PROTOENSTATITE AND NATIVE COPPER NANO-CRYSTALS IN OREGON SUNSTONES: TEM INVESTIGATION AND THEIR IMPLICATIONS

Oregon sunstone is an uncommonly colored gem-quality plagioclase feldspar (~An₆₅) found in small thin lava flows in south central Oregon. Some sunstones display dichroic red/green colors along different crystallographic directions. Copper platelets oriented along crystallographic planes of (010) and (100) cause a schiller effect. High-resolution transmission electron microscopy (HRTEM) was used to examine colored sunstones with and without schiller to determine crystallographic orientation of nanocrystals of copper and previously undetected low Ca, high Al nano-pyroxene precipitates within the plagioclase. High resolution images, electron diffraction patterns (ED), and electron energy dispersive spectroscopy (EDS) reveal pyroxene phases of protoenstatite, enstatite domains, and stacking disorder structures along the a-axis in association with native copper nano-crystals. Protoenstatite requires high T (T> 1000C) and rapid cooling in order to preserve protoenstatite at room temperature. Most pyroxene precipitates containing disordered stacking along the a-axis display streaking along the a*-direction in diffraction patterns. It is proposed that Al-bearing enstatite and intergrown domains with stacking disorders result from a protoenstatite phase transition during rapid cooling of the host plagioclase. Electron diffraction patterns from the precipitates show preferred orientation between the pyroxene phases and the host plagioclase. Plagioclase with weak and diffuse b-reflections also indicates fast cooling / quenching. Some linear features of Cu/pyroxene precursors can be seen in TEM images based upon strain contrast and rapid amorphization of the plagioclase under the electron beam. EDS spectra from these areas show relatively higher concentrations of Cu, Mg, and Fe with respect to the neighboring plagioclase. It is proposed that the Al-bearing protoenstatite precipitated (or exsolved) from the plagioclase due to P-T condition changes. The pyroxene precipitates and other defect sites may serve as nucleation sites for Cu precipitates. Preferred orientation of the pyroxene may contribute to observed pleochroic colors (pink/green) of plagioclase.