Corundum Crystallography
Corundum crystallizes in the trigonal system, but the habit varies with locality and/or color. In many crystals the pinacoid is developed at the expense of the pyramid and rhombohedron; bipyramidal habit is also common. Clues to origin rather than diagnosis may be provided by some corundum crystals.
Combinations of any of these three main habits are also possible. Ruby and sapphire crystals often show different habits. Rubies from Myanmar may occur as hexagonal prisms terminated at both ends by the pinacoid, with rhombohedral faces at alternate corners. The rhombohedral faces may be more or less developed or entirely absent, especially in the large, and usually opaque, crystals from Tanzania and Madagascar.
In many ruby crystals the prisms are flattened and, although they may be of large diameter, are relatively thin. Such crystals often exhibit a stepped or platy appearance, as though the crystal was composed of a number of thin plates. Basal planes of many crystals are traversed in three directions by fine parallel striations, which take the form of hair-like lines crossing at angles of 60/120°, dividing the area into small triangles.
Sapphire (and some ruby) takes the form of a hexagonal bipyramid of twelve triangular faces, six above and six below, meeting at a girdle. This habit may occur in combinations of bipyramids of different inclinations, with the girdle sometimes formed by a narrow hexagonal prism. The ends of many bipyramidal crystals are capped by the flat basal face to give a barrel-shaped habit. The hexagonal bipyramidal habit is common in sapphire crystals from Sri Lanka, and in this form the faces are often deeply striated horizontally, owing to repeated oscillation between different pyramids or between the pyramid and the basal pinacoid.
Sapphires from Yogo Gulch, Montana, as described by Clabaugh in Corundum Deposits of Montana (US Geological Survey Bulletin 983), 1952, showed a number of forms whose true nature was obscured by alteration
of plane surface and original faces by etching. A basal pinacoid and rhombohedron were the two forms identified in a study by Pratt in Corundum and Its Occurrence in the United States (US Geological Survey Bulletin 269), 1906. In general, corundum crystals show predominating pinacoid and pyramid forms; tourmaline might be suspected when a red crystal with predominating prism forms is encountered and red spinel when triangular faces are all that can be seen.
Title: Unveiling the Brilliance: Exploring Corundum Crystallography
Corundum, renowned for its captivating hues and remarkable durability, stands as one of the most prized gemstones in the world. Its allure is not merely skin deep; rather, it emanates from the intricate world of crystallography that underlies its formation and properties. Delving into the realm of corundum crystallography unveils a fascinating narrative of symmetry, structure, and brilliance.
Understanding Corundum Crystallography:
Corundum belongs to the trigonal crystal system, characterized by a threefold symmetry axis. Chemically, it is composed of aluminum oxide (Al2O3), wherein aluminum atoms are surrounded by oxygen atoms in a hexagonal arrangement. This hexagonal close-packed structure manifests in the crystal lattice of corundum, contributing to its exceptional hardness and optical properties.
Symmetry and Forms:
Corundum crystals typically exhibit a variety of forms, ranging from hexagonal prisms to barrel-shaped crystals. The primary habit of corundum is a six-sided prism terminated by a flat basal plane. However, variations in growth conditions often result in diverse crystal habits, including tabular, bipyramidal, and rhombohedral forms.
Twinning and Growth Patterns:
Twinning, a phenomenon where two or more intergrown crystals share a common plane or axis, is relatively common in corundum. Penetration twins, characterized by interpenetrating crystal lattices, are frequently observed. These twinning patterns influence the gem’s optical properties, often creating distinctive features such as the “star effect” in star sapphires.
Color and Impurities:
The vibrant colors exhibited by corundum are primarily attributed to trace elements present during its formation. Chromium impurities yield the iconic red hue in rubies, while iron and titanium contribute to the spectrum of colors seen in sapphires, including blue, yellow, pink, and green. The absorption spectra of corundum provide valuable insights into the types and concentrations of impurities present in each specimen.
Optical Properties:
Corundum’s optical properties are a testament to its crystalline perfection. It boasts high refractive indices and exceptional dispersion, lending brilliance and fire to faceted gems. Moreover, its pleochroism—the ability to exhibit different colors when viewed from different angles—adds depth and complexity to its visual appeal.
Durability and Applications:
The hardness of corundum, ranking 9 on the Mohs scale, makes it exceptionally resistant to scratching and abrasion. This durability, coupled with its stunning aesthetics, renders corundum an ideal choice for a myriad of applications beyond jewelry. Industrial-grade corundum finds use as abrasives, cutting tools, and even in the manufacturing of precision optics.
Enhancements and Treatments:
To optimize their appearance, corundum gems may undergo various treatments, including heat treatment, diffusion, and fracture filling. These processes aim to improve color, clarity, and overall marketability. While common and accepted within the industry, disclosure of such treatments is crucial to maintaining transparency and consumer trust.
Conclusion:
Corundum crystallography unveils a world of mesmerizing beauty and scientific intrigue. From its symmetrical crystal forms to the diverse colors imparted by trace impurities, each facet of corundum tells a story of geological processes and mineralogical phenomena. Beyond its aesthetic allure, corundum’s durability and optical properties solidify its status as a timeless treasure cherished by gem enthusiasts and scientists alike.
As we continue to unravel the mysteries of corundum crystallography, we deepen our appreciation for this extraordinary gemstone, celebrating its elegance, resilience, and enduring legacy in the world of gemology.
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