Weathered Cortex of Eluvial–Deluvial Jadeite Jade from Myanmar: Its Features, Formation Mechanism, and Implications

Abstract

Myanmar is the principal provider country of high-quality jadeite jade in the world, including so-called primary and secondary stones. The secondary stones occur as rounded pebbles, boulders, and blocks in eluvium–alluvium and often hold varying degrees of weathering. Unlike common rocks, such as granite, gabbro, schist, gneiss, and amphibolite, secondary jadeite stones frequently have weathered cortexes that vary in appearance, depth, texture, and mineral components compared with those of inner primary bodies. In this study, representative samples of secondary eluvial–deluvial jadeite stones with varying weathered cortexes were selected, and their appearances, textures, mineral components, and chemical composition features were analyzed. Their weathered cortexes were red, yellow, white, or black, and were 0.01–1.80 cm thick. The cortexes were opaque, often with soil luster and a fansha phenomenon. The body of the jade was usually translucent, and green and white in color. Along the border between the weathered cortex and the body of a certain jade stone, the textures were the same for the successive grain sizes. The only difference was that there were more cracks, cleavage planes, and fissures in the cortex. Jadeite was the main mineral component of both; however, minor late-stage supergene minerals (such as gibbsite, kaolinite, and halloysite) and Fe-bearing colloidal minerals were identified along the grain boundaries in the cortex. Studies of the textures and mineral components of weathered cortexes have gemmological applications including the identification and grading raw jadeite, as well as its design and carving. Moreover, such studies might provide information for improving our understanding of the unique weathering processes of monomineralic aggregates relative to multiple-mineral rocks, as well as gambling jadeite jade pieces through analyzing their cortex.

1. Introduction

The Myanmar jade tract, which has been mined for ~300 years [1], produces high-quality primary and secondary jadeite jade pieces, which have by far the highest commercial value in the world. The secondary pieces are usually composed of a weathered cortex, which are referred to as the weathered and/or weathering-influenced layer, surrounding an intact jade body [2]. In the Chinese jade market, unprocessed jadeite jade is called raw jadeite. Some of these pieces have weathered cortexes and are often called as gambling jadeite stone because there are so many uncertainties caused by the weathered cortex obscuring the jade body [3]. Although some excellent jade dealers can lessen some of these uncertainties by means of practiced trading experiences, these uncertainties still puzzle most dealers and those who have an interest in jade. In the past 40 years, many gemologists have attempted to utilize the mineral components, textures, colors, compositional evolution, and supergene geochemistry of weathered cortexes to evaluate the jade body within [4,5,6,7,8,9]. Unfortunately, due to the lack of the systematic comparison studies, using only these features of the cortex to determine the connection between the weathered cortex and jade body is still not convincing.

In petrology, jadeite jade is a type of jadeitite- and jadeite-related pyroxenite. Jadeitite is a virtually monomineralic aggregate and is commonly composed of more than 90% jadeite. Observations of other weathered monomineralic aggregates, such as nephrite, chalcedony, and serpentinite, have revealed that they tend to consist of an even, weathered cortex surrounding a fresh core, while some common multiple-mineral rocks, such as granite, gabbro, schist, and amphibolite, do not form a complete weathered cortex. The latter mostly exhibit uneven weathered surfaces due to the different resistances of the various minerals to weathering processes. Studying the changes of this mono-jadeite rock during weathering process may serve as an example case of the weathering of monomineralic rock, which can be expected to be different from common multiple-mineral rocks.

In this study, the appearance, texture, mineral components, and chemical compositions of the weathered cortexes and intact jade bodies of secondary eluvial and deluvial pieces of jadeitite were analyzed and compared. Based on the physical geographic conditions of the Myanmar jadeite deposit, the formation mechanism of the weathered cortex and its implications were investigated.

