JADEITE FROM BURMA

This article is a continuation of Episode 64, featuring ja­deite from Burma, contributed by HL Chhibber in 1934.

PETROLOGY OF THE JADEITE-ALBITE ROCKS.

100 100.04 99.75 100.23 99.93 99.71 99.72

Density _ 3.276 3.348 2.685 _ _ 3.284

1. Theoretical jadite. NaA1 (SiO2)3.

2. Jadeitite with jadeite in long white rods, Tawmaw; analyst, M Raoult.

3. Apple green jadeitite, Sietaung.

4. Highly albitic part of a jadeitite from Kadon Dwin.

5. White jadeite in long white rods. Analyst, Raoult.

6. White jadeite, spotted green, Dwingyi.

7. Nepheliniferous jadeite, Tawmaw.

Physical Properties: Prismatic cleavage in Burmese jadeite is very well-marked, and partings in several directions are also distinct, including those parallel to (100) and (010).

Interpenetration twinning on a prism face, and simple twining are observed under the micro­scope.

The specific gravity of the ja­deite ranges from 3.264 to 3.336, and the average of 12 determina­tions was 3.31. Prismatic, colum­nar, massive, fibrous, granular and compact habits of jadeite were observed.

Colour: Jadeite varies from pure white to various shades of green. Not infrequently, green spots or streaks are observed in the white vari­eties. Other less common tints are amethystine, light-blue, bright-red, brownish and black. The bright-red and brownish tints are observed in a thin outer zone of jadeite boulders embedded in red earth, and the colour is due to the dissemination of ferruginous matter by percolating water. About one-third of an inch from the surface, the red colour entirely disappears. Thin sections of red jadeite are seen to be stained red and yellow with haematite and limonite, respectively.

Microscopic Characters: Thin sections of jadeite are seen to consist of interlocking, hypidiomorphic, rather irregular prismatic sections. It is this interlocking arrangement of the crystals that makes the mineral so tough. Idiomorphic sections show the development of prisms commonly and of pinacoids rarely. Sometimes the jadeite shows granoblastic structure, at others, it is mylonitised, and in places, radially arranged, sheaf-like aggregates are to be seen. The average double prismatic cleavage angle is 87.3, while it varies from 85.2 to 89.0, depending upon the angle at which the crystal is lying in the section. Jadeite is normally colourless in thin sections, but the bright green variety exhibits a pale-green colour under the microscope and is very slightly pleochroic. The birefringence is strong. The maximum extinction angle observed is 43.5· while the minimum is 27·. Undulose extinction is very characteristic, indicating that the mineral has under­gone considerable strain. Inclusions of amphibole are not infrequent, and occasionally those of albite too. The mineral becomes cloudy and opaque on account of meteoric weathering, but under the microscope, it is also seen altered to colourless amphibole.

Origin of the jadeite-albite rocks: Rosenbush noticed that jadeite may have been formed by the addition of one molecule of albite to one of nepheline. Such a composition is comparable with that of a leucocratic nephe­line-syenite, almost exclusively sodic. The density of jadeite is much greater than that of albite; therefore, high pressure must have played its part in its forma­tion.

Pirrson, Iddings and Grubenmann considered jade­itite to be an orthoschist, the last named referring to the rock to the deepest zone of dynamo-met­amorphism, the Tiefeste Zone, the corresponding rock type from intermediate depths being nephe­linegeneiss (Mesoalkaligneiss).

Noetling and Bleeck agree in considering the whole of the Tawmaw sodic rocks as forming a dyke of eruptive origin. Bleeck, however, considers that, from the genetic point of view, the jadeitite and albitite must be separated from the amphibolite, the first two having been intruded into the latter, which he believes to be genetically related to the peri­dotites. In support of this view, he stresses the occurrence of xenoliths of the amphibolite in the jadeite-albite rocks, the en­closures being aligned parallel to the margins of the dyke, while a “ribboned texture” is frequently developed here. Bleeck explains the presence of chromite in both the peridotites and the jadeite-al­bite rocks as due to their close magmatic relationship, the ser­pentinised peridotites and the gabbro (in the course of being transformed into crystalline schists) being products of differ­entiation from the same body of magma, and this gave rise, as an end member of extreme com­position, to a nepheline-aplite. This nepheline-aplite, instead of appearing in its original form, has been changed after intrusion into jadeite-albite rock, the change being in the nature of an exomor­phic transformation effected by granite magma intruded under high pressure following tectonic activity.

Professor Lacroix thinks that the jadeite, albite and am­phibolite are linked together in a different way from that suggest­ed by Bleeck. It is important to note that the composition of the amphibolite is unique: its low con­tent of lime and alumina will not allow one to regard it as derived by regional metamorphism from the gabbro. Further, it is very rich in magnesia and, like the white rocks of the dyke, is characterised by a high content of soda and low content of potash. Thus, in some respects, it appears to be linked with the albite-jadeite rocks, and in others, with the peridotites.

Lacroix does not think that all the Tawmaw rocks can have been produced by the normal dif­ferentiation of a nepheline-rich magma on the following grounds: all the known examples of such magmas are characterised by the absence of chromite and low con­tent of magnesia in the leucocrat­ic rocks formed from them, while the mesocractic and melanocrat­ic members, such as theralites, are rich in lime and iron as well as in magnesia. This contrasts strongly with the amphibolites of Tawmaw, the analyses of which in fact do not correspond with any other known rock. On the other hand, the peridotites into which the Tawmaw dyke was intruded are destitute of alkalies and prac­tically without lime; they consist essentially of magnesia and iron, and contain a little chromite. It is noteworthy that all the rocks of dyke contain chromite; Lacroix states that the green spots in the dyke rocks are indubitably due to chromite.

Lacroix claims that the dyke fissure was originally filled with a highly aluminous and richly sodic hololeucrocratic magma, which stopped its walls, the amphibo­lite representing endomorphosed portions of these. The amphibo­lite, on this view, is certainly not crystalline schist which has been metamorphosed by the magma, but represents peridotite that has contributed magnesium, iron and chromium, while the magma has contributed soda and silica. Fur­ther, it is evident that to provide the requisite silica to effect the transformation, a more highly silicate magma than the nephe­line-bearing type postulated by Bleeck must have been involved. It seems reasonable to suppose that this magma had the compo­sition of granite-aplite. Bleeck pointed out that there is a narrow zone of chlorite-schist, bearing chloritoid and chrome-epidote, between the peridotite and the dyke rocks. This he regarded as part of the metamorphic aureole of the dyke. Lacroix states that no such rocks were found among those which he examined and finds it difficult to understand how a purely sodic magma could, by interaction with peridotite es­sentially free from lime, give rise to lime-rich minerals such as zoisite, tawmawite and others.

Due to limited space, the Ja­deite by Dr Chhibber can be de­scribed in brief. This is the end of Episode 65. The author is highly grateful to the Editorial Depart­ment for its constant permission.

References: Chhibber, HL, 1934: The Mineral Resources of Burma, Macmillan and Co Limited, St Martin’s Street, London.