THE AUSTRALIAN GEMMOLOGIST | Staurolite and its Role as an Ornamental
Staurolite and its Role as an Ornamental
Introduction
Staurolite is an uncommon ornamental mineral and rarely fashioned as a gemstone. For mineral collectors, staurolite is appreciated for its neat cruciform twinned crystals, and cabinet display specimens of staurolite crystals-in-matrix are sought after items for their aesthetic mineralogy.
Staurolite is an intriguing and complex mineral, and features in numerous research articles over many decades concerning its detailed mineralogy, interpretations of its many twinned crystal patterns, its crystal system, chemistry with element partitioning, and its definitive formula. As a mineral that has a long history as an object provoking curiosity to the finder and with crystal forms that have instilled talismanic and amulet usages, many articles about staurolite are historical, anecdotal, and continue to embrace metaphysical attributes.
Figure 1. Hand specimen with one single and two twinned staurolite crystals in a fine white mica schist host rock from Keuvy, Russia. The presentation style of specimen is typical from this location. Photo courtesy of Craig Bosel.
Figure 2. Three twinned staurolite crystals.
Left: a right-angled penetration cross twin (12mm height). Right: a diagonal cross of two penetrant crystals (15mm maximum dimension). Centre: a diagonally penetrant twin (10mm maximum dimension). Specimen courtesy of GAA (WA) Collection.
History and Lore
Staurolite was scientifically described as a mineral in the late 18th Century by the French mineralogist Jean Claude Delamétherie. (Delamétherie, 1792). He named the mineral, referring to its common cross formation penetration twins, combining stauros from the Greek word denoting cross or a stake and the common suffix ite from lithos meaning rock. His scientific description follows other earlier accounts of the crystals found in Brittany (Gautron de Robien, 1751) as these were well known and collected objects in countries of Europe where they were commonly used as religious symbols.
Two of the most collectable twinned forms are those in which two component interpenetrating crystals are either diagonally or at right angles to one another. When two crystals are diagonally disposed, they form the so-called St Andrew’s cross, described as a cross saltire in heraldry terminology. Intersection angles between crystals demonstrate variation of this habit (Figure 2, right).
Rarer is the right-angled cross, formed by two crystals at approximate angles of 90° to one another and referred to as a Greek cross when the arms are of similar lengths; this style of cross is described in heraldry as a cross couped (Figure 2, left). When one of the twin crystals is noticeably longer than its twin component, the group is termed a Latin cross; this is the rarest formation and most sought after as it depicts the Christian symbol.
Prior to its scientific recognition, twinned staurolite crystals were known colloquially by descriptive terms, some related to the regional source. Examples include: Pierre de Croix (Brittany, France. Lit. stone cross), Pierre de la vraie croix (Brittany, stone of the true cross), Lapis crucifer (Latin, rock cross, St Gotthard, Switzerland) and Basler taufstein (German reference to a baptismal stone) all of which denote the religious symbolism created by the crystal’s twinned forms.
Figure 3. Two twinned crystals from Brittany, originally boxed as part of a 500 numbered mineral collection of the early 19th Century. The collection has its original hand written catalogue compiled in 1839. Specimens from a private collection in Western Australia.
Figure 4. Staurolite crystals from Brittany in host rock, with accompanying single crystals and constructions. Hand-coloured copper plate illustrations by James Sowerby (1811). Plate copy from the original reference in a private collection in Western Australia.
In the early 20th Century, the cruciform staurolite crystals found in Virginia, USA have been described as fairy crosses and fairy stones noting their surprisingly naturally neat, twinned crystal habits. Fairy Stone State Park, Patrick County, Virginia, USA continues as a site where the collecting of staurolite crystals is permitted (but not involving any digging), and commercial stalls outside the park provide information and maps for fossickers and collectors.
20th Century accounts of staurolite crystal collecting in the USA demonstrate that this popular activity reaped loose single crystals and twins as floaters “by the millions” over decades in the area of Fairy Stone State Park. In another account the “sackful’s of staurolite crystals” were reported from Fannin County, Georgia, USA (Moore, 2016). Many staurolite crystal constructions in mineralogical textbooks feature crystals from both these sources (Dana, 1920).
