Ceylonite
General
Category	Oxide minerals Spinel group Spinel structural group
Formula (repeating unit)	(Mg,Fe)Al2O4
Crystal system	Isometric
Identification
Color	Various; red to blue to mauve, dark green, brown, black
Crystal habit	Cubic, octahedral
Cleavage	Indistinct
Fracture	Conchoidal, uneven
Mohs scale hardness	7.5–8.0
Luster	Vitreous
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	3.6–3.9
Optical properties	Isotropic
Refractive index	1.770-1.780
Pleochroism	Absent
Solubility	none
Other characteristics	Nonmagnetic, non-radioactive, sometimes fluorescent (red)

Ceylonite (first cited 1793) and pleonaste (first cited 1801) or pleonast^[1] are dingy blue or grey to black varieties of spinel. Ceylonite, named for the island of Ceylon, is a ferroan spinel with Mg:Fe from 3:1 and 1:1, and little or no ferric iron. Pleonaste is named from the Greek 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.^[2]

Composition

The mineral ceylonite has the chemical composition of (Mg, Fe²⁺) Al₂O₄, putting it into a group of minerals known as the spinel group, or the oxide spinels. The oxide spinels have a formula of the model [A][B]₂O₄; where [A] is commonly Fe²⁺, Mg²⁺, or Mn²⁺, and [B] is Fe³⁺, Al³⁺, or Cr³⁺. It is an iron-rich variety of the spinel mineral species.

Structure

Ceylonite has the structural formula [A]Al₂O₄, where [A] cation is Mg²⁺ or Fe²⁺. This formula creates a face centered cubic Bravais lattice, 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 was exposed, on the other, a graphic granite vein.

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 rocks, as well as, metamorphic rock. 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.