THE AUSTRALIAN GEMMOLOGIST | Some Encounters with Halite and Gypsum – Notes on their Crystallography and Gemmology
Some Encounters with Halite and Gypsum – Notes on their Crystallography and Gemmology
Background
Following on from a field trip to collect gypsum crystals from one of the numerous salt lakes in Western Australia, and having had an interesting discovery concerning some salt crystals, the author was motivated to write a short report about these two minerals and their relevance to aspects of gemmology.
References to both salt and gypsum are not common in gemmology texts as neither mineral has obvious ornamental appeal. Salt, commonly termed rock salt, is the mineral halite. Gypsum, a correct mineral name, is best known in the form of its ornamental variety, termed alabaster. Both minerals certainly have relevance to crystal growth in a hydrous environment, and their development, which is at low and ambient temperatures, can be observed easily which makes them accessible to study.
Both halite and gypsum have important commercial and industrial uses which are detailed in Tables 1 and 2.
In the study of gemmology both minerals are of interest for several reasons:
- they are common minerals to collect and are easily available;
- crystals of both halite and gypsum can be grown at home at ambient temperature;
- many aspects about their formation, growth, crystal morphology, and how inclusions become incorporated into crystals, are exemplified by minerals in general.
By far the greatest number of gemstones discussed in gemmology courses are silicates; more rarely, carbonate minerals such as malachite, calcite and aragonite are used as gem materials. Gypsum is the only sulphate mineral described as a gem material. Halite is a chloride mineral and its applications in the decorative arts are rare due to its solubility and corrosive properties; however, it does feature in some rare and interesting objects or jewellery items.
Halite
Occurrences
The mineral halite is a common secondary evaporite mineral that crystallises from saline solutions and can be found currently forming encrustations on salt lakes in arid, hot areas (Table 1). Halite is also found interlayered in sedimentary evaporitic rock sequences that are widespread and were formed throughout geological time. Tourists will be familiar with some of the spectacular salt mines open to visitors in many sites in Europe, with the UNESCO World Heritage Site at the Wieliczka salt mine near Krakow in Poland as an excellent example. Here, Miocene age salt deposits, several hundreds of metres thick, were mined through a network of shafts and tunnels, almost continuously for 700 years from the 13th Century until the late 1990s. Several of the large caverns within the salt strata are now used as concert venues and church services, and carvings made out of the salt.
Uses in gemmology
Two items in the GAA gemmology syllabus refer to halite. The first is the use of a saturated solution of salt water (brine) for testing the specific gravity (SG) of amber and copal resins; and the second is the occurrence of microscopic crystals, of a few microns in size, within multi-phase inclusions of emeralds from Muzo and Chivor Mines in Colombia (GAA, 2021). Their cubic shape is easily recognisable and the occurrence of halite is part of the paragenesis of the host rocks, having derived from evaporitic brines and organic substances in black shales. Octahedral halite crystals have more recently been described in the three-phase inclusions from Muzo and this different isometric habit is explained by the space constraints (Koivula, 1998). More recently (Saeseaws et al., 2014), emeralds from specific locales in Zambia, Afghanistan and China are also reported to contain multi-phase inclusions, including some daughter crystals, as cubic, isotropic crystals, identified as halides which pose a need for greater caution concerning questions of provenance.
Home-grown crystals
The author’s interest in halite was triggered by a surprising culinary crystallography discovery. Whilst shaking a bottle of fish sauce found in the kitchen cupboard (years beyond a sell-by date!), it produced a rattling sound. Within the bottle was an aggregate of light, brown-coloured crystals of mixed sizes, from a few millimetres to the largest about 15mm. Having retrieved the crystals and examined them, from taste (a rare test in mineralogy) and an accurate refractive index (RI, by oil immersion), they were confirmed as halite. However, the crystals were not distinctly cubic in habit but had cube faces modified by octahedra and were strongly resolved (Figure 1).
Figure 1. Single halite crystal with combined cubic and octahedral faces. A transparent crystal with dark fluid inclusions (fish sauce); central zone has a phantom cubic form also with bevelled edges. Length of crystal: 10mm.
Table 1. Mineral properties and applications of halite.
