Ametrine
and Brazil Law Twinning in Quartz
 Photo: E. Skalwold Natural Ametrine Crystal Slice (looking down c axis) | ||
| In this basal section through an ametrine quartz crystal, the amethyst sectors show angled color zoning and display strong eye-visible pleochroism as well (note the two bottom L/R sectors which are only moderately contrasting in this image; as the piece is tilted back and forth, they will shift from the bluish violet color seen in the beveled lower area to deep purple). Natural ametrine occurs as long prismatic crystals which usually have deeply etched prism faces and are full of healed and partially healed fractures (as seen above). This piece has been polished, trimmed and beveled. | ||
 Photo: E. Skalwold The ametrine slice between crossed polars. (slice has been flipped over to other side). | ||
| Placing the slice between two polarizing filters in the crossed position results in spectacular colors. The angled color zoning seen in the previous image reveals itself to be Brazil law twinning, which occurs under the crystal's major rhombohedron faces ( r ). Notice the lone twin boundary angling up to the left into the citrine sector which is under the minor rhombohedron ( z ). Long sought-after, this endlessly fascinating ametrine quartz found its way to me via Alfredo Petrov all the way from a mine in Bolivia, one of the only places in the world natural ametrine has been found (ametrine has been induced in natural sectored quartz and it has been synthesized for the gem market - see below). Be sure to read Alfredo's paper on the Anahi Mine in Lithographie #12, Bolivia. | ||
 Photo: E. Skalwold A slice of sectored quartz. The crystal has an overgrowth of carnelian cryptocrystalline quartz. Each amethyst sector shows Brazil law twinning when viewed between crossed polars. (looking down c axis; eye-visible inclusions of goethite and hematite) | ||
 Photo: E. Skalwold Natural Ametrine Yellow citrine and purple amethyst sectors. This is a polished basal section which has been cut into a hexagonal shape. | ||
 Photo: E. Skalwold The ametrine slice between crossed polars. At right, black Brewster fringes (extinction bands) indicate the net zero effect of R and L twinning along the boundaries between the lamellae. | ||
 In quartz, twinning of various types is the rule rather than the exception, and in amethyst Brazil law twinning is almost ubiquitous (again, as in the purple sectors of ametrine, it forms under the major rhombohedron). It is found in other colors as well, but seems to be limited to quartz which has formed in low temperture environments with fluctuating growth conditions. This type of twinning has been found to be triggered by solid inclusions (notably goethite) or from dislocations produced by the inclusion itself (1,2 Taijing, 1990).The dramatic colorful patterns seen with a polariscope are related to both the twinning and the optical activity of quartz. The left and right twin lamellae vary in thickness so quartz's rotational polarization of light through them also varies, producing contrasting colors as it exits the quartz in each layer: the shorter the wavelength, the greater the rotation; that is, violet light is rotated more than red light (in the same way, thickness or tilting of a specimen creates variations in path length which affect the exiting color). The left and right handedness of quartz may be determined via observation with a conoscope, and in turn, the handedness of each lamellae in the twinned sectors. The black Brewster fringes (extinction bands) indicate the interface between lamellae and are caused by the net zero effect of their right and left twinning - or as McLaren (1982) writes - in the optic axis direction light passes through equal thicknesses of each twin layer resulting in "essentially zero optical rotation." Brewster fringes are not always visible in Brazil law twinning; the fringes themselves are composed of many twin boundaries, not just one, and have an intricate structure (for detailed discussion, see McLaren, 1982 and 1,2 Taijing, 1990). Turning the upper polarizing filter to make lamellae of one hand or the other extinct will cause the black fringes to disappear from view. | ||
 Photo: E. Skalwold Faceted Ametrine. | ||
 Photo: E. Skalwold A faceted ametrine under crossed polars. The amethyst portion at right shows classic Brazil law twinning. | ||
| With a cut gem of ametrine or amethyst, it is not always easy to tell from what sector in the original crystal the gem has been fashioned. This may make it difficult to interpret the optical twinning correctly. Additionally, the angle of observation may distort the presentation entirely, leading to misinterpretation. Examples: it may contain only a small portion of the twinned area; it may be missing untwinned areas if cut from near the termination (image below); it may be composed solely of an untwinned sector of the original crystal further away from the termination (for images of many variations, see: Koivula, 1989 and Schmetzer 1986, 1987, 1989). | ||
 Photo: E. Skalwold Brazil law twinning in amethyst quartz. | ||
| Any twinning which is observed optically in quartz using polarized light is Brazil law twinning (Dauphine twinning can not be seen in polarized light). This twinning occurs in amethyst (or amethyst sectors of zoned quartz such as ametrine or colorless/amethyst) under the major rhombohedron ( r ) of the quartz crystal. There are many different presentations of this effect to be seen, including that which is observed in synthetic ametrine or amethyst grown on twinned seeds. The latter can look very different or very alike Brazil law twinning in natural quartz and therefore can no longer be used alone to determine natural origin. Also, yellow citrine produced by heat-treatment of amethyst will often show Brazil law twinning, though in some cases the annealing process may alter or destroy the twinning while the color-zoning remains visible (Schmetzer, 1989). Green prasiolite quartz will often exhibit twinning as well, revealing its origin as amethyst which has been treated. | ||
 Photo: E. Skalwold Brazil law twinning as seen along c-axis cross-sections of quartz inclusions in chalcedony. The left and right twin lamellae are very fine and more difficult to discern than those with Brewster fringes. (see more) | ||
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| This slice of synthetic ametrine was cut from a "bar," a hydrothermally grown quartz crystal with a tabular habit (see an entire synthetic quartz crystal). Originally the purple sectors were colorless; the amethyst color has been induced by post-growth irradiation (the citrine remains unaffected). Natural ametrine crystals occur in the prismatic habit commonly seen in quartz; that is, elongated along the c axis with three major and three minor rhombohedrons at the termination. The most frequent method for growing ametrine utilizes a natural quartz seed plate cut parallel to the basal pinacoid and elongated in the trigonal prism. This orientation optimizes the color distribution and yield, resulting in a long bar-shaped crystal which is then cut perpendicular to its length (like bread) into several slices as seen above. Since the colors can range from weak to strong in saturation, color alone can not be used to indicate synthetic origin. There are some major differences resulting from the morphology of the cystals though: in synthetic ametrine the boundary between amethyst and citrine is parallel to the rhombohedral faces (in natural ametrine it is parallel to the c axis, which is also the optic axis in quartz); determining the optic axis in relation to the boundries will help separate natural from synthetic. Note that, as an indication of direction of growth, color zoning within the amethyst sectors is parallel to rhombohedral faces (visible in the image, under the rhombohedral face at top left) and the color zoning within the citrine sectors is parallel to the basal pinacoid (visible in the image just a short distance out from the seed plate. While not all synthetic quartz is grown using the same kind of seed plate orientation (resulting in different crystal habits), understanding the directions of growth -as indicated by zoning - and orientation to the optic axis in various presentations of synthetic quartz versus that of natural quartz is useful when looking at rough or fashioned ametrine. There is an abundance of synthetic quartz on the market, including fantastic "mineral specimens," which at first glance look like quite natural looking crystal clusters. Seeded on natural quartz druzy, a closer look reveals very bizarre crystal habits! | ||
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| Suggested Reading: (note: Taijing, 1990 updates McLaren,1982)
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