage where the specimen microscope slide is mounted and examined, and the analyser is generally located within the body tube of the microscope above the stage. Both prism systems may be rotatable; ‘crossed Nicols’ describes the situation when the analyser prism is oriented at right angles to the polarizer. Throughout the 19th and early 20th to mid-20th Centuries the polarizer and analyser prisms were constructed of transparent, untwinned calcite crystals. An antique petrological microscope equipped with Nicol prisms is illustrated in Figure 14 and remains in working order 125 years after its construction.
Figure 14. Antique petrological microscope, circa 1900, by Swift and Company, London. The microscope is fitted with two Nicol prism polarizers, one, the polarizer, is housed below and the other, the analyser, is positioned in a flip-over mount above the eyepiece (arrowed). Photos courtesy of the author.
Calcite as a Gemstone
Calcite is a rarity as a gemstone. Its properties make fashioning difficult, requiring an expert lapidary. Transparent calcite has been faceted for exhibition and as a challenge (Figure 15). Discussion of the complexities of this type of cutting, taking account of the orientation of twin planes, the cleavage directions, strong birefringence and low hardness (Mohs 3) is described by Hurlbut and Francis (1984) in the process of faceting a large calcite gemstone. Faceted calcite may display a multitude of coloured images which change with the direction of viewing as a result of the high dispersion and birefringence, compounded by the large number of multiple facet images caused by the twinning.
Calcite, as a massive fine-grained material does have many ornamental uses: as facing stone (dimension stone), marble for sculpture and objets d’art, and many other possibilities considering the colourful and textural varieties that are available.
Figure 15. Exceptional faceted calcite gemstone at the Geological Museum, Madrid, Spain. Photo courtesy of the author.
Conclusions
“Calcite is in a scientific context the most important mineral species…the history of calcite is the history of mineralogy” attributed to Gustav Tcshermak (Austrian mineralogist)
(Wikipedia, 2024b)
The quote provides an apt summary of the important legacy that the discovery of transparent crystals of calcite has contributed to geoscience. Gemmology and the instruments used to determine optical properties of gemstones developed from those used for mineralogy. Although increasingly advanced technical instrumentation is used for mineral identification, the fundamentals by optical means remain those that were developed centuries ago.
References
Cairncross, B., 2022. Minerals & Gemstones of Southern Africa. Century City, South Africa: Struik Nature.
Calcite CaCo3, 2005. Calcite. [online] Available at: https://www.handbookofmineralogy.org/pdfs/calcite.pdf [Accessed: 20 September 2024].
Deer, W., Howie, R. and Zussman, J., 1972. Rock forming minerals Volume 5. Non-silicates. London: Longman Group.
Encyclopædia Britannica, 2024. Erasmus Bartholin. [online] Available at: https://www.britannica.com/biography/Erasmus-Bartholin [Accessed: 20 September 2024].
Falcon-Lang, H., 2012. The Geological Society of London – Double-crossed Nicol. The Geological Society. [online] Available at: https://www.geolsoc.org.uk/Geoscientist/Archive/Dec11-Jan12/Double crossed-Nicol [Accessed: 20 September 2024].
Hurlbut, C. and Francis, C., 1984. An extraordinary calcite gemstone. Gems & Gemology, 20(4), pp.222-225. DOI.10.5741/gems.20.4.222.
Kristjánsson, L., 2002. Iceland spar: the Helgustadir calcite locality and its influence on the development of science. Journal of Geoscience Education, 50(4), pp.419-427. DOI:10.5408/1089-9995-50.4.419.
Kristjánsson, L., 2010. Iceland spar and its influence on the development of science and technology in the period 1780 -1930. Institute of Earth Sciences, Science Institute University of Iceland. Third Edition.[online] Available at: <http://erewhon.superkuh.com/library/Physics/Iceland%20Spar%20and%20its%20influence%20on%20the%20development%20of%20science%20and%20technology%20in%20the%20period%201780-1930_%20Leo%20Kristjansson_%202010.pdf> [Accessed: 20 September 2024].
Kristjánsson, L., 2012. Iceland spar and its legacy in science. Hist. Geo Space. Sci., 3, pp.117–126 DOI.org/10.5194/hgss-3-117-2012.
Nicol, W., 1829. On a method of so far increasing the divergency of the two rays in calcareous spar, that only one Image may be seen at a time. Edinburgh Philosophical Society, p.8384.
Nicol, W., 1839. Notice concerning an improvement in the construction of the single vision prism of calcareous spar. Edinburgh Philosophical Society, pp.332-333.
Russell, D., 2008. Helgustadir Iceland Spar Mine. [online] Available at: https://www.mindat.org/article.php/190/Helgustadir+Iceland+Spar+Mine#:~:text=The%20Iceland%20Spar,Daniel%20E%20Russell [Accessed: 20 September 2024].
Scottish Geology Trust (n.d.) [online] Available at: https://www.scottishgeologytrust.org/geology/scotlands-geology/famous-scottish-geologists/william-nicol/ [Accessed: 20 September 2024].
Walker, G., 1958. Eastern Iceland geology of the Reydarfjordur area. Quarterly Journal of the Geological Society. 114(1-4), pp.367-391. DOI:10.1144/gsjgs.114.1.0367.
Wikipedia, 2024a. René Just Haüy. [online] Available at: https://en.wikipedia.org/wiki/Ren%C3%A9_Just_Ha%C3%BCy [Accessed: 20 September 2024].
Wikipedia, 2024b. Gustav Tschermak von Seysenegg. [online] Available at: https://en.wikipedia.org/wiki/Gustav_Tschermak_von_Seysenegg [Accessed: 20 September 2024].
Wood, E., 1977. Crystals and light: An introduction to optical crystallography. 2nd ed. New York: Dover Publications Inc.