Less common crystal forms
A literature search provided explanations about the modification of crystal faces within the chemical environment (Sunagawa, 2005). Halite may develop more complex faces when grown in an aqueous solution containing impurities including organic compounds such as urea and formaldehyde. The addition of Pb or Mn ions also results in the formation of octahedral faces on halite. In the late 18th Century, French mineralogist and crystallographer, Jean-Baptiste Romé de l’Isle, observed that the habit of a salt crystal changed after the addition of a small amount of an impurity. His observations were made when urea was added to an aqueous solution and he concluded that impurity ions adsorb selectively on a particular crystal face, leading to retarded growth of those faces and a change of habit. Impurities in solution may act either to promote or retard crystal growth rates (Aquilano et al., 2016).
Discussions about these interesting crystal modifications with an avid mineral collector prompted mention of another halite experience. He had discovered a collection of well-formed cubic crystals from the bottom of a jar in which olives had been preserved. The preserving solution was a salt solution (brine) that also contained a measure of vinegar. These salt crystals, with a perfectly formed cubic habit and of pellucid transparency, are shown in Figure 2. Examination of one of these crystals delivered a surprise, organic inclusions – two entrapped black beetles. The beetles, one enclosed in a crystal and the other partly embedded, are identified as a pest species of grain beetles (Oryzaephilus sp.) and were presumably already dead before becoming biological inclusions as there was no indication of a struggle. The beetles are likely to have provided a nucleating point for the development of the crystals.
These examples demonstrate the ease of growing halite crystals and provide neo-crystallographers with an introduction to the significance of the chemistry of the crystallising solution as one of the factors that influences which crystal faces become pre-eminent, and why crystal habits may vary in mineral occurrences worldwide. It also gives some idea of the chemistry of fish sauce!
Salt lakes in Western Australia
The State of Western Australia has many salt lakes, and some are sites of commercial ventures as sources of the most common lake precipitates: salt and gypsum. On the surfaces of some of these lakes, crusts of predominantly white, and sometimes pink, efflorescent salt crystals glisten in bright sunlight from reflections off their crystal faces (Figure 3).
Figure 2. Transparent crystals of halite with insect inclusions extracted from the brine in the preserving jar. Longest dimension of the crystal held by the tweezers is 10mm.
Figure 3. Salt crystal crust on Lake Negri, Rottnest Island, Western Australia. The salt crystals are colourless with many showing cubic form. The crystals are on the fine-grained, pink-coloured silt surface of the lake. Field of view: 80mm.
These brine lakes formed in a variety of ways. Some were in areas where shallow coastal lagoons were stranded or barred from the Indian Ocean and, over geologic time, have formed marine brine lakes. Others, also called playas or salinas, which number hundreds in the south and eastern part of the state, are shallow inland lakes. Most formed when old river systems, that had originally drained eastwards to the sea in the area that is now the Nullarbor Plain, became stranded. These paleo-riverine systems are commonly referred to as Tertiary channels, formed in wetter times, and have been infilled by relatively recent accumulations of clays and sands over the past few thousands of years. Concentrations of salts greatly exceed those of sea waters. Sea water averages 30,000 total dissolved solids (TDS) whereas some of the salt lake systems contain levels of up to 330,000 TDS.
Some of these salt lakes extend for tens of kilometres and are well-known as scenic sites. Lake Ballard, approximately 800km north north-east of Perth, for example, is the site of an extensive outdoor artwork, designed and constructed by Anthony Gormley in 2003, comprising 51 figural statues cast in steel (Figure 4).
Some of the brine lakes have spectacular, pink-coloured salt crusts and have become sites of tourist interest. Hutt Lagoon, near Port Gregory in Western Australia, borders the Indian Ocean and has striking pink-coloured salt crusts and waters (Figure 5). In addition to the various salts, this lagoon has populations of halophilic (salt tolerant) extremophiles (organisms that withstand extreme conditions) that include specialist algae and bacteria. The microalga, Dunaliella salina, is harvested from a series of artificial ponds as a commercial source of beta-carotene, a natural food-colouring agent, and also source of vitamin A (Figure 6). Pink colouration of the halite crystals can also be attributed to other causes. A bacterium (Salinibacter ruber) may also be a source (Cassella, 2016).
Figure 4. One of 51 steel figures by artist Anthony Gormley at Lake Ballard, a salt lake with a pink coloured crust in Western Australia.
Figure 5. Aerial view northwards of the Hutt Lagoon “Pink Lake” and Indian Ocean views. Drone photo courtesy of John Mill.
Figure 6. Aerial view of Hutt Lagoon with infrastructure showing ponds for the extraction of carotenoid-producing algae. Drone photo courtesy of John Mill.