2. Geologic Background

The Myanmar jade tracts are located in northern Myanmar and are mainly distributed along and to both sides of the Uru River. They are distributed in the NNE direction, within an area that is about 18 km long and 3.4 km wide, and are basically distributed along the Sagaing strike-slip fault zone (Figure 1). Petrologically, the jades consist of jadeitite, omphacitite, and kosmochlor rock. The primary jadeitite and related rocks occur as veins and blocks in the Hpakan–Tawmaw serpentinite mélange. They are closely related to the ultramafic rock belt, ophiolite, blueschist, and eclogite (Figure 1). They formed during the Jurassic under low-temperature and high-pressure conditions within a subduction zone and were exhumed to shallow depths at ~45 Ma [10,11].

The mines are ~200–600 m above sea level and contain primary and secondary deposits. The secondary jadeite jade was formed from the primary jadeite. Most of its surrounding rocks, i.e., serpentinite and other ultramafic rocks, have been weathered into a soil layer due to their relatively poor resistance to erosion by sun light irradiation, air, water, and/or biological activities. The soil layer is up to ~300 m thick. It is clearly segmented vertically, the colors of the soil layers are distinct, often yellow, red, and black sub-layers from the top downward (Figure 2). The boundaries between the sub-layers are not distinct. It should be noted that the cortex color of the second pieces is often the same as that of the sub-layer.

3. Materials and Methods

Out of more than 50 secondary jadeitite samples from Myanmar, 8 representative samples (Figure 3) collected from eluvium and deluvium around the open pit shown in Figure 2 were selected for analysis in this study. The other weathered jadeite stones from the alluvium and diluvium in Myanmar were not used in this investigation because most of them had smooth surfaces without cortexes or only had a very thin cortex. These samples were almost completely comprised of jadeite (~95 vol.% Jd). The characteristics of the weathered cortexes and jade bodies of the studied samples are summarized in a Table S1 in the Supplementary Materials. Among them, sample MJC-1 was collected from the bottom layer; samples MJC-3, RD-24, MJC-7, and MJC-8 were collected from the middle layer; and samples MJC-2, MJC-4, and MJC-5 were collected from the top layer (Figure 2). All the samples had distinct weathered cortexes (0.5–1.4 cm thick) with unweathered bodies. The grain size was 0.05–1.50 mm, and most of the samples had a granoblastic texture, but a few parts had a nematoblastic texture.

Optical observations were performed using a Leica S6D gem microscope and Leica DM750P polarizing microscope at the China University of Geosciences, Beijing (CUGB). The X-ray diffraction (XRD) analysis was conducted using a Rigaku SmartLab X instrument with Cu Kα radiation operated at 45 kV, 200 mA, continuous scanning, a scanning speed of 2 × 10/min, a slit of IS = RSI = 2/3, and an RSI of 0.3 mm.

The chemical compositions were acquired using an EPMA-1600 with a voltage of 15 kV, a beam current of 10 nA, and a spot size of less than 5 μm at the CUGB. The BSE images of the weathered cortexes and line scans were conducted using a MIRA3 XMU field emission scanning electron microscope (SEM) in BSE mode and an Oxford X-MAX energy dispersive spectrometry analyzer at the CUGB.

The clay minerals were selected under the optical microscope and were analyzed via powder diffraction using an Rigaku Oxford diffraction XtaLAB PRO-007HF rotating anode microfocus X-ray source (50 kV, 24 mA, 1.2 kW, MoKα) with a hybrid pixel array detector and a single-crystal diffractometer at the CUGB.

4. Results

The cortexes were mostly opaque, although their body counterparts could be transparent. They were white, yellow, brown, and black in color. The surfaces of the cortexes were rounded, and most of them exhibited what jade dealers refer to as a fansha phenomenon, which means that the crystal grains are loose and will fall off when the sample is touched and rubbed with the naked hand. The grains that fall off are not well rounded, and one would feel slight pain if rubbing the grains hard with one’s naked fingers. The jade bodies were fine-grained to coarse-grained and transparent–translucent. On a cut plain, a transition zone from the body to the cortex could be observed. This zone is called the fog zone by jade dealers.