The interest and use of staurolite crystals as ornaments, was described as an active cottage industry in the 1930s in Virginia, USA (Moore, 1937). These crystals, ranging from a few millimetres to approximately 40mm in length, were collected mostly from loose surface occurrences, derived from the nearby horizons of biotite chlorite schist, a sericite staurolite schist, and an iron-stained quartz mica schist. The majority of staurolite crystals were twinned, approximately 75% of which were the diagonal pattern St Andrew’s cross of penetration twins and less than 5% were the ~90° twinned angles Greek cross, with few occurring as single forms. Crystal surfaces were also noted as ‘pitted’ with castes formed by garnets. Petrographic study of a range of these crystals established that staurolite crystals enclosed a significant volume of inclusions of up to 50% made up of quartz, garnet, muscovite, biotite, magnetite, tourmaline and graphite (Moore,1937).
Several regions in northwest France have historically been a source of cross-styled crystals (Figures 3 and 4). Local names given to the crystals include Breton crosslets and Coadry stones being those gathered near the village of Coadry, near Scaer in Brittany.
Possession of these crystals was believed to bring protection from many conditions and events including illness, shipwrecks and rabies (Roberts,1934). Coadry stones found in remote places, including Reunion Island in the French Mascarenes, are thought to have been brought there by Breton seamen.
Whilst cross-styled crystals have always been popular amulets, single crystals with their hexagonal-shaped transverse section, were not as commercial because they resemble coffins (Moore, 2016; Figure 5).
Figure 5. View of a single prismatic staurolite crystal showing its common hexagonal (coffin-shaped) termination. Surfaces are encrusted with mica crystals. Crystal length 7mm. Specimen courtesy of GAA (WA) Collection.
Figure 6. Staurolite crystal in a fine white micaceous host rock. The crystal is partly exposed after grinding tools were used to expose the termination. Crystal length 25mm. The crystal surface is pitted where contacting minerals have left castes. Small staurolite crystals are embedded in the schist host. Specimen courtesy of Western Australian Museum.
Figure 7. St Andrew’s cross twinned staurolite crystals. Two opaque staurolite crystals in a fine mica host prepared as a typical display skimming stone shaped specimen from the Kola Peninsula, Russia. Longest crystal is 52mm. Specimen courtesy of Clive Daw.
Figure 8. Imitation staurolite cross (centre) purchased at Tucson Gem and Mineral Fair, featured together with natural crystals collected from stream cuts at Blanchard Dam, Morrison County, Minnesota, USA. Field of view 10cm. Photo courtesy of William Cordua and published with permission.
Specimen Preparation
Well-formed twinned crystals of staurolite continue to be appreciated as objects for modern pendant necklaces and charms, and are generally used as whole crystals with minor surface finishing, fashioning and treatments.
Cabinet specimens in which host rocks containing crystals are displayed are commonly prepared for sale using air blasting, grinding and water gunning tools in order to expose crystals within their rock matrices (Figure 6).
Specimens originating from Russia are commonly prepared in this way and feature the large staurolite crystals within their white micaceous host rock (Figure 7). Specimens supplied to the commercial market available on the internet generally exhibit staurolite crystals in their host rocks; some sourced from the European Alps may feature large and strikingly beautiful staurolite crystals often together with well-formed crystals of both kyanite and garnets embedded in silky, lustrous white mica (namely muscovite and paragonite) schist. Many of the staurolite crystals developed as single crystals and have bright lustrous crystal faces.
Imitations
Fake cruciform crystals are also purposefully carved from soft rocks such as talc schist, steatite and soapstone. With Mohs hardness of 2.5-3, these rocks are easily filed into shape and, with an oiled finish, the original grey and green colours of the rocks are disguised. Small holes are also drilled into the imitations to create a similar ‘holey’ surface as the natural rock originally contained small garnet crystals. The prevalence of these fakes, described in the 1920s (Roberts, 1934), exemplifies how the trade of imitation goods shadows any commercial gemstone enterprise (Figure 8). The imitations were marketed as fairy crosses and lucky crosses, and advertorial claims about these crystals purport “to have originated from crystallised tears at the time of the crucifixion” or “from falling stars” and to bring good fortune to wearers of these charms (Roberts,1934).
