Gem Elbaite as a Recorder of Pegmatite Evolution: In Situ Major, Trace Elements and Boron Isotope Analysis of a Colour-Zoning Tourmaline Crystal

Beiqi Zheng,Meihua Chen

doi:10.3390/cryst11111363

Abstract

Few studies have focused on gem-quality tourmaline acting as a petrogenetic recorder, and the colour genesis of pink elbaite is still controversial. We carry out in situ major, trace element and boron isotope composition analyses on a single tourmaline crystal. This crystal is characterized by sudden transformation from colourless to pink, which can represent full pegmatite magma evolution. According to the analysis results, all spots are divided into alkali groups according to X-site occupancy and subdivided into elbaite series. The pink part accommodates higher concentrations of volatile and incompatible elements. The result is most consistent with successive pegmatite evolution in which the colourless part crystallized from the early stage, while the pink part crystallized from the late stage. The relatively consistent δ11B value between the colourless and the pink part suggests no fluid exsolution occurred during pegmatite evolution. The slight increase of δ11B values within the pink part and the colourless part may be due to mica crystallization. The combination of (Li++Mn2+) (Al3++Xvac)-1 and the exclusive positive linear relationship of Mn2+ vs. Ti4+ indicate that Mn2+ is the main cause of pink, while Mn2+-Ti4+ intervalence charge transfer also plays an important role.

Highlights

Tourmaline is distinguished from other gem minerals by its unique splendour and changeable colour, even within individual crystals
The single tourmaline crystal in this study is recognized as a pink part and a colourless part with chemical and boron isotope differences between these two parts
Based on detailed morphology and element and boron isotope analysis, we suggest that this tourmaline can represent an entire evolution process of pegmatite magma

Summary

Introduction

Tourmaline is distinguished from other gem minerals by its unique splendour and changeable colour, even within individual crystals. As schorl is the most widespread tourmaline which has a black appearance, quite low percentages of tourmaline can reach gem quality [6]. It is considered an attractive mineral and collectable gemstone. As a common accessory mineral in pegmatite, tourmaline, especially gem tourvolatile components. As a common accessory mineral in pegmatite, tourmaline, especially gem tourmaline with zonation, is a better carrier than other rock-forming minerals, such as feldspar and muscovite. Very poor cleavage and flawlessness under the 10 magnifying glasses are shown in this entire crystal.

Methods

Boron Isotope Analyses

Conclusions