THE AUSTRALIAN GEMMOLOGIST | A Photomicrographic Compilation of Inclusions in Quartzes
A Photomicrographic Compilation of Inclusions in Quartzes
Summary
Silicon dioxide, SiO2, is one of the most abundant and least expensive gem materials, and is greatly appreciated by lapidaries. Varieties include macro-crystalline versions such as colourless rock crystal, citrine, amethyst, smoky quartz, rose quartz and prasiolite. There are also the translucent cryptocrystalline varieties of chalcedony and chert which include agates, jaspers and flints.
Much macrocrystalline quartz is often ‘flawed’; numerous inclusions of foreign materials, or internal disruptions, are contained in transparent examples, most of which cause the lapidary to reject the piece as unsuitable (or undesirable) for either cutting en cabochon or faceting, except under certain circumstances. These include, for example, where a particular inclusion adds special interest, like a single black schorl tourmaline crystal that can be oriented from the culet to the centre of the table of a faceted brilliant cut, thus developing many attractive and symmetrical internal reflections.
However, many inclusions can be amazing subjects for inspection with the microscope and camera so that searching for, and photographing, interesting inclusions can be a fascinating pastime. Frequently, more beauty and interest can be found in the microscopic views of inclusions than in the cut and polished stone. The nature of the inclusions also may suggest the conditions of the specimen’s geological formation.
Presented here is an eclectic selection of photographs of quartz inclusions with brief descriptions of their identity or origin.
Photography
The photomicrographs were taken with a stereomicroscope and a computer-interfaced USB microscope-camera. Darkfield plus brightfield LED illumination, or long wave ultraviolet light (LWUV), and polarisation were used where appropriate to minimise double imaging from birefringence. The field of view (FOV) of each image is indicated as its width in millimetres.
Figure 1. Quartz crystal (origin, China) with polished prism faces.
Left: there is an aggregation of opaque, black inclusions which are possibly stibnite (Sb2S3), or other sulphide, or silicate minerals in the centre of the crystal. An assemblage of other pale brown crystal inclusions (possibly muscovite mica) is observed at the base of the crystal. An associated internal fracture was observed to show coloured interference fringes (irisations), similar to those of soap bubbles or of oil films on water. Fractures can develop due to stresses from differential contraction of the quartz and its inclusions. Note that the origin of these irisations is quite different to the diffraction colours seen in iris agate which is due to its lamellar microstructure. (FOV 50mm).
Right: magnified interference fringes developed at the internal fracture with its thin film of air, the different colours representing the different thicknesses of the air film. (FOV 5mm).
Figure 2. Irisations at an internal fracture in two quartz samples.
Left and right: two different magnified interference fringes, or irisations, at an internal fracture with its thin film of air. The different colours reflect the varying thicknesses of the film and localised fracture widths, and reveal the patterns of the varying air-film thickness in the rippling fractures. (FOV 5mm).
Figure 3. Pink fire quartz, also called pink shimmer quartz, from Minas Gerais, Brazil.
Left and right: the material contains numerous planar and parallel-oriented, hexagonal platelets of covellite, a rare copper sulphide (CuS) which reflects pink light when viewed from the correct angle. Massive covellite is a deep blackish-blue colour with a metallic lustre and shows iridescence in reflected light, or reportedly, deep green in thin sections by transmitted light. (left, FOV 30mm; right, FOV 5mm).
Figure 4a. Inclusions of ripidolite, an uncommon variety of chlorite.
Left: ripidolite is a mica/clay progenitor which resembles squirming worms or snail droppings, and is usually a dull ferrous-green colour but may also take on rusty red colours. (FOV 15mm).
Right: individual coiled cords of ripidolite consist of numerous parallel, circular platelets of the same size strung tightly together with the appearance of randomly twisting tangled cords. (FOV 5mm).
Figure 4b. Red and green individual coiled cords of ripidolite which are oxidised (rusty) ferric-red ripidolite (left of image) and its less oxidised ferrous-green state (right of image). (FOV 2mm).
Figure 5. An unusual crystallographically-oriented, skeletal-looking inclusion of the uncommon sulphide mineral, tetrahedrite (Cu6(Cu4Fe2)Sb4S12S). Tetrahedrite is an opaque sulphide of mostly copper, iron and antimony which has a metallic lustre. (FOV 5mm).
Figure 6. ‘Strawberry’ quartz (also called ‘Tangerine’ quartz) specimen, possibly from Brazil, Mexico or Tanzania. ‘Strawberry quartz’ is not a strictly defined gemmological term but generally refers to a single, colourless quartz crystal (rock crystal) which contains numerous inclusions to give it a pink-red appearance.
Left: low magnification indicates the relative orientation and abundance of coloured platelets in the rock crystal. The array of colouring particles are myriads of non-aligned and translucent pinkish-brown sub-hexagonal or trigonal platelets, perhaps hematite (ferric oxide, Fe2O3), but possibly flakes of lepidolite (pale-pink translucent lithium mica) or alurgite (a brownish-pink manganiferous variety of muscovite mica). (FOV 10mm).