Figure 9. An impressive cabinet specimen from alpine Switzerland. The host rock is coarse quartz mica schist with large single crystals of staurolite and kyanite (both of several centimetres in length). Specimen is part of a donated collection of specimens formerly owned by Professor Robert Andrew Howie, co-author of a series of seminal mineralogy reference works. Photo courtesy of the Matlock Museum, Derbyshire, England.
Figure 10. Chromolithographic illustration of a hand specimen of staurolite and kyanite crystals in white mica schist from alpine Switzerland, shown natural size. Illustration from a 1903 antiquarian mineralogical reference work by Reinhard Brauns in a private collection in Western Australia.
Geological Origins and Geographic Occurrences
Staurolite is not a rare mineral and rock types hosting crystals occur worldwide in regionally geologically extensive lithological units, many of great thicknesses and covering many tens of kilometres.
Staurolite is a metamorphic mineral formed in metasedimentary rocks, typical of medium temperature and pressure conditions, and commonly associated with mica and chlorite together with garnet and kyanite. The primary source host rocks of collectable staurolite crystals are predominantly mica schists and paragneisses.
Staurolite specimens from alpine Switzerland have a history from the late 18th Century as an important source of collectable museum quality display specimens with their large staurolite, kyanite and garnet crystals, within quartz lenses in a fine white mica matrix (Figures 9 and 10). The high-grade metamorphic conditions ascribed for the rocks from the Alpe Sponda staurolite-bearing schists in the Ticino area of the Central Alps of Switzerland quote temperatures of 600°C and pressures of 700MPa under the conditions of continental collision for their development (Bucher and Grapes, 2011; Figure 9).
Unlike most other gemstone materials, there are no staurolite mines as such and specific locations where crystals have been collected are detailed in few accounts where an area has become a popular collecting and fossicking site. Such sites are where relatively large crystals have been weathered from their host rocks and released into local soils. Mining and collecting of crystals are commonly carried out by individuals or small groups on small scale workings using manual methods and simple digging tools to extract crystals. Excavators are used for deeper works to remove unconsolidated soils in some working sites.
There are many worldwide sites where crystals have been collected and some remain current as collecting areas, notably in Brittany, France; the USA, especially at many sites in the Eastern States; areas of the Kola Peninsula of Russia; areas of eastern states in Brazil which is a source of some facet quality staurolite; and several regions of alpine Europe, particularly Switzerland. Collecting sites in Europe likely have the longest history, with crystals sought after as amulets for several hundreds of years although crystals would have been appreciated as items of curiosity wherever they are found.
Figure 11. Surface of a portion of a single crystal of staurolite showing a deeply indented texture with resinous lustre. Field of view 5mm. Specimen courtesy of GAA (WA) Collection.
Figure 12. A single well-formed staurolite crystal, with chisel-shaped terminations, and rough matte surface texture; 42mm length x 18mm width from Offin River, Ghana. Courtesy of the Western Australian Museum specimen S2015 (Simpson Collection).
Staurolite Mineralogy
The mineral staurolite is the ferroan variety of a three- member group and has a formula [Fe2+2Al9Si4023(OH)]. Magnesiostaurolite (Mg) and Zincostaurolite (Zn) are the other group members. Common staurolite is also a complex mineral with isostructural cation substitution, variable hydrogen content and some atomic sites partially occupied; Zn, Co, Mg, Mn, Li, V and Cr are the most common substitutional elements.
Staurolite is a durable and relatively hard mineral with a Mohs hardness of 7 to 7.5. The physical characteristics record its cleavages as b pinacoid [010] distinct, m prism [110] traces, and with a sub-conchoidal fracture. It is the chemical stability and durable nature of staurolite that explains its presence as a common alluvial, eluvial and detrital mineral derived from weathering of geologically metamorphic terrains.