An unusual gem material in jewellery
Of gemmological interest is a custom-made ring, displayed in the Spring edition of Gems and Jewellery (Angelett, 2017) with its centre set with a raw piece of a group of pink halite crystals from Port Gregory. This unusual piece of jewellery, which also features diamonds from Argyle, Western Australia, serves as a reminder that there is almost no limit to the variety of materials that may be featured in jewellery and artworks.
Formations in other situations
Halite crystals can form in many unusual sites. Figure 7 shows a weighty growth of halite crystals coating a chain that was found in an abandoned mine shaft located south of the town of Coolgardie in Western Australia.
Figure 7. Large salt crystal mass that had grown naturally on a chain, formerly at a mine site and, pictured in a rock shop at Coolgardie, Western Australia.
Gypsum
Introduction
The GAA course notes have several references to gypsum described under the varietal names: selenite, satin-spar and alabaster (GAA, 2021).
Transparent crystals of gypsum are described as selenite; groups of gypsum crystals that are fibrous or narrowly prismatic in parallel groups are termed satin-spar; and alabaster describes gypsum of massive habit and is utilised as an ornamental gem material. Transparent gypsum is rarely faceted as it lacks durability, possessing low hardness and perfect cleavage. Satin-spar is usually not faceted but as a gemmological material it is commonly seen in mineral retail outlets as ‘wands’ and decorative items displaying chatoyancy, such as spheres and statuary. Alabaster is used as a carving and building stone and its uses date to prehistory. In a broader sense, stone workers also apply the term ‘alabaster’ to include ‘onyx marble’ which is compositionally a fine-grained banded limestone. The properties and applications of gypsum are detailed in Table 2.
Occurrences
Gypsum, occurring in veins and commonly as chemically deposited evaporite strata together with halite associated with calcareous shales, muds and limestones, is found throughout geological time. A common bulk source of gypsum in Western Australia is from the salt lakes, where many thousands of tonnes have been commercially extracted (Simpson, 1952).
Gypsum, as spectacular gigantic single crystals, is recorded to lengths of almost twelve metres at the Naica mine in Mexico and of three metres at the Braden granitic breccia pipe in the El Teniente copper mine in Chile (Gübelin and Koivula,2008).
Like halite, gypsum is an evaporite mineral and forms at ambient temperatures. Crystals can be laboratory-grown or observed on site in saline pools, and growth rates can be measured. Controlled experiments on the growth of gypsum at a site in South Australia established rates of around 0.5mm per day (Corbett, 1966). Under ideal conditions of evaporation rates, the chemistry of salts, their concentrations and fluid movement in the lake systems, gypsum and salt crystals can be formed episodically, but may not occur in the same locales. The growth of small gypsum crystals can be observed under the polarising microscope when a drop of sea water, placed on a glass plate, is allowed to evaporate. This simple experiment will also produce the twinned habits of gypsum (Donnelly,1967).
Collecting crystals
Collecting gypsum crystals from salt lakes requires minimal equipment and nil experience, only a digging tool and a few sample bags for the crystals (Figure 8).
On a visit to Lake Cowan in Western Australia, the geometric forms of the gypsum crystal terminations could be seen outlined in the fine mud surface of the lake. The crystals occurred in clumps and were easily dug out and the mud washed off to free the crystals. The crystals shown in Figures 9 and 10 were taken just at, and just below, the lake’s surface.
Gypsum crystals from salt lakes within Australia show a wide variety of crystal forms including several twinning habits with different orientations and penetration forms. Gypsum crystals from a salt lake in South Australia are described with eight examples of different morphologies of twinning (England, 1982). The common ‘swallowtail’ habit, from Lake Cowan, is likely to have the configuration where the 100 (a) plane is the contact twin plane and the pairs of hemi pyramid faces form to create the re-entrant angle. However, contact twining with the twin plane [001] also would produce twins with this ‘swallowtail’ appearance.
Figure 8. Exploring for, and extracting, gypsum crystals from Lake Cowan, Western Australia. Photo courtesy of Ida Newton.
Figure 9. Lake Cowan, near Norseman, Western Australia. The surface shows patterns of end-on terminations formed by aggregated gypsum crystals that had grown vertically below the lake’s surface.
Figure 10. Gypsum crystal mass from Lake Cowan. This group formed as one mass together with mud in the lake and were cleaned for display. Length of longest crystal: 90mm.
Table 2. Mineral properties and applications of gypsum.
The crystals from the Lake Cowan site had similar forms. A few had a bladed habit and most have a flattened conical shape with distinctly “V”-shaped terminations. These are twinned crystals with obvious re-entrant angles producing arrow-head shapes (Figure 11).