Right: at higher magnification, typical morphology and diaphaneity of individual platelets, or flakes, can be seen in the rock crystal. Consistent with a previous report, the orangish-pink, translucent-transparent inclusions are of identical morphology, orientation, colour and translucency to that of the rare manganiferous variety of muscovite mica, alurgite (Lin et al., 2021)1. (FOV 2mm).
Figure 7. A tiny, glassy, transparent and doubly terminated crystal of petroleum quartz from Baluchistan, Pakistan.
Left: seen here on the author’s fingertip, the crystal contains isolated, irregular but rounded cavities with a yellow-brown liquid of petroleum oil which shows blue fluorescence under LWUV (see Figure 13, below). (FOV 15mm).
Right: the crystal also contains saturated brine with a gas bubble, possibly CO2, CO, N2, CH4 or H2, together with (usually) colourless and low-relief evaporitic crystals of gypsum, calcite, aragonite or other water-soluble salts. (FOV 10mm).
Figure 8. Surface feature of the same quartz crystal shown in Figure 7.
Left: a small, angled depression is seen in the centre of its uppermost face. (FOV 4mm).
Right: on several of the faces within the depression are observed parallel and linearly growth-striated planes that generate diffraction colours. (FOV 1mm).
Figure 9. Magnified ‘fingerprint’ inclusion in the same quartz crystal shown in Figures 7 and 8.
Left: zone of curved, parallel trails of equally-spaced, spheroidised gas or liquid-filled bubbles. The gas or liquid is likely of significantly lower refractive index to the host quartz to give such high optical contrast. (FOV 2mm).
Right: the parallel trails probably arise from a healed (but originally rippled) growth fracture that entrapped lines of gases and liquids before progressively sealing them off and spheroidising them during subsequent crystal growth. Rippled fractures and surface growth in twinned amethyst are well-known as ‘tiger stripes’ or ‘zebra stripes’ (FOV 0.4mm).
Figure 10. A small cluster of grey-green, transparent quartz crystals protruding from massive white quartz.
Left: a central dark-coloured crystal of the cluster is intensely penetrated by an array of crystallographically-aligned and translucent yellowish-brown crystals. (FOV 50mm).
Centre: the yellowish-brown crystals protrude well beyond the surface of the quartz crystal. Their identity is uncertain but is thought to be rutile (titanium dioxide, TiO2). It displays an unusual configuration, different to the usual appearance and structure of rutilated quartz (often known as ‘Venus hair stone’) where the rutile fibres are not strictly crystallographically-oriented but often curved. (FOV 3mm).
Right: the crystal more highly magnified, showing the assemblage of the crystallographically-aligned and straight yellowish-brown crystals of rutile. (FOV 1mm).
Figure 11. Quartz cabochon (28 x 25mm) from Sri Lanka that shows strong 6-pointed asterism under a concentrated point light source.
The asterism is due to the myriads of crystallographically-aligned needles of a second mineral thought to be sillimanite (aluminium silicate, Al2SiO5) which developed epigenetically. Sillimanite is a polymorph of andalusite and kyanite.
Figure 12. Larger, doubly terminated petroleum quartz crystal from Pakistan than shown in Figures 7, 8 and 9.
Left: showing a prolific suite of multiphasal inclusions including numerous golden oil globules, brine plus gas bubbles filling many irregularly-shaped cavities. (FOV 30mm).
Right: magnified image of a single multiphase cavity. It contains a gas bubble with the yellow petroleum oil, colourless brine, and some dark opaque globules thought to be impure sulphur which is insoluble in the hydrocarbon liquid and the aqueous solution. (FOV 3mm).
Figure 13. The isolated multiphase inclusion cavity shown in Figure 12 (right) illuminated by LWUV (385nm) plus white light.
Left: illuminated by white light and LWUV showing the blue-green fluorescence of the oil relative to other features. (FOV 6mm).
Right: illuminated only by LWUV showing that the hydrocarbon oil is the sole fluorescent component. (FOV 6mm).
Figure 14. Two magnifications of the single multiphase cavity in the doubly terminated petroleum quartz crystal shown in Figure 12 (right).
An unusual single inclusion with the appearance of stacked cards of unexplained origin has grown into the internal surface of the cavity and partially surrounds the spherical gas bubble. Its substantial optical contrast from its quartz host and its conspicuous reflectivity suggest that it possesses a significantly different refractive index than quartz. (left, FOV 1mm; right, FOV 0.7mm).
Figure 15. Transparent and colourless crystal of uncertain identity and crystal system (possibly orthorhombic topaz) which adjoins the cavity in the petroleum quartz crystal, shown in Figure12 (right).