Staurolite occurs in colours of various intensities of brown, as well as orange, yellow, red-brown and almost black. Some transparent crystals also may display hourglass colour zoning.
The crystals display a wide range of lustre. Vitreous lustre is mostly observed on gem quality crystals with some transparency, and different lustres can be observed on different crystal faces with [010] pinacoid faces often brighter.
Crystal surfaces commonly have resinous, dull, matte, or an earthy lustre and are often textured by rippled effects and pitted surfaces caused by concurrent growth with other minerals in contact at the time of their formation (Figure 11). Weathering of the crystals also results in dull and pitted surfaces, with small pits marking sites where garnets have been removed.
Diaphaneity also is varied from opaque to transparent. Opacity of crystals is often the result of host-rock mineral inclusions as well as the intense dark brown to black body colour. Single crystals are rarely transparent but are those most likely to show some translucence.
Pleochroism is distinct with a scheme of colours variable with body colour. Some lighter-coloured crystals are trichroic displaying light yellow, colourless, and golden yellow. Intense coloured brown staurolite may show only dichroism with an intense reddish orange colour.
Colour change has also been recorded with the lighter coloured zincian staurolite (common staurolite with trace zinc) reportedly showing red-brown colour in incandescent light and yellowish-green in fluorescent light (Webster, 1975).
Refractive indices quoted for common ferroan staurolite are 1.736 (lowest α) -1.761 (highest γ) and biaxial positive (2Vγ, 80-90°). The optic sign has also been reported as biaxial negative (Drysdall and Stillman, 1963). Refractive indices increase with both iron and titanium content. Birefringence is [ 0.011- 0.014] with magnesiostaurolite having a lower refractive index range. Dispersion is moderate at 0.023 for the B to G interval (Deer et al., 2016).
Gem quality staurolite may display fine absorption bands at 632, 610, 578, 532 and 449nm.
Scientifically, staurolite is classified as a monoclinic mineral with (β, 90-90.68’) but is also recorded in many references as pseudo-orthorhombic as its symmetry is commonly observed as such. The proof of the crystal system designation has been one of the research challenges and was confirmed only more recently (Donnay and Donnay, 1983).
One of the interesting physical properties that can produce a range when testing is specific gravity. The quoted range is 3.74 to 3.83 (Deer et al., 2016). The lowest SG recorded in an empirical test is 3.2 and the explanation for this is reported in a later section.
Figure 13a. Left: twinned staurolite crystal (12mm height) in the style termed a Greek cross or a cross couped. Specimen courtesy of GAA (WA) Collection. Right: diagrammatic sketch of the construction of the Greek cross. Crystal face notations: prisms (m); basal pinacoids (c); clinopinacoids (b); hemidomes or orthodomes (r). Crystal construction courtesy of Karl Schmetzer.
Figure 13b. Left: twinned staurolite crystal (15mm height) in the style termed
St Andrew’s cross or a cross saltire. Specimen courtesy of Geert Buters.
Right: diagrammatic sketch of the construction of the St Andrew’s cross.
Crystal notations: prisms (m); basal pinacoid (c); clinopinacoids (b). Crystal construction courtesy of Karl Schmetzer.
Staurolite Crystals, Habits and Forms
Single crystals
Commonly, staurolite crystals display a simple habit as prismatic forms with compressed six-sided transverse sections terminated by basal pinacoids [001], and open forms of prisms [110] and pinacoids [010] with hemidomes [101] sometimes developed (Figure 12).
Double twinned crystals
Interpenetration twinned crystals usually show the faces of single habit crystals and are twinned on planes not commonly observed, including [231] to form a diagonal cross in style and [031] to form a right-angled cross form (Figure 13a and b).