Microscopic observations of the studied samples revealed that there were reddish-brown substances on the surfaces of the weathered cortexes (MJC-1, MJC-4, and MJC-7), in the cracks of samples MJC-2 and MJC-3, and the material filling the cracks (MJC-1, MJC-3), all of which were identical to the color of the cortex (Figure 4). However, their surface features were different with respect to the color of the cortex. The black cortexes were thinner than the cortexes with other colors. They still exhibited the distinct but not sharp edges, angles, and crystal plains of the individual grains, but they did not exhibit the fansha phenomenon. Although some red-brown substances were observed between the crystal grains on the surface, the filling in the open cracks inside was still black (Figure 4a,b). The samples with white–brown cortexes were often thicker, especially the white cortexes. Most of their surfaces exhibited apparent fansha phenomenon.

The microstructures were observed in thin sections that crosscut the junction between the jade body and the weathered cortex. It was found that the jade body and adjacent weathered cortex had similar textures and mineral grain sizes. However, the grain boundaries in the weathered cortex were more distinct than those in the corresponding jade body. Wider grain boundaries, more cracks, and cleavage fissures in the grains were observed in the weathered cortex, and most of the spaces were filled with dark and yellow substances. It was commonly found that a fine-grained body was covered by a fine-grained cortex. However, the coarse-grained bodies were not always covered by coarse-grained cortexes. For example, some of the white cortex samples (MJC-2) had finer-grained fresh bodies (Figure 5). One reason for this is that the white color is inferred to be caused by the fine grains, i.e., such as a streak flaw color.

According to the microscopic observations and comparison, the sizes and shapes of the jadeite grains in the weathered cortexes and jade bodies were almost the same and were successive, but there were much more cracks, cleavage plains, and fissures in the weathered cortexes than those in the jade bodies. This may lead to a misunderstanding that the grain size of the weathered cortex was smaller than that of the jade body (Figure 5). Late-stage supergene minerals with different colors occurred as thin films filling the cracks, cleavage fissures, and grain boundaries in the weathered cortexes. Gibbsite, kaolinite, and halloysite were present as powders, and mineral clusters mainly occurred on the surfaces of the weathered cortexes and in the open cracks that reached the surface (Figure 3). The gibbsite, kaolinite, and halloysite powders were usually opaque (Figure 5), and their chemical compositions were usually impure, resulting in many different colors (Figure 5).

The XRD phase analyses of the weathered cortexes of the eight samples revealed that the main diffraction peaks of all the samples matched those of standard jadeite. However, the weathered cortex of MJC-5 contained a small amount of albite; and the weathered cortexes of MJC-3 and MJC-7 contained gibbsite (Figure 6). In addition, they all contained unknown impurity peaks, for which there is no specific mineral match; however, the peaks indicate that these substances are clay minerals and Fe-bearing colloidal minerals (Figure 6). These clay minerals were determined to be kaolinite and halloysite using an X-ray single crystal diffractometer (Figure 7).

The main chemical compositions of the weathered cortexes and jade bodies were basically identical (

Table 1. Representative chemical compositions of the minerals in the studied samples.