Left: crystal viewed under white light. (FOV 0.5mm).
Right: crystal viewed under LWUV (395 nm) showing lack of fluorescence. (FOV 0.5mm).
Figure 16. Oil, brine, gas bubble and colourless crystals contained within the cavity in the petroleum quartz crystal.
Left: inclusion showing an unusual, serrated border, or striations, of uncertain nature and origin. (FOV 2mm).
Right: only the oil globule within the cavity displays blue-green fluorescence under LWUV (395nm). (FOV 2mm).
Figure 17. Heavily included, irregular and striated portion of amethyst crystal.
Left: unusual white, opaque rings (arrowed) of unidentified nature and composition. As far as the author is aware, these have not been reported in the literature (FOV 40mm).
Right: the magnified zone contains the randomly-oriented white rings (at least eight have been observed in this specimen, each about 0.2mm diameter), together with many iron-rich, brush-like inclusions. (FOV 5mm).
Figure 18. Two magnifications of the white and randomly dispersed rings in the amethyst shown in Figure 17.
Left: the rings are clearly seen accompanied by divergent sprays or brushes of goethite (iron oxide, FeO(OH), commonly known as rust!). (FOV 2.5mm).
Right: a single white ring at maximum available magnification under polarised darkfield illumination displays a multi-particulate microstructure of almost spherical bubble-like structures or equant microcrystals.2 (FOV 0.5mm).
Figure 19. Two divergent sprays or brushes of goethite.
Goethite was previously (and often still is) confused with another iron mineral, cacoxenite, which is an iron aluminium phosphate
(Fe3+24Al(PO4)17O6(OH)12·17(H2O)). Goethite and the similar limonite (FeO(OH).nH2O; both are mixtures of various hydrous ferric-iron oxy-hydroxides) are commonly seen in amethyst which is an iron-rich quartz. Amethyst owes its purple colour to some iron in its crystal structure (similarly for citrine, and into which amethyst is convertible by mild heating). (left, FOV 2mm; right, FOV 3mm).
Figure 20a. Goethite and other iron oxide inclusions in amethyst.
Left: fibrous goethite sprays or brushes in the amethyst crystal shown in Figure 19 nucleating from deposited discontinuities on a single growth plane (arrowed). (FOV 5mm).
Right: another morphology of goethite (or limonite, or possibly anhydrous ferric iron oxide, hematite) which often accompanies the brushes in natural amethyst, and is known colloquially as ‘beetle legs’. This formation is so-called because of the strip-like ribbons which are often serrated, their translucence and dark red-brown colour, which are analogous to the platy legs of some brown beetles. These are often seen but only in natural amethyst. (FOV 5mm).
Figure 20b. Various morphologies of iron oxide inclusions in amethyst
Left: transversely-striated, dark-brown, single ribbon of (probably) goethite within the amethyst crystal, and in the ‘beetle leg’ morphology seen in Figure 20. (FOV 4mm).
Right: various networks, dendritic-type structures, and patterns of isolated particles of iron oxide minerals in amethyst, deposited syngenetically. (FOV 10mm).
Figure 20c. Additional morphologies of goethite inclusions in amethyst.
Left: further example of the ‘beetle-leg’, serrated ribbon of goethite. (FOV 5mm).
Right: flake-like morphology of the translucent, red-brown goethite. (FOV 10mm)
All photos courtesy of the author.
Footnotes
1 The conclusion of alurgite was made entirely by visual comparison and analogy with the description and photomicrographs in the paper by Lin et al. (2021), and from several alternative descriptions of ‘strawberry‘ quartz being also coloured by this mineral, not by direct energy dispersive X−ray, spectrometric or other analytical determination. Therefore, there is the possibility, in some circumstances, of the inclusions being another mineral such as hematite. However, the author considers this unlikely. Furthermore, the platelets are also certainly not any of the hydrous ferric oxy-hydroxy compounds, such as lepidocrocite, goethite, limonite, or any of the various other related ferric hydrous oxide compounds which are also mostly extremely dark brown and almost opaque except in very thin section.
2 The possibility that these rings are either micro-spheroidal cavities (‘bubbles’) or equant microcrystalline inclusions is left open since it is difficult to visually distinguish between either due to the particle size and the morphology limitation with the available stereomicroscope magnification. A larger photomicrograph (not shown) of the single ring, however, does show evidence of short, straight edges and apparently corners on individual particles, suggesting the ring is a concentrated assemblage of tiny crystals, but it is far from certain either way. Furthermore, spheroidal bubbles are unlikely since this macrocrystalline quartz was generated hydrothermally (i.e. not from a melt), evidenced by its array of the thermally-sensitive hydrous ferric oxy-hydroxy inclusions of sprays of goethite.
Reference
Lin, S., Li, Y. and Chen, H., 2021. Pink aventurine quartz with alurgite inclusions. Gems & Gemology, 57(3), pp.282-283.