Many papers are published concerning twin laws to account for both the diagonally disposed crystals occurring at intersecting angles ~60° and for ‘right angled’ twins, issues that have addressed the change in crystal face notations following the designation of staurolite from the orthorhombic to the monoclinic system in the 1950s (Donnay and Donnay, 1983). It is known that the 90° cross habit, can be accounted for by two different twin laws and, the diagonal cross, by five (Hurst et al., 1956). The relationship for the 90° penetration twins, with the twin planes on [031], can be envisaged as a 90° rotation of one of the crystals on an [100] face.
In a common variation of this pattern the 90° cross is colloquially named bow tie-type whereby one smaller crystal is angled at ~90° within a larger host crystal. In this form the smaller crystal appears to be made up of four triangular portions, as in the heraldic style, cross patée (Figures 14-16). The angular relationship is the same as that for a Greek cross.
Figure 14. Group of twinned staurolite crystals from the Barossa Valley, South Australia. Measurement of largest twin group is 15mm x 12mm. Several twinning patterns are shown including the bow tie habit.
Crystals courtesy of Sue Koepke.
Figure 15. Bow tie habit twinned staurolite crystals from the Barossa Valley, South Australia. The smaller enclosed crystal is symmetrically disposed within the larger one. Crystal measures 10mm x 8mm. Specimen courtesy of Sue Koepke.
Figure 16. Bow tie twinned crystal of staurolite. Left: the two crystals are slightly askew. Right: transverse section of the same crystal specimen showing the relationship between crystals. 28mm x 22mm; height 26mm, matte lustre. Specimen from USA. Courtesy of the Western Australian Museum Collection.
Triple and multiple twinned crystals
Rarer are the groups of three penetration crystals, termed trillings and, sometimes, sixlings. These groups have been featured in text diagrams from sources in Fannin County, USA (Dana, 1920); Slatovsk, Urals Russia; and Australia (Petersen and McColl, 1982), and are considered as collectors’ specimens (Figure 17).
Staurolite in Gemmology
In gemmology studies staurolite is seldom featured as it is a common mineral, seemingly unattractive, and rarely occurs as transparent specimens. Most suitable crystals for faceting and fashioning are single crystals that are transparent and of less intense colour but these qualities are rare.
Fashioned staurolite is advertised commercially and images show brown-coloured gemstones, both faceted and cabochon cut, from sources usually quoted as Brazil or Myanmar. Gemstone sizes available for sale are in the order of less than 2ct, but there are exceptions (Figure 18).
An early 20th Century account states that some staurolite originating from the Ticino Canton of Switzerland was also cut as gemstones (Bauer, 1904). This is the same general area as the Alpe-Sponda specimens.
Cabochons cut from staurolite crystals originating in Myanmar are described as reddish-brown in colour and of translucent quality (Hlaing, 2001). These were sourced from alluvial gravels derived from a metamorphic rock domain containing schists in the central northeast Shan State of Myanmar in the areas west of Lai Hka and Mong Keng townships. Of particular interest is that the crystals are described as prismatic in habit and untwined which makes them suitable for fashioning when they are sufficiently translucent. Analyses also revealed niobium (Nb) as a trace element component in the staurolite (Hlaing, 2001).
Gemmological testing of one faceted oval-cut staurolite was carried out (Figure 18). The country of origin of this gem was declared as Brazil, but this is unsubstantiated as the gemstone was purchased from an international mineral fair. The gem is of an intense reddish-brown colour, transparent, measures 9.14mm x 7.05mm x 2.89mm and weighs 1.66ct. Pleochroism is strong from very light pink-brown to an intense reddish orange brown. Refractive indices measured on a table model refractometer are recorded as nα 1.736, nβ 1.740, n 1.746 with biaxial character and positive optical sign.
Specific gravity measured by hydrostatic means is 3.78. Microscope examination revealed numerous small zircon inclusions each with discoidal stress fracture halos, and some with multiple intersecting haloes. A small group of zircon crystals showed stress fractures in parallel. The zircons are identified only from their common rounded habit, high relief, characteristically surrounded by discoidal stress fractures and their anisotropism. No other mineral inclusions were found by optical microscope examination and no twinning or other structural features were observed. The ~578nm absorption band was apparent in the gemstone, although noted from its relative position and not by measurement.
