Polysynthetic Twinning of Diopsides in the Niewang and Tatliksu Nephrite Deposits, Xinjiang, China

Abstract

Diopside, an important component of the crustal and upper mantle, plays an important role in the formation of nephrite. Polysynthetic twinning in natural diopside, especially from skarns, has rarely been systematically researched. Here, the polysynthetic twinning of natural diopside was investigated in two skarn-type nephrite deposits (Niewang and Tatliksu) in Qiemo, Xinjiang, China. Petrographic observations revealed periodic alternations of dark–light lamellae under cross-polarized light and parallel striations under plane-polarized light, whereas backscattered electron images indicated high homogeneity, which suggested a type of mechanical polysynthetic twinning. According to the optical indicatrix, twins were predominantly oriented as a (100)[100] system. Raman spectra and chemical data show that pyroxenes in nephrite are close to the Di end-member with 0.90–1.04 a.p.f.u. Mg and 0.98–1.05 a.p.f.u. Ca. Both diopside and tremolite from Tatliksu contain slightly higher Fe than those from Niewang. Given the seismic origin of mechanical clinopyroxenes twins, twins of studied diopsides are likely to be attributed to Paleozoic-Mesozoic paleoearthquakes in the Altyn Mountain. A shear stress ≥ 140 MPa and a differential stress greater than 280 MPa are suggested as conditions linked to generation of the twinning of diopsides in the nephrite deposits. The replacement of diopside by tremolite along its twin planes highlights the potential gemological implications, as such replacement by tremolite probably facilitated the formation of dense, fine fibre textures in the nephrites.