The gypsum crystals collected from Lake Cowan have rough, irregular surfaces and are intergrown with the fine sediment of the lake and small gypsum crystals making them appear semi-opaque. The remnant planar crystal surfaces can be seen from their reflections and the typical pearly lustre of gypsum’s pinacoid faces is apparent.
As well as sediment particles, rough crystal surfaces are caused by solution action as crystals are dissolved by the changing chemistry of rainfall and ground waters. Originally these “V”-shaped crystals would have been more tabular in habit but have been partly dissolved by percolating rainwater. The surfaces are showing resorption features and have therefore presented an opportunity of viewing contemporary resorption activity.
Crystals from South Australia
Gypsum from salt lakes can occur in a variety of forms; the largest single crystals are selenite and bladed crystals with lengths of up to 37.5cm that have been recorded from a salt lake near Whyalla, South Australia (Corbett, 1966).
A student of crystallography would appreciate many aspects about crystal development and crystal habit variations with just a glance at the many crystal habits and twinning configurations of gypsum (Figure 12).
Figure 11. a. Idealized crystal construction of a twinned gypsum crystal; b. Idealized crystal construction showing features of dissolution; c. Gypsum crystal in a state of dissolution from Lake Cowan, shown in the orientation found with the swallow-tail termination almost at the lake’s surface. Length of crystal: 52mm. Crystal constructions adapted from Sutton (2005).
Figure 12. A variety of gypsum crystal growth habits from Lake Gilles, South Australia.
Crystal constructions and descriptions adapted from England (1982).
‘Desert Roses‘
‘Desert roses‘ is a colloquial term describing aggregates of disc shaped gypsum crystals grouped in clusters that may resemble flower-like forms. These groups make attractive and interesting mineral specimens (Figure 13). The individual disc-shaped crystals within these aggregations show a biconvex habit, having developed through enlargement of the [011] crystal faces (hemi clinodomes) relative to the slower growth rates of the [010] faces (clinopinacoids), and can be found clumped together in bizarre shapes.
There are many sites worldwide where “desert roses” crystal clusters develop. A common environment for their formation are the intertidal beaches of coastal zones in areas where there is seepage of sea water and the climate is hot and arid, providing conditions of high evaporation rates such as certain areas in coastal zones, termed sabkhas, of Saudi Arabia and Qatar along the Arabian Sea (Almohandis, 2002). The National Museum in Doha, Qatar, owes its inspired and unique architectural design to the mineral gypsum and this development of disc form crystals (Figure 14).
As a micro mount mineral
Gypsum crystal groups are also collected for micro mount specimens. Caution is required in handling these specimens as they are fragile and the gypsum crystals brittle and delicate, but they are of interest as aesthetic mineralogical items.
Groups of tiny gypsum crystals can be found having developed on pieces of vegetation debris that provide nucleation points; crystallisation has proceeded outwards from a point producing a series of radially lath-shaped crystals in a pompom shape and as branch-like aggregations (Figure 15).
Figure 13. ‘Desert rose‘ cluster of disc-shaped gypsum crystals from NSW displaying random orientations. Width of cluster: 150mm.
Figure 14. View of the National Museum in Qatar. Its plate-like forms are inspired by the shapes of aggregated gypsum crystals. (Exterior of National Museum of Qatar in Doha, 2019)
Figures 15. Two groups of micro mount specimens of blade habit selenite gypsum crystals, collected from the surface of Lake Cowan as grown on small twigs.
a. Pompom group, diameter: ~15mm; b. Branch group, field of view at the widest point: ~23mm. Photos courtesy of Ted Fowler.
Conclusion
From antiquity, halite and gypsum have been important minerals and closely associated in human history. Halite is a dietary requirement and essential to life, and gypsum is known to have been used for millennia as a decorative white render in plasterwork on Neolithic structures and on ancient buildings dating to 7000 BC (Rollefson, 1990).
Both halite and gypsum are common minerals and can be closely studied as their crystals are comparatively easily cultured and accessed. As transparent crystals, both minerals can be seen to contain liquid and solid mineral inclusions. The halite crystals described not only demonstrated different crystal habits in certain growth environments but also contained biological inclusions, all features that have relevance in the study of minerals in general and are analogous to many observations made when studying gemstone minerals. The variety of crystal growth forms as shown by gypsum as single, twinned and aggregated forms provide specimens of interest for crystallographers.
All photos courtesy of the author unless otherwise stated.
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