Cathodoluminescence petrography of P–type jadeitites from the New Idria serpentinite body, California

Abstract

Jadeitite from the New Idria serpentinite body of California is a fluid precipitation–to–metasomatic product. Optical cathodoluminescence (CL) microscopy of the jadeitite revealed that vein–filling ‘pure’ jadeites (mostly 97–99.9 mol% jadeite) exhibit bright luminescence, whereas ‘impure’ jadeites (mostly 75–95 mol% jadeite) in pale–greenish matrix show dark luminescence. The ‘pure’ jadeites in the veins are composed of mixtures of red, blue and dull blue CL–colored domains, showing growth textures (oscillatory bands). The ‘impure’ jadeites in the pale–greenish matrix with dark luminescence have a higher augite component (up to 5.37 wt% FeO), implying that the CL property is due to significant amount of Fe²⁺ to act as a quencher. CL spectra of the blue CL–colored domains of the vein–filling ‘pure’ jadeite have a doublet broad emission peak centered at ∼ 320 and ∼ 360 nm in the ultraviolet (UV) to blue region. In the red CL–colored domains, a broad asymmetric emission peak at ∼ 700 nm is also recognized together with the doublet UV–blue emission peak. Comparing monochromatic CL images in the UV–blue (300–400 nm) and red (650–750 nm) emission regions with X–ray elemental maps, luminescence centers contributing the UV–blue and red CL emission peaks were assigned. The red emission peak of the ‘pure’ jadeite with subtle augite component would be attributed to lattice defects related to Ca²⁺, Fe²⁺ (or Fe³⁺) and Mg²⁺ deficiency and/or excess centers in M1 or M2 sites. Alternatively, transition metal ions (Mn²⁺ and Fe³⁺) or rare earth elements in the M1 and M2 sites as impurity centers, might contribute to the red emission peak. As the UV–blue emissions correlate with Al³⁺ content, i.e. purity of jadeite component, they might be related to Na⁺ and/or Al³⁺ defect centers.