Discovery of a new Mineral Metamict Allanite

This article is reproduced with kind permission from the National Gem and Jewellery Authority.

This ia a black amorphous mineral from Sri Lanka-D.P Gunasekera, FGA ,President: Crystals gallery-Gem Lab.

Chemical analysis was performed on a JEOL 733 electron microprobe equipped with Tracor Northern 5550 and 5600 automation. Data reduction was done with a PAP routine in the programme XMAQNT.

The operating voltage of the electron probe was 15kv ,the beam current was 20nA and the beam diameter was 30 microns.

The emperical formula derived from the analysis is :

[Ca 1.24 Mn 0.06 Y 0.08 La 0.07 Ce 0.19 Pr 0.03 Nd 0.12 Sm0.05 Gd0.03 Dy0.02 Th0.12]Sigma 2.01 (Fe2+0.86 Mg0.07 Ti0.04 ) Sigma 0.97( Al 1.82 Fe3+ 0.14 ) Sigma 1.96 O.OH (Si2.06 07) (Si04).

The formula calculation was based on 8 cations and 13 anions H2O was calculated by stoichiometry from the allanite formula.

The mineral is therefore confirmed as a metamict allanite with rather standard composition.

Allanite is not a new mineral ,but this one is interesting for its metamict state (related to its thorium content) and for its numer of rare earth elements (REE).A simplified way of writing the formula is :

(Ca, Mn, Y, REE, Th)2 (Fe2+,Mg, Ti)(Al Fe 3+)2OOH (Si2O7)(Sio4)

Electron microprobe analysis of Metamict Allanite from Goluwahela ,Badulla district, Sri Lanka.

Na2O 0.00 (Weight in%) Na+ 0.000
K2O 0.00 K+ 0.000
CaO 11.73 Ca++ 1.238
MgO 0.48 Mg++ 0.070
MnO 0.70 Mn++ 0.058
Al2O3 15.64 Al3+ 1.816
FeO 10.41 Fe2+ 0.858
Fe2O3 1.85 Fe3+ 0.137
Y2O3 1.59 Y3+ 0.083
La2O3 2.03 La3+ 0.074
Ce203 5.28 Ce3+ 0.190
Pr203 0.69 Pr3+ 0.025
Nd203 3.44 Nd3+ 0.121
Sm203 1.40 Sm3+ 0.048
Gd203 0.98 Gd3+ 0.032
Dy203 0.76 Dy3+ 0.024
TiO2 0.59 Ti4+ 0.044
Th02- 5.36 Th4+ 0.120
UO2 0.00 U4+ 0.000
SiO2 31.07 Si4+ 3.061
H2O* 1.52 H+ 1.000
Total 95.52 CATSUM 8.000

*Determined by stoichiometry

Mrs Willow Wight, Research associate/gemmologist

CC: Dr Joel D Grice, Canadian museum of nature. Po Box :3443 STN D,Ottawa ON KIP 6P4 Canada.

Date 29th June 2001.

H=6

SG: 3.40

RI: 1.680 single refraction

Streak: Gray

Irregular rough stones with brownish coat.

Locality: Goluwahela, Badulla District

Comments: Allanite is very dark in colour and seldom cut .The content of rare earth and radioactive elements causes it to become metamict with severe damage to the chemical crystalline structure .Colour encyclopedia of gemstones ,Joel E Arem ,PhD 2nd edition 1987 page 90.

Discoverer : Dunil Palitha Gunasekera,Dip Gem, FGA.

Sri Lankan Gemmologist

References:

Green,G :Discovery of metamict allanite in Sri Lanka by DP Gunasekera ,Midland Focus ,7th issue ,winter 2005 ,pp 28 -29.

Muller H.D ,Metamict allanite, Mineralian welt 4/2006 pp47.

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