Staurolite inclusions in other gemstones
Rarely is staurolite reported as a mineral inclusion in other gemstones. However, it has been documented in four host gemstone minerals as typically well-formed (euhedral) brown-orange coloured crystals in kyanite, zoisite, ruby, and in one diamond crystal (Daniels et al., 1996).
Kyanite, zoisite and ruby are gemstones of metamorphic derivation and it is not unexpected that staurolite could become an inclusion during their crystal development. The rarity is finding sufficiently transparent quality host gemstones for identification purposes. In a geological context staurolite develops commonly in close association with garnet and other common accompanying minerals within the host rocks.
Figure 17. Constructions of a range of staurolite three component twinned crystals. Images a and b from Fannin County Georgia, USA; image c from Queensland, Australia. Modified from Dana (1920) and Petersen and McColl (1982).
Figure 18. Left: an oval cut faceted staurolite reported to be from Brazil measuring 9.14mm x 7.05mm x 2.89mm depth, weight 1.66ct. This quality transparent staurolite is considered fairly rare and faceted gemstones are relatively costly. The eye-visible inclusions are zircon crystals with several decrepitation haloes. Right: high magnification image, largest zircon halo inclusion measured as 0.9mm diameter.
Figure 19. A well-formed inclusion of a staurolite crystal at x10 magnification, in a faceted kyanite gemstone. Specimen from Brazil. Photo courtesy of John Koivula (2005) and republished with permission.
Figure 20. A mineral crush made from a millimetre-sized piece of one staurolite crystal. Field of view 2mm. Left: mineral grains in RI oil matched to quartz (and therefore not visible in plane polarized light). Individual staurolite grains have irregular conchoidal fractured borders; staurolite shows in high relief with light yellow-brown body colour with lighter and darker body colours caused by different orientations under plane polarized light. Right: same mineral crush shown under crossed polarized light. Shards of quartz now show between grains of staurolite that are not in extinction. Quartz shards appear to display apparent high interference colours (due to their comparative thicknesses). It can be seen that quartz forms a high proportion of the original crystal of staurolite. Magnification x200.
Inclusions of euhedral staurolite in kyanite gemstones originating in Brazil from Barra do Salinas in Minas Gerais (Gübelin and Koivula, 2005; Figure 19) and in zoisite (variety tanzanite) from Tanzania (Gübelin and Koivula, 1986) are also rare examples. More recent reports include two finds of staurolite crystals in rubies from Mozambique and Madagascar (Ahline, 2020; Hughes, 2020). The identities of these finds were confirmed by Raman spectrometry analyses.
Staurolite in all these host minerals displays an intense orange-brown body colour and well-formed prismatic habit. One example of a staurolite inclusion in a diamond was described from the Dokolwayo diamond deposits of north eastern Swaziland (Daniels et al., 1996). Report of this rare find is of particular interest and regarded as highly unusual in a mantle origin mineral as staurolite is a crustal origin metamorphic mineral. The interpretation given is that staurolite was contained in the protolithic crustal material within a subduction zone and introduced into the upper mantle where it was incorporated into a developing diamond crystal (Daniels et al., 1996).
Staurolite Crystal Examinations and Texture Investigations
Following SG tests performed on several whole natural staurolite crystal specimens the results produced values of 3.32 to 3.36 which compare unfavourably to its quoted range of 3.74 to 3.83. Investigations to examine the integrity of these crystals and explain the SG disparity were carried out using mineral crushes and petrographic sections. Both methods require the sacrifice of some material.
Mineral crushes
Mineral crushes are a simple means to gain quick results – a few milligrams scratched from a sacrificial crystal are crushed to produce a fine powder between glass sheets used to avoid contamination. A small portion (pinhead size sample) of the resulting powder is covered by a drop of oil of known RI and examined using a petrological microscope. A mineral crush results in fracturing grains which then appear with broken margins but will contain the included ‘guest’ mineralogy. This simple low-cost method for mineral identification can be useful. A series of crushes is then examined using a range of RI oils to establish the identity of each of the associated minerals, where possible. The most common inclusion within the staurolite crystals is quartz, and rounded grains of hematite are enclosed by both the quartz grains and the staurolite host (Figure 20). The mineral crush featured was from a Western Australian crystal.
