Pyroxene Spectroscopy at Visible Wavelengths: Effect of Iron Content on Spin Forbidden Absorption Features

Pyroxene spectroscopy has been the focus of much research since the 1970’s [e.g. 1,2-4]. Most of this research has centered on the behavior of the prominent 1 and 2 μm crystal field (CF) absorptions that easily distinguish pyroxene from other minerals [1,5]. In addition to these absorption bands, there are a number of spin-forbidden crystal field absorptions in visible spectra of Fe-bearing pyroxenes. Although these features are orders of magnitude weaker than those in the nearinfrared, they have been clearly observed on Vesta and other smaller asteroids [6]. With the exception of early transmission spectroscopy of lunar and terrestrial pyroxenes [2,3,7,8], there has been little systematic characterization of visible wavelength pyroxene features with chemical compostion. We focus on visible reflectance spectra of a series of synthetic Ca-free pyroxenes to investigate variations in spin-forbidden features with Fe content. These features are of interest because sensitive high spectral resolution detetectors are available to detect their presence in remote observations. Background: Absorption features at visible wavelengths are attributed to four processes [e.g. 5]: (1) Oxygen metal charge transfers (OMCT); (2) Intervalence charge transfers (IVCT); (3) Spin-allowed crystal field transitions (CFT); and (4) Spin-forbidden crystal field transitions (SFT) [5]. CF transitions are responsible for the prominent 1 and 2 μm Fe absorptions in pyroxenes and may also produce absorptions in the visible when Ti or Cr are present [e.g. 5]. OMCT may be 1-2 orders of maginitude more intense than spinallowed crystal field absorptions, and are responsible for the strong absorption edge seen in Fe silicates near 300-400 nm [5]. SFT produce a series of sharp absorptions that are several orders of magnitude weaker than CFT. For example, polarized transmission spectra of a clinopyroxene from Apollo 11 show seven total absorption features between 400 and 700 nm, five of which have been assigned to spin-forbidden transitions in Fe and two of which are assigned to CFT in Ti [3]. These assignments are based on comparison of the positions of the observed bands with energy level diagrams for Fe in octahedral coordination. Spectra of terrestrial and lunar pyroxenes may exhibit four or fewer spin-forbidden features [2,3,7,8]. In terrestrial pyroxenes, these are most likely due to the presence of Fe, which causes an Fe-Fe IVCT band at a similar wavelength, obscuring the weak spin-forbidden features. In extraterrestrial materials, space weathering or shock effects may mask or erase the weaker spin-forbidden features [9]. Approach: This study focuses specifically on the Ca-free orthopyroxene sequence, allowing the effects of Fe substitution on the spin-forbidden features in pyroxene to be explored (Ca-bearing pyroxenes will be included later). Through the generosity of Donald Lindsley and Allan Turnock, we have acquired the set of synthetic Ca-Fe-Mg pyroxenes described in [10]. Pyroxenes for this study were synthesized by a variety of methods that were chosen to prevent nucleation of olivine or pyroxenoids. Purity of samples was verified by X-ray diffraction and electron microprobe [10]. Although all of the samples were prepared as fine powders, all pyroxenes were sieved to <45 μm grain size. Spectra were collected from 410-590 nm with a 0.5 nm resolution using the bidirectional reflectance spectrometer at RELAB. Position, width, and intensity of absorption bands were determined using the Modified Gaussian Model (MGM) [11]. Although the MGM was created to model CFT absorption features, SFT are also dependent on the crystal field environment, and SFT absorption energies may be similarly related to the average bond length. Preliminary Results: High-resolution visible spectra of the Ca-free synthetic pyroxene suite are shown in Figure 1. Spectra have been scaled to endmember enstatite at 525 nm. The strongest discernable absorption feature occurs near 506 nm. Absorptions near 425 and 550 nm are also present in all but the endmember enstatite spectrum. With increasing Fe content, additional bands between roughly 450-490 nm become more prominent.