Sample	MJC-1					MJC-2			MJC-3	MJC-5		RD-24		MJC-7
No.	1	2	3	4	5	1	2	3	1	1	2	1	2	1	2	3	4	5	6
SiO2	28.13	58.94	59.24	59.02	58.53	59.62	58.84	58.42	0.68	57.74	59.17	59.08	58.45	58.85	58.75	59.18	59.14	57.49	58.90
TiO2	0.00	0.07	0.00	0.03	0.07	0.00	0.12	0.00	0.09	0.00	0.02	0.00	0.11	0.09	0.07	0.00	0.03	0.19	0.02
Al2O3	16.00	23.43	23.74	23.78	24.03	23.78	22.23	23.65	64.35	20.40	23.81	23.90	24.74	24.38	24.02	24.83	24.58	20.80	24.33
Cr2O3	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	2.69	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
FeO	36.20	0.45	0.52	0.22	0.09	0.33	1.07	0.45	0.42	0.41	0.57	0.21	0.32	0.18	0.39	0.19	0.22	1.55	0.30
MnO	0.45	0.03	0.00	0.00	0.00	0.00	0.00	0.08	0.00	0.13	0.08	0.11	0.00	0.00	0.09	0.00	0.00	0.00	0.08
MgO	3.69	0.94	0.55	0.65	0.70	0.62	1.45	0.66	0.00	1.69	0.59	0.64	0.10	0.26	0.15	0.08	0.06	2.11	0.33
CaO	0.38	1.38	0.70	0.92	1.05	1.20	1.97	1.17	0.00	2.23	0.81	0.91	0.24	0.54	0.24	0.14	0.22	3.01	0.62
Na2O	0.37	14.71	15.11	15.24	15.07	14.88	14.42	14.78	0.00	14.24	15.10	15.15	15.48	15.24	15.18	15.51	15.52	13.60	15.22
K2O	0.00	0.01	0.01	0.00	0.22	0.32	0.00	0.26	0.00	0.00	0.02	0.03	0.22	0.09	0.00	0.00	0.03	0.00	0.00
Total	85.22	99.96	99.87	99.86	99.76	100.75	100.10	99.47	65.54	99.53	100.17	100.03	99.66	99.63	98.89	99.93	99.80	98.75	99.80
Si	3.34	1.99	2.00	1.99	1.97	2.00	1.99	1.98		1.98	1.99	1.99	1.97	1.99	2.00	1.99	1.99	1.98	1.99
AlIV	0.66	0.01	0.00	0.01	0.03	0.00	0.01	0.02		0.02	0.01	0.01	0.03	0.01	0.00	0.01	0.01	0.02	0.01
Ti	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00		0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
T	4.00	2.00	2.00	2.00	2.00	2.00	2.00	2.00		2.00	2.00	2.00	2.00	2.00	2.00	2.00	2.00	2.00	2.00
AlVI	1.58	0.92	0.94	0.93	0.92	0.94	0.88	0.93		0.80	0.93	0.94	0.95	0.96	0.96	0.97	0.97	0.82	0.96
Fe	3.59	0.01	0.01	0.01	0.00	0.01	0.03	0.01		0.01	0.02	0.01	0.01	0.01	0.01	0.01	0.01	0.04	0.01
Cr	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00		0.07	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Mg	0.00	0.05	0.03	0.03	0.04	0.03	0.07	0.03		0.09	0.03	0.03	0.01	0.01	0.01	0.00	0.00	0.11	0.02
M1		0.98	0.98	0.97	0.96	0.98	0.98	0.97		0.97	0.98	0.98	0.97	0.98	0.98	0.98	0.98	0.97	0.99
Mn	0.05	0.00	0.00	0.00	0.00	0.00	0.00	0.00		0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Mg	0.65	0.00	0.00	0.00	0.00	0.00	0.00	0.00		0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Fe		0.00	0.00	0.00	0.00	0.00	0.00	0.00		0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Ca	0.01	0.05	0.03	0.03	0.04	0.04	0.07	0.04		0.08	0.03	0.03	0.01	0.02	0.01	0.01	0.01	0.11	0.02
Na	0.09	0.96	0.99	1.00	0.99	0.97	0.95	0.97		0.94	0.99	0.99	1.01	1.00	1.00	1.01	1.01	0.91	1.00
K	0.00	0.00	0.00	0.00	0.01	0.01	0.00	0.01		0.00	0.00	0.00	0.01	0.00	0.00	0.00	0.00	0.00	0.00
M2		1.01	1.02	1.03	1.04	1.02	1.02	1.02		1.02	1.02	1.02	1.03	1.02	1.01	1.02	1.02	1.02	1.02
Quad		0.05	0.03	0.03	0.04	0.04	0.07	0.04		0.08	0.03	0.03	0.01	0.02	0.01	0.00	0.01	0.11	0.02
Jd		0.94	0.96	0.96	0.96	0.95	0.90	0.95		0.91	0.95	0.96	0.98	0.98	0.98	0.99	0.99	0.85	0.97
Ae		0.01	0.01	0.01	0.00	0.01	0.03	0.01		0.01	0.02	0.01	0.01	0.01	0.01	0.01	0.01	0.04	0.01
Mieral	Chl	Jd	Jd	Jd	Jd	Jd	Jd	Jd	Gbs	Jd	Jd	Jd	Jd	Jd	Jd	Jd	Jd	Jd	Jd

Note: the chemical calculation of chlorite is based on 10 cations, and that of jadeite is based on six oxygen atoms. Mineral abbreviations: Chl—chlorite; Jd—jadeite; Gbs—gibbsite. The nomenclature for pyroxene is from [16].

