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 , which means ''spine'' in reference to its pointed crystals.
Properties
S ...
. Ceylonite, named for the island of
Ceylon
Sri Lanka (, ; si, ශ්රී ලංකා, Śrī Laṅkā, translit-std=ISO (); ta, இலங்கை, Ilaṅkai, translit-std=ISO ()), formerly known as Ceylon and officially the Democratic Socialist Republic of Sri Lanka, is an ...
, is a ferroan spinel with Mg:Fe from 3:1 and 1:1, and little or no
ferric
In chemistry, iron(III) refers to the element iron in its +3 oxidation state. In ionic compounds (salts), such an atom may occur as a separate cation (positive ion) denoted by Fe3+.
The adjective ferric or the prefix ferri- is often used to spe ...
iron. Pleonaste is named from the
Greek
Greek may refer to:
Greece
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 ...
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, Fe
2+) Al
2O
4, putting it into a group of minerals known as the spinels, or the oxide spinels. The oxide spinels have a formula of the model
B]
2O
4; where
is commonly Fe
2+, Mg
2+, or Mn
2+, and
is Fe
3+, Al
3+, or Cr
3+.
Structure
Ceylonite has the structural formula
l
2O
4, where
cation is Mg
2+ or Fe
2+. 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 + n_ ...
, 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 Mg
2+ ions occupy tetrahedral 4c symmetry positions, and the Al
3+ 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 Fe
2+Al
2O
4. 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 ( ) 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. Gneiss forms at higher temperatures an ...
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 data, as in design and manufacture, ...
granite
Granite () is a coarse-grained (phaneritic) 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 cools and solidifies undergro ...
vein
Veins are blood vessels in 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 the pulmonary and umbilical veins, both of which carry oxygenated b ...
.
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 (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main The three types of rocks, rock types, the others being Sedimentary rock, sedimentary and metamorphic rock, metamorphic. Igneous rock ...
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, causin ...
. 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