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In the Myanmar jadeitite area of Pharkan, amphibole felses occur between jadeitites and serpentinized dunites. These so-called amphibole fels boundary zones were studied optically and by electron microprobe, and found to include the six amphibole species magnesiokatophorite (Mg-kat), nyböite (Nyb), eckermannite (Eck), glaucophane (Gln), richterite (Rich) and winchite (Win). In most samples, the two main amphibole species Mg-kat and Eck coexist with amphiboles containing variable amounts of components of the remaining four species, as well as with the clinopyroxenes jadeite (Jd), omphacite (Omp) and kosmochlor (Ko). However, Mg-kat, Nyb and Eck are also present as separate phases as well as in zoned porphyroblasts with Mg-kat in the core, Nyb in the inner rims, and Eck in the outer rims. The analytical data on such zoned amphiboles reveal that the chemistry changes from core to inner rim by virtue of the substitution NaAlCa-1Mg-1 (glaucophane vector), and from the inner to the outer rim along MgSiAl-1Al-1 (tschermak vector). The overall substitution from core to outer rim is, therefore, along NaSiCa-1Al-1 (plagioclase vector). Based on the Si content, three groups can be distinguished within Eck: Eck coexisting with Nyb has low Si contents of <7.6 a.p.f.u., Eck rimming Nyb has higher Si contents of 7.6–8.0 a.p.f.u., and fine-grained Eck in the matrix has Si contents of 7.9–8.0 a.p.f.u. Plotting the amphibole analyses in a compositional volume with the axes (Na+K) in A, Na in M(4), and tetrahedral Si shows that three groups of amphibole compositions can be distinguished, one being subdivided into three subsets. Group A contains Rich and Mg-kat, B comprises of Win and Gln, whereas the subsets C can be defined as follows: C1: high-Na amphiboles with low tetrahedral Si; these are mainly amphiboles from the Eck field but overlap with the two fields of Gln and Win; C2: high-Na and low-Si Ecks overlapping to high-Si Nybs; this group is midway between Eck and Nyb end members; C3: high-Na Mg-kats. Textural observations indicate three stages of sodic and sodic–calcic amphibole growth: stage 1 are amphiboles of group A (Mg-kat+Rich), stage 2 are amphiboles of group C2 (Nyb+Eck with Si<7.6 a.p.f.u.), and stage 3 are amphiboles of groups C1 and B (Eck with Si>7.6 a.p.f.u., +Gln+Win). Based on the subdivision into the compositional groups A–C, the only hint to a miscibility gap is provided by the large gap in the (Na+K) content on the A site which may point to a possible solvus in the system Eck–Win. Overall, the amphiboles investigated here show discontinuities in their growth compositions, rather than miscibility gaps. Textural observations suggest amphibole formation during fluid infiltration in the contact zone between the jadeitite bodies and the surrounding peridotite under high-pressure conditions (>1.0 GPa) and rather low temperatures of about 250–370 °C. Based on compositional trends within the amphiboles as well as phase-equilibrium constraints between amphibole and coexisting pyroxene solid solutions, the chemical composition of zoned amphibole porphyroblasts indicates two growth episodes—increasing pressures from stage 1 to stage 2 lead to the formation of Nyb from Mg-kat, and subsequently decreasing pressures lead to the formation of stage 3 Eck from Rich. [ABSTRACT FROM AUTHOR]

The jadeite mines of Myanmar (Burma) are the principal source of top-grade jade, including Imperial jadeite. Petrologically, Imperial jadeite is a fine-grained, Cr-bearing jadeitite. However, it is unclear how Cr3+ from chromite impurities became incorporated into the jadeite. We have studied the textures and compositions of kosmochlor and chromian jadeite aggregates (including maw-sit-sit) collected from the Myanmar jadeitite area to explore how the best-quality jadeite formed. There are four distinct textures of kosmochlor and chromian jadeite: (1) spheroidal or ellipsoidal aggregates surrounding relict chromite; (2) spheroidal or ellipsoidal aggregates with a core of low-Cr jadeite; (3) granoblastic textures in undeformed coarse-grained clinopyroxene rocks, and (4) recrystallized fine-grained aggregates of deformed low-Cr jadeitite (see figure). Electron-microprobe analysis revealed four compositional pairs of coexisting kosmochlor (Ko) and chromian jadeite (Jd) along the Ko-Jd join. Sharp compositional boundaries between them suggest the possibility of miscibility gaps or different stages of replacement of kosmochlor by jadeite. However, replaced textures of kosmochlor by jadeite exclude the possibility of miscibility gaps. The correlation between the textures and compositions of kosmochlor and jadeite is more likely associated with replacement at different stages of formation or spatial differences in the chemical environment. The presence of relict chromite in spheroidal or ellipsoidal aggregates with kosmochlor indicates a metasomatic origin of the jadeitites from original peridotites that reacted with an aqueous solution rich in Na, Al, and Si at a minimum pressure of 1.0 GPa and temperatures of 250-370°C (Shi et al., 2005). Recrystallization during later ductile deformation of the clinopyroxene rocks formed fine-grained aggregates of chromian jadeite, including the Imperial jadeite. The textural and compositional features of the studied samples suggest that the chromium in the jadeite came from chromite in the adjacent host serpentinite. The chromium was incorporated into the jadeite via the following sequence: (1) metasomatic reactions of chromite to kosmochlor, forming spheroidal textures or granoblastic textures; (2) replacement reactions of kosmochlor to chromian jadeite, accompanied by metasomatism; and (3) replacement reactions of chromian jadeite to Cr-bearing jadeite (about 0.3 wt.% Cr203 in Imperial jadeite), accompanied by recrystallization induced by deformation. Further evolution of the final sequence led to the formation of light green jadeite with lower Cr contents. These processes were influenced by the local mineral assemblage, the characteristics of deformation/metamorphism induced by shearing, the pressure-temperature conditions, and local fluid compositions. [ABSTRACT FROM AUTHOR]