). The dark clustered substance in the weathered cortex of MJC-3 (Figure 4) was determined to be gibbsite based on the XRD analysis results (Figure 6).

The gibbsite and dense tiny intergranular fissures are shown in the BSE/SEM images (Figure 8a,b). The results of the line scan over the intragranular fissures exhibit a clear tendency. That is, the Na and Si contents of the material in the intragranular fissures are much lower than those of the grains. On the contrary, the Al contents of the material in the intragranular fissures are higher (Figure 8c).

5. Discussion

Not all rocks have cortexes after weathering. Most igneous rocks, sedimentary rocks, and metamorphic rocks are comprised of multiple minerals, so when they are weathered, they usually became loose and break into individual grains, or even into dust and soil. During these processes, these rocks seldom form a weathered cortex, which is a transition state often observed in monomineralic jadeitite in Myanmar.

An increase in the number of open cracks, widened gaps between grains, and cleavage fissures occurred in the weathered cortexes of the Myanmar jadeitite. Even high-quality jadeite jade with shape preferred orientation (SPO), crystallographic-preferred orientation (CPO), and homogranular texture can have open cracks, widened gaps between grains, and cleavage fissures [1,5,7,9]. The cracks, widened gaps between grains, and cleavage fissures were distinctly observed in the thin sections under a microscope. The open cracks were approximatively straight, cutting both through the grains and between the grains, exhibiting an open outline, and were often distinct on the surface. In the fresh jade domain, the jadeite grain boundaries were indistinct, and the gaps between them were almost invisible. In contrast, in the successive domain in the weathered cortexes, the jadeite grain boundaries were distinct, and the gaps between the grains were very clear (Figure 5). Such phenomena indicate that widening of the gaps between the grains took place during the weathering processes. In some domains, the grain boundary was sufficiently wide for the jadeite grains to become loose and for some of them to fall off under shearing and pressure.

More cleavage fissures developed inside the individual jadeite grains in the cortexes of the Myanmar jadeitite. It is well known that all pyroxenes have perfect {110} cleavage plains that intersect at nearly 90° angles. In the weathered cortexes, almost every single jadeite grain contained several cleavage fissures (Figure 5). If the cleavage fissures were enlarged due to later chemical and biological weathering, the weathered grain would be broken up into several fragments; therefore, the grains in the cortexes would become smaller and smaller as the weathering progressed. Considering that dense cleavage fissures were observed in some of the domains, the surface of the domain of the cortex would have a fine-grained appearance similar to that of kilned lime.

It was found that the outlines of the jadeite grains and/or fragments in the cortexes of the Myanmar jadeitite were more rounded than those in the inner transition zone and those of the fresh jadeite domain. By scrutinizing the microphotographs, it was found that the intersections between the gaps and the open cracks were larger than those in the adjacent areas. Similarly, the intersections between the gaps between the grain boundaries and the cleavage fissures were also larger (Figure 5). These larger intersections reduced specific surface area of the grains and/or fragments, making their edges less sharp.

It is well known that the transparency and luster of a weathered cortex are lower than that of its jade body for the jadeitite from Myanmar. The textural changes in the cortex cause optical interfaces and greatly decrease the transparency due to attenuation of the light. Most gemologists believe that the influence of the texture of the jadeite jade on the weathered cortex is crucial, and may be reflected in the thickness, color, luster, fansha, grain size, and orientation, since the texture is a very important parameter in terms of jade quality. However, no obvious differences in the thicknesses, colors, lusters, or fansha were observed between the samples with different textures.

The existence of open cracks, widened gaps between grains, and cleavage fissures in the weathered cortexes of the Myanmar jadeitite clearly indicate that some elements and parts of the minerals may have been removed from the cortex, especially for the larger gaps at the intersections between open cracks (Figure 9).

