Ceylonite (first cited 1793) and pleonaste (first cited 1801) or pleonast are dingy blue or grey to black varieties of

spinel Spinel () is the magnesium/aluminium member of the larger spinel group of minerals. It has the formula in the cubic crystal system. Its name comes from the Latin word , a diminutive form of ''spine,'' in reference to its pointed crystals. Prop ...

. Ceylonite, named for the island of

Ceylon Sri Lanka, officially the Democratic Socialist Republic of Sri Lanka, also known historically as Ceylon, is an island country in South Asia. It lies in the Indian Ocean, southwest of the Bay of Bengal, separated from the Indian subcontinent, ...

, is a ferroan spinel with Mg:Fe from 3:1 and 1:1, and little or no

ferric In chemistry, iron(III) or ''ferric'' refers to the chemical element, element iron in its +3 oxidation number, oxidation state. ''Ferric chloride'' is an alternative name for iron(III) chloride (). The adjective ''ferrous'' is used instead for i ...

iron. Pleonaste is named from the

Greek Greek may refer to: Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group *Greek language, a branch of the Indo-European language family **Proto-Greek language, the assumed last common ancestor of all kno ...

for 'abundant,' for its many crystal forms, and is distinguished chemically by low Mg:Fe ratios of approximately 1:3. It is sometimes used as a gemstone.Precious stones and gems, their history and distinguishing characteristics By Edwin William Streeter, p.33-34

Composition

The mineral ceylonite has the chemical composition of (Mg, Fe²⁺) Al₂O₄, putting it into a group of minerals known as the

spinel group The spinels are any of a class of minerals of general formulation which crystallise in the cubic (isometric) crystal system, with the X anions (typically chalcogens, like oxygen and sulfur) arranged in a cubic close-packed lattice and the cations ...

, or the oxide spinels. The oxide spinels have a formula of the model B]₂O₄; where is commonly Fe²⁺, Mg²⁺, or Mn²⁺, and is Fe³⁺, Al³⁺, or Cr³⁺. It is an iron-rich variety of the spinel mineral species.

Structure

Ceylonite has the structural formula l₂O₄, where cation is Mg²⁺ or Fe²⁺. This formula creates a face centered cubic

Bravais lattice In geometry and crystallography, a Bravais lattice, named after , is an infinite array of discrete points generated by a set of discrete translation operations described in three dimensional space by : \mathbf = n_1 \mathbf_1 + n_2 \mathbf_2 ...

, with a space group of Fd3m. The point symmetry can be three possibilities; 4*3m, 3*m, or mm. Within the oxygen sub-lattice of ceylonite, the Mg²⁺ ions occupy tetrahedral 4c symmetry positions, and the Al³⁺ ions occupy octahedral 8f site. This allows for the remaining octahedral site to be open for defects, causing a variety in structure and physical properties. Ceylonite can actually undergo a solid solution series in which the composition changes, but it is still ceylonite. Magnesium rich ceylonite can undergo an increase in iron that will replace the magnesium cation in the A-block, therefore making the new composition Fe²⁺Al₂O₄. The chemistry changes, but the mineral technically remains ceylonite.

Occurrence

The ceylonite first discovered on the island of Ceylon, modern day Sri Lanka, was found imbedded in calcareous spar, and accompanied by pyrite and micas. The crystals were located in a low lying dried up river, and were relatively shallow in the soil; 8 to 10 inches. On one side of the bank a mass of

gneiss Gneiss (pronounced ) is a common and widely distributed type of metamorphic rock. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. This rock is formed under p ...

was exposed, on the other, a

graphic Graphics () are visual images or designs on some surface, such as a wall, canvas, screen, paper, or stone, to inform, illustrate, or entertain. In contemporary usage, it includes a pictorial representation of the data, as in design and manufa ...

granite Granite ( ) is a coarse-grained (phanerite, phaneritic) intrusive rock, intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly coo ...

vein Veins () are blood vessels in the circulatory system of humans and most other animals that carry blood towards the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are those of the pulmonary and feta ...

. The grains in the first discovered ceylonite looked of compacted soils, leading one to believe they were perhaps sedimentary in formation. This was an early hypothesis, and did not hold true after further analysis. The Mg-ceylonite has a much more reliable study of its geological occurrence. It is found in Mg and Al rich

igneous rock Igneous rock ( ), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rocks are formed through the cooling and solidification of magma or lava. The magma can be derived from partial ...

s, as well as,

metamorphic rock Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock ( protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, caus ...

. Like many rocks and mineral, ceylonite is weathered and can be found in sedimentary rocks.

History

Ceylonite and pleonaste are historical terms in mineralogy. Nevertheless, both terms are used in the current mineralogical literature.

References

{{Reflist *Ball, J.A., Murphy, S.T., Grimes, R.W., Bacorisen, D., Smith, R., Uberuaga, B.P., Sickafus, K.E., June 2008, Defect processes in MgAl2O4 spinel, Solid State Sciences, 10, 6, 717-724. *Desa, M., Ramana, M.V., Ramprasad, T., 30 June 2006, Seafloor spreading magnetic anomalies south off Sri Lanka, Marine Geology, 229, 34, 227-240. *King, R. J. (2004). Minerals explained 40: The spinels. Geology Today, 20(5), 194-200. *Sickafus, K. E., Wills, J. M. and Grimes, N. W. (1999), Structure of Spinel. Journal of the American Ceramic Society. *Skeen, George J.A. (1887) Journal of the Ceylon Branch of the Royal Asiatic Society, Volume IL-Part L No. 4, 97-98 *Spinels, 1992, Physics and Chemistry of the Earth, 18, 105-125.
Gemdat ceylonite
Spinel gemstones