Figure 21. Two views of a petrographic section of a staurolite crystal, 1.8mm field of view. One crystal edge shows in the bottom RHS of both images. Left: plane polarized light view, shows the significant amount of irregular shaped quartz within the staurolite host. A few rounded hematite grains and brown mica are also visible. Right: same section viewed under crossed polarized light, highlighting the quartz inclusions.
Figure 22. Petrographic section of a staurolite crystal. Left: plane polarised light sieve texture showing staurolite matrix (yellow colour- uniform pleochroism as the crystal is in optical continuity) with rounded quartz and opaque mineral grains throughout the staurolite host. Right: same view under cross polarised light, the staurolite host crystal shows an orange-yellow colour, in optical continuity with low birefringence grey colour quartz grain inclusions throughout.
Field of view 2mm. Magnification x200.
Figure 23. Petrographic sections with a lined overlay to show a Greek cross style of a twinned staurolite crystal under plane polarised lighting in two positions shown by a rotation difference of 90°. Sections show the narrow central zone of intersection between individual crystal components of the twin. Views demonstrate the triangular areas in optical continuity of the individual twin components of the staurolite and the poikiloblastic nature of the quartz-staurolite intergrowth.
Field of view 4mm. Magnification x100.
Petrographic sections
Examination of three petrographic sections, one from eastern Australia and two from Western Australia, demonstrates that the staurolite crystals had developed to enclose varying amounts of other host rock minerals.
In one crystal specimen (Figure 21) the most common inclusions are quartz, as irregular grains with a few platelets of brown-green biotite mica, often pock-marked with zircon haloes, and scattered grains of rounded hematite. The rounded opaque grains mineral were identified by polished section ore microscopy as hematite with grey to dark grey anisotropism (M. Wort, 2021, pers. comm.).
In another Western Australian example, the quartz and staurolite have developed as a sieve texture with droplet-shaped rounded inclusions of quartz inclusions that do not show any crystallographic relationship to the staurolite grain host (Figure 20).
The developmental growth of staurolite shows no disruption or alteration by the encapsulation of these mineral inclusions and the crystals display regular anisotropism when viewed by rotation under crossed polarisers (Figure 22).
The petrographic section cut through the centre of a twinned crystal sectioned from one of the Barossa Valley crystals of a bow tie style of twin, shows the centre section as the junction of four triangular zones composed of two crystals of staurolite, and displays the pleochroic responses shown as pairs of triangles, e.g., north and south or east and west. Under crossed polarized light the crystals are alternatively either in extinction or out on rotation. The section also shows the high degree of mineral intergrowth between staurolite (showing shades of yellow) and quartz blebs (Figure 23).
Conclusion
Staurolite develops in pelitic metamorphosed rocks and generally encloses a proportion of host rock mineralogy, commonly quartz and mica. The crystal borders vary from well- to poorly formed; well-formed examples are classed as collectable when their development has been as porphyroblasts occurring as single crystals and in twinned habits. Occurrences of single staurolite crystals of sizes suitable for gemstones without inclusions and of high transparency are rare.
Variations of SG determination values that are lower than the stated range is an indication that the staurolite under test is either impure by hosting inclusions of other minerals, or replaced and pseudomorphed during retrograde metamorphism and/or weathering. Whichever its development, staurolite characteristically shows good form, distinctive brown hues and is an easily recognised ornamental material in its twinned crystal habits.
Although determination of SG values is not routine testing for the identification of crystals, the method establishes that the SG values that are out of the normal range for that mineral require investigation and, in this example, indicated that the crystals are naturally contaminated in their growth development.
Copper plate and lithograph illustrations of staurolite crystals in matrix from selected antiquarian literature dating to the early 19th Century show that the most well-known and appreciated specimens have remained as those sources of interest to current mineral collectors.
All photos courtesy of the author unless otherwise stated.
References
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