Gibbsite, kaolinite, halloysite, and Fe-bearing colloidal minerals were identified in the cortexes and fog zones of the Myanmar jadeitite, but these minerals were not observed in the corresponding fresh jadeite jade domains. In the weathered cortexes and fog zones, these minerals were observed to occur in the open cracks, widened gaps between grains, and cleavage fissures. Gibbsite, kaolinite, and halloysite could be residual phases in the jadeite since some of these minerals or similar minerals (e.g., diaspore) were found in fresh-jadeitite-related rocks from Myanmar in the metasomatic phase [17]. However, they could also be exotic phases incorporated from the surrounding environment during weathering. The Fe-bearing colloidal minerals in the open cracks, widened gaps between grains, and cleavage fissures in the cracks in the weathered cortexes and on the surfaces of the weathered cortexes are considered to be exotic filling phases.

The changes in the texture and mineral components of the cortex of a jadeite piece from Myanmar compared with the weathered cortex have gemological implications. Although yellow-brown and black weathered cortexes may be meaningless in predicting the color of the jade body within, based on the color of the weathered cortex, it is possible to roughly estimate the depth- and oxidizing-reducing conditions of the location from which the jade piece was mined. By scrutinizing the texture of the cortex, the grain size of the inner fresh jade can be predicted, and consequently, the texture-related quality of the jade can be roughly estimated. Mostly, the grain sizes and textures of the weathered cortex and jade body of a piece of jadeite are basically identical. In addition, the weathered area of the jadeite jade piece does not always decrease the quality of the jade. In some domains or the same jade piece, the slightly weathered domain exhibited a bright color and a much better luster (Figure 10). For example, in some Chinese jade cravings, the weathered cortex has been used to create marvelous artworks using a particular carving technique, which is called Qiaodiao in Chinese (e.g., Wang and Shi [18]).

Although every jadeite jade piece is unique in texture, both on its surface and inside, the weathered cortex and area along the open crack were revealed to have similar characteristics in this investigation. The changes in the textures and mineral components of the weathered cortex compared with the fresh jade body revealed in this investigation provide some evidence of the quality of the jadeite jade body inside the weathered cortex. Studies of the textures and mineral components of weathered cortexes have gemological applications in terms of identifying and grading raw jadeite and in the design and manufacture of jade pieces from Myanmar. Moreover, such studies can improve our understanding of the changes in jadeite jade caused by weathering from other localities, such as Guatemala (e.g., Xing et al. [19]), and of the changes in other rough jade pieces due to weathering, such as nephrites from China and Russia (e.g., Jiang et al. [20], Wang and Shi [21]), as well as the differences from the weathering of multiple-mineral rocks.

Jadeite stones from other localities may have weathered cortexes formed in a similar way. For instance, some jadeite stones from Guatemala have also been observed to have weathered cortexes. However, this issue has not received much attention and related research has not been thoroughly carried out. Studies of the weathering process on jadeite stones from other localities would have new implications for identifying their origin, which gemmologists and archaeologists consider of critical concern.

6. Conclusions

In this study, the weathered cortexes of eluvial–deluvial jadeite jade, including the aspects of appearance, texture, mineral components, and composition, were analyzed and the following findings were obtained.

• The textural and mineralogical changes in the cortex cause optical interfaces and greatly decrease the transparency, indicating that some elements and components of the minerals have been removed from the cortex. While some neo-minerals (gibbsite, kaolinite, and halloysite) formed as the residual phases of the jadeite, the Fe-bearing colloidal minerals are inferred to have been incorporated from the surrounding environment during the weathering process.
• The yellow-brown and black weathered cortexes may be meaningless in terms of predicting the color of the inner jade body based on the color of the weathered cortex. However, they contain valuable information for estimating the depth and oxidizing-reducing conditions of the location from which the jade piece was mined.
• Since the grain sizes and textures of the weathered cortex and jade body of a piece of jadeite are basically identical, it is possible to predict the grain size of the inner fresh jade by scrutinizing the texture of the cortex. However, too-small size in a certain cortex is not a necessary indication for very fine grain texture of its inner jade body.
• Studies of the weathering process on monomineralic jadeite stones would have implications for identifying their origin, understanding their gemmological utilities, and understanding the differences in the weathering processes from rocks consisting of multiple minerals.