Dunilite- Discovered by Mr Dunil Palitha Gunasekera (FGA)

Reproduced with kind permission from the National Gem and Jewellery Authority

By Dunil Palitha Gunasekera Dip. Gem, F.G.A,-President Crystals Gallery -Gem Lab, 28,Batugedara, Ratnapura, Sri Lanka.

Long mistaken for green peridot, dunilite was identified in 2000 as a new gem variety from olivine group. It is an orthorhombic , magnesium, iron, manganese silicate. Though the x-ray diffraction pattern shows that the line spacing is similar to peridot . These spacing , however are not identical with those for peridot( By Dr Mary L Johnson , GIA July 8 1998).

The colour of dunilite varies from  light greenish yellow to brownish green, the depth of colour being related to the iron content. Dunilite is not quite as hard as quartz and thus is not one of the more durable gems. It does not however ,have cleavage, and offers no problems for the lapidary.

Dunilite occurs in only one locality called, “Katukubura mountain” Kolonne, Sri Lanka. It has been found as tabular rectangle shaped crystals in white calcite rock . The first morphological data for dunilite was obtained from a 0.97-carat faceted gemstone from GIA 10318180 Dated March 13th, 1998 confirms the stone as a new member of the olivine group.

Faceted stones have a light greenish yellow appearance resembling yellow chrysoberyl or  sinhalite . The usual size range is from 0.05 to 1 carat . Tiny faceted dunilite are cut from the rough , transparent crystals. The largest dunilte olivine aggregate crystal weighs 4.485 and a ferroedenite homblende crystal 4.735 are now displayed at the Natural History Museum , Los Angeles county, 900 Exposition Boulevard, Los Angeles, California 9007, USA.

Few cabochon cut stones show four weak silver rays.

Dunilite

Chemical Composition: Magnesium, Iron, Manganese, Silicate

Formula: (Mg,Fe,Mn)2SiO4

on a basis of 4 anions :

CaO: 0.09, MgO:38.87, MnO: 0.31, FeO: 21.69, SiO2: 37.61. Total 98.57 wt%

Group: Olivine

Crystal System : Orthorhombic

Occurrence: In a huge calcite rock with black amphibole rock

Locality: Top of Katukubura Mountain, Kolonne, Sri Lanka

Appearance and Physical Properties

Habit: Tabular, rectangle shaped aggregate crystals. Colour: Grey Green. Crystals very small to large size.

Form: Prismatic with six sided rhombic crystal faces on corner . Not seen in other olivine crystals

Colour: Light Greenish Yellow.

Lustre: Vitreous

Fluorescence: None

Hardness: 6  1/2

Fracture: Uneven

Transparency: Transparent to translucent

Cleavage: None

Specific Gravity: 3.48

Optical Properties of Dunilite

Refractive Indices: a= 1.677, B=1.700, y=1.718,

Birefringence : 0.041

Optic Sign: Bi- axial Negative

Dispersion: 0.020

Absorption Spectrum: Diagnostic, three strong bands at 524,493 and 470nm( By Mr Nick Michailidas, Imperial Gem Instruments ,2455-23rd Street CA 90405.

Identification

The refractive indices of dunilite are higher than those for, peridot and sinhalite. Dunilite has large birefringence . Absorption spectrum is much similar to peridot , but different nm lines. The properties of gemstones that might be confused with dunilite are listed below:

Refractive Index

Forsterite   a=1.635

Peridot     1.654

Dunilite   1.677

Sinhalite    1.669

Double Refraction

Forsterite  B= 1.651, y=1.670

Peridot     1.671       1.689

Dunilite   1.700         1.718

Sinhalite    1.689         1.706

Absorption Spectrum

Forsterite  0.035+

Peridot     0.036(+) 3 lines at 493,473 and 453nm

Dunilite   0.041(-)3 lines at 524,493  and 470nm

Sinhalite   0.037(-) 4 lines at 493, 475,463 and 4520nm

Specific Gravity

Forsterite  3.22

Peridot     3.34

Dunilite   3.48

Sinhalite

Dichroism

Forsterite  weak

Peridot     weak

Dunilite  strong yellow/green

Sinhalite   strong

Trade Name

The discoverer of this new olivine gemstone has proposed that it be named “Dunilite” , after his first name Dunil . As this is not a new mineral, this would of course be a trade name only and such usage will be at the discretion of the gem trade.

The Sri Lanka Gem and Jewellery Authority has accepted the name “Dunilite” for this gem and allows its use for gem exports . At last I would like to thank Mrs Willow Wight of the canadian museum of nature, Ottawa, Ontario, Canada.

References

Arem. J .E.,Olivine -Peridot Forsterite , Color encyclopedia of gemstones 2nd edition, 1987 , p137-139, sinhalite ,p 173 .

Burford .M., Gunasekera ,D.P.,(2000) An unusual olivine group of gemstone from Kolonne, Sri Lanka.Trade name Dunilite.

Canadian Gemmologist ,vol 21 no3 ,pp 84-90.

Galser S.I., Gunasekera .D.P., Rare Dunilite from Kolonne , Sri Lanka, Midlands focus, 6 th issue ,summer 2005 ,p27-29

Graziani.G, Martarelli. L, Zoysa G. E (2002). The peculiarity of olivine from Sri Lanka , z. Dtsch Gemm ., Ges 51 (1) p 29-39.

Muller. H. D., Haltern .S.J, . Kolonne bel Embilipitiya auf Sri Lanka -eine interessante Fundstelle fur olivine ,Homblende und spinelle , Mineralien Weit , 4/2006 , p.44-51.

Schemetzer .K., Boosshart.G., Bernhardt.H.J., Gubelin.E.J., Smith.C.P.,(2002) Serendibite from Sri Lanka , Gems and Gemmology , vol 38, No 1 p 73-79.

Webster.R., Gems their sources , descriptions and identification (1983) Peridot p160-164.