A Spectroscopic and Chemical Study of the Coloration of Feldspars by Irradiation and Impurities, Including Water

Citation

Hofmeister, Anne Marie (1984) A Spectroscopic and Chemical Study of the Coloration of Feldspars by Irradiation and Impurities, Including Water. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/bj75-5674. https://resolver.caltech.edu/CaltechTHESIS:10172018-114703911

Abstract

Natural smoky color or smoky color induced by ionizing radiation develops only in potassium feldspars (KAlSi₃O₈) free of water bound in the feldspar structure. Neither fluid inclusion water nor ≡SiOH have an effect. The optical absorption spectra of the smoky color consist of polarized bands at 11600, 16200, 19100, and 27200 cm^-1, whose integrated intensities are linearly correlated with the integrated intensity of a broad, asymmetric first derivative at g_eff = 2.027 in electron paramagnetic resonance (EPR) spectra. This hole center forms only in KAlSi₃O₈ without structurally bound H₂O, and in microcline is resolved into an asymmetric six-line pattern at g_eff = 2.024 and a single derivative at g_eff = 2.009 which are Si-O^- -K and a hole shared between two nonbonding oxygens (NBO) on Si. In analogy to coloring in quartz and glass, the 11600 cm^-1 band is caused by a hole trapped between two NBO's on silicon, the 16200 and 27200 cm^-1 bands are due to the Si-O^- -K center, and the 19100 cm^-1 band results from a hole trapped on an oxygen attached to two aluminums. Smoky centers do not develop in feldspars with structural water because irradiation mobilizes protons which, while diffusing, destroy centers in their path, and finally then settle in sites similar to their original site. Smoky color also develops in sodic plagioclases, but high Al content inhibits its formation in labradorite.

Amazonite color is intrinsic and controlled by an absorption minimum between three overlapping bands in the ultraviolet and a broad band in β at 630, or one UV band and a broad band in β at 720 nm, or both superimposed. Comparison of EPR to optical integrated intensities shows that all three colors are connected with a first derivative at g_eff = 1.56 and two satellites of about 1/7 intensity at g_eff of 1.83 and 1.39. Analysis of the EPR pattern shows that this center is Pb³⁺ 31% of the time, with the hole located on coordinating oxygens for the remaining 69%. This center is only produced in samples which have in addition to Pb, H₂O structurally bound in the lattice. The dependence of color intensity on the smaller molar concentration of structural water or lead implies that lead and structural water in a 1:1 ratio produce color centers in amazonite. The first order reaction kinetics of amazonite color formation by irradiation and the observation that water is not consumed in the process suggests that Pb²⁺ is oxidized to Pb³⁺ by the product OH of the irradiation-induced dissociation of water while H concurrently destroys a hole center on an oxygen, and is followed by the regeneration of the water molecule. The kinetics also show that the radiation necessary for the coloration is provided by internal decay of ⁴⁰K. The two end-member color types (630 or 720 nm) occur for microcline or orthoclase local structure, respectively. Al/Si disorder increases first locally, and then overall as larger amounts Pb or H₂O are incorporated, so that crystals with intermediate Pb contents have both color types. A spectrally similar blue radiation color also occurs for Pb-bearing sodic plagioclases.

Gemmy labradorite phenocrysts from one Steens Mountain basalt flow in Rabbit Basin, Oregon, sometimes possess a pink schiller, or more rarely a transparent red or green coloration. Direct microprobe analysis of the schiller flakes show that these are metallic copper. XRF analysis of the different colored zones revealed that only the copper content varies with color: colorless samples, or sections of crystals, have 0-35 ppm Cu; greens average 80 ppm Cu; reds average 135 ppm Cu; while schiller bearing labradorites have 50 to 240 ppm Cu. Spectral similarity of the red color to copper-ruby color of glass shows that the red arises from the intrinsic absorption of colloidal Cu^o particles that are too small to scatter light (ca. 4 to 22 nm). Spectra from the green regions strongly resemble that of amazonite. Because the temperature of exsolution is subsolidus and proportional to Cu content, diffusion proceeds more rapidly for crystals with higher Cu content and results in formation of larger particles. The Cu^o reduction at low temperature (800°C) involves formation of hole center (O^-) that is captured by Pb²⁺ to form the green amazonite color (Pb³⁺). At high temperatures (~ 900 to 1100°C) the reduction of Cu is controlled by whatever reactions occur in the basalt to keep fO₂ along the QFM buffer. Migration of Cu^o may cause the variation of Cu concentrations in a single sample; but the variation of Cu content among different crystals suggests that the composition of the megacrysts was not constant and changed in response to an increasing copper content in the melt as crystallization of the labradorite proceeded.

The coloration process in feldspar strongly resembles that in glasses for both radiation colors (smoky) and exsolution phenomena (Cu^o colloids, Cu^o schiller) and also that of radiation colors in other crystalline solids (smoky quartz, Pb³⁺ or Tl²⁺ in KCl). Although quartz and glass are structurally and chemically similar to feldspar, KCl is not, suggesting that for the most part it is the behavior of the chemical impurity on an atomic level which controls the coloring mechanism.

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