Inclusions in Taaffeites From Sri Lanka

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N O T E S

A N D

N W

T E C H N I Q U E S

INCLUSIONS IN TAAFFEITES FROM SRI LANKA

B y

M a h j n d a G u n a w a r d e n e

Th is article reports on a gemological and min-

eralogical st ud y of incl usions i n taaffeite s from Sri

Lanka w it h photomicrographs of som e of the

m os t characteristic occurrences. Th e presence of m in -

erals such as apatite phlogopite spinel and musco vite

is noted and their signific ance in th e genesis of

taa f fe i te

is discussed.

Taaffei tesirom Sri Lanka, China, the USSR, Aus-

tralia, and even Burma have been the subject of

considerable interest with regard to their rarity,

their chemical composition, and the problems of

nomenclature that have arisen (Schmetzer, 1983).

However, the internal paragenesis of taaffeites has

not yet been investigated in great detail.

The first gem taaffeite reported is believed to

be of Ceylon (now called Sri Lanka] origin (Ander-

son et al., 1951; Anderson, 1968). Most of the

gem-quality taaffeites encountered in the trade to

date originate from this South Asian locality (fig-

ure 1).However, natural counterparts of this min-

eral have been found in Hunan Province, China

(Peng and Wang, 19631; in Ladoga Lake, USSR

[Kozhevnilzov et al., 1977); n Mount Painter Prov-

ince, Australia (Teale, 1980); in Casey Bay, Ant-

arctica (Grew, 1981);and, most recently, in Burma

(Spengler, 1983). In 1981, Moor e t al. described a

new mineral of the taaffeite group under the name

taprobanite, but this has since been proved to be

the red variey of taaffeite (Schmetzer, 1983). The

object of the present study was to examine gem-

quality taaffeites from the locality with which

they are most closely associated-Sri Lanka-to

determine both what inclusions are characteristic

Figure 1 . 0.67-ct taaffei te from Sri Lanka.

Ston e c ourte sy of

W

Taylor Finest Faceted Gem s.

of this rare gem material and what these inclusions

might tell us about the genesis of their host.

S MPLE STONES N D EXPERIMENTS

For this study, 46 gem taaffeites were carefully

obtained from different gem deposits within the

ABOUT THE AUTHOR

Mr. Gunawardene s president of the Lanka Foundation for the

Research of Gemstones Colombo Sri Lanka.

Acknowledgments: The author thanks Many Gem s o Sri Lanka

tor their help in collecling the sample stones and for provid-

ing transportation to the various depo sits. Thanks also go to

Dr. G. Lenzen who provided the laboratory facilities at the

Deutsche G emmologische G esellschaft for this study.

984Gemological Institute of America

Notes

and New

Techniques

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Highland region of Sri Lanka Zwaan, 1982), as

listed in table

1.

Because of the costliness of the

gem taaffeites, only six samples could be prepared

for microprobe analysis. The stones were sliced to

expose the inclusions at the surface. Qualitative

analyses were performed to identify the various

minerals rather than calculate their exact chemi-

cal compositions. A microscope with crossed po-

larizers and a magnification range of l o x to lOOx

Figure

2.

Dis tin ctl y developed crystal of apatite

i n

taaffeite from Sri Lanka. Dark-f i eld

i l luminat ion, magnif ied

45

x.

60

x

was the maximum used in the investigations)

was used to study the optical characteristics and

crystal morphology of the inclusions; in some in-

stances, the stones were examined while they

were immersed in methylene iodide nD=1.735).

INCLUSIONS IN TAAFFEITE

Several inclusions were observed in the stones

studied. The most important ones are described

below.

Apatite Long prisms of apatite were commonly

observed in most of the taaffeites studied figure 2) .

The apatites varied in color from yellowish and

opaque to near-colorless and transparent. The

well-formed shape of th e included crysta ls

strongly suggested that they were apatite, al-

though the crystals themselves were not analyzed

with the microprobe. In larger samples 3 ct), this

hexagonal inclusion could be seen with the l o x

lens. Dr. Edward Giibelin also noted the presence

of apatite in taaffeite Koivula, 1980-81).

Phlogopite Another frequently observed inclu-

sion in taaffeite is phlogopite, species of mica. A

few of these colorless pseudohexagonal tablets

were exposed at the surface of taaffeites from

Kuruwitta. The polished surfaces analyzed with

TABLE 1

Deposits represented

by

the

46

Sri Lankan taaffeites studied and their most important inclusions.

Gem deposit

Number of Color Size range Inclusions observed

stones (in carats)

Hattota Amuna

(North Central Province)

Kuruwitta

(Sabaragamuwa Province)

Deraniyagala


Balangoda


Horana

(Western Province)

Baduraliya

(Western Province)

Okkampitiya

(Uva Province)

Amerawewa

(Uva Province)

Deniyaya

(Southern Province)

17 Mauve to violet

Pinkish purple and

violet-brown

2

Mauve

1 Nearly colorless

7 Mauve to violet

Purplish to light

mauve

2 Yellowish mauve

1 Purplish with a

tinge of brown

0.40 to 267

0.32 and 0.72

0.87

1

OO

to

4.00

0.60 to 1.50

0.87

and 1.02

0.37

1.59

Fingerprints of negative crystals and

small spinels; well-formed apatite

prisms, phlogopite mica, and zircon

crystals

Phlogopite; fingerprints of negative

crystals

Spinel fingerprint; garnet crystal

Phlogopite; fingerprint of negative

crystals

Negative crystals, apatite, and spinel;

partly healed liquid fractures

Phlogopite, apatite, and zircon;

fingerprints of negative crystals

Partly healed fracture

Muscovite mica

Phlogopite and fingerprints

160

otes and ew Techniques

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GEMOLOGY

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Figure 3. Tabular pr isms o f pseudohexagona l

appearance iden t i f i ed as ph logop i te i n a taa f fe i te

f rom Sri Lanka . Dark- f ie ld i l lum ina t i on

m a g n i f i ed .6 0x

the microprobe confirmed the presence of K Mg,

Si and A1 corresponding to the formula of

phlogopite figure 3 . Moreover, the polarizing

microscope revealed the characteristic strain

Figure

5

Characte ris t ical ly orie nted f ingerprint-

l i k e ea ther cons i s t ing of m i n u t e s p i n e l i n c l u s i o n s

i n a t a a f fe i t e f ro m t h e H a t t ot a A m u n a g em

depos i t Sri Lanka . Dark- fie ld i l lumina t ion

m a g n i f i e d 30

X

Figure 4 Phlogop i te i n taa f fe i fe from Sri Lanka as

observed w it h crossed polarizers . Magnif ied 25 x .

colors and structure of mica figure41 Only rarely

was the euhedral form evident; in most instances,

these inclusions were anhedral.

Spinel The fine crystallites shown in figure 5 were

quite difficult to identify even under higher mag-

nification. However, the microprobe confirmed

tha t they are spinel. The detection of iron in addi-

tion to the Mg and A ratios indicated tha t these

minute crystals formed before the host gem.

Muscovite. One flat taaffeite plate obtained at

Amerawewa contained crystals of muscovite fig-

Figure

6

Mu s co v i t e m i ca o b s erved i n a t a a ff e i ie

f o un d i n t h e A m er a w ew a g e m ar ea

of

Sri Lanka.

Immers ion microscope magni f ie d 40

X

Notes and New Techniques

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Figure 7 . Fingerprints of negative crystals such as

this are observed in a taa ffe ite rom Kurzzwitta

in Sr i Lanka and were comm only encoun teredin

th e s tones s tud ied. Darlz-field i l lum inat io n,

magnif ied 20 X

ure 6), another species of mica. The inclusion be-

came apparent when the Beck line method was

used. Since the refractive index of muscovite is

lower no 1.60) han that of the host taaffeite, the

ring of light passes into the mica inclusion once

the objective of the microscope is turned toward

the gem. Because the microprobe revealed no Mg

in the inclusion, phlogopite was ruled out in this

instance.

Negative Crystals. With dark-field illumination,

as in figure

7

most of the taaffeites found in Sri

Lanka showed numerous small inclusions in a

fingerprint-like pattern seealso, Fryer et al., 1982).

At higher magnification, the minute inclusions

appeared to be negative crystals figure 8) .

DISCUSSION

Although apatite is a common phosphate mineral

that crystallizes early in the rock-forming phase of

the igneous cycle, under various circumstances i t

can be widely distributed in other rock types. Spe-

cifically, in metamorphic rocks, apatite has been

reported as an inclusion in spinel Zwaan, 1965).

During metamorphism, recrystallization of preex-

isting rock types can give rise to various schist-

type deposits. Mica schists are a classic example of

a new generation of mineralization; in fact, taaf-

feite has been reported to occur in association with

spinel-phlogopite schists in Australia Teale,

Notes and New Techniques

Figure 8. Wh en the s tone p ictured in igure was

i m m e r s ed i n me t h y l e n e i o d i d e a n d t h e i n c lu s i on

examined at a higher magnif icat ion 30

x ,

t h e

negative crystals in the fingerprint were clearly

evident .

1980).The presence of spinel in taaffeite confirms

their occurrence in a metamorphic rather than ig-

neous environment.

In fact, the spinel inclusions provide impor-

tant clues to the occurrence of taaffeite in Sri

Lanka. Specifically, Sri Lanka has shown an abun-

dance of spinel MgA1204)and chrysoberyl

BeA1204).Gem spinel is a classic accessory min-

eral to corundum in zones of contact metamor-

phis m Dahanayake and Ranasinghe, 198 1;

Dahanayake et al., 1980). Be-rich fluids in peg-

ma t i t e~an enrich the basic zones of country rock

to create environmental conditions conducive to

the formation of chrysoberyl. The dominant rock

type in the Highland region is a crystalline lime-

stone; introduction of Be to such a metamorphic

environment would allow for a composition in-

termediate between spinel and chrysoberyl,

namely, BeMgA140a, or taaffeite, to form. We

would expect other minerals such as phlogopite,

apatite, graphite, and spinel to be present in this

same environment, and this is supported by their

occurrence as inclusions in the taaffeite.

REFERENCES

Anderson

B.W.119681

The first two taaffeites.

A

historical note.

Gems

a )

Gemology, Vol. 12,No. 9, pp

259-262.

AndersonB.W. ayne C.J. Claringbull G.F. ey M H 1951)

Taaffeite a new beryllium mineral found as a cut

gemstone.

Mineralogical Magazine,

Vol.

29,

No.

15,

pp. 765-772.

GEMS GEMOLOGY

Fall 1984

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Inclusions in Taaffeites From Sri Lanka