1 Introduction

The ISO mission revealed the presence of crystalline silicates around both evolved and young stars. Comparing the spectra to laboratory data suggests magnesium-rich olivine $\rm (Mg,Fe)_2SiO_4$ and pyroxene $\rm (Mg,Fe)SiO_3$ to be the major crystalline silicate components of most objects (see, e.g., Waters et al. 1996; Waelkens et al. 1996; Jäger et al. 1998; Waters et al. 1999; Molster 2000).

For radiative transfer computations needed to model spectra of circumstellar envelopes and disks, optical constants are always required. They serve as the basis for the calculation of the necessary absorption and scattering efficiencies for different size and shape distributions of the dust particles. In order to broaden the database of optical constants, we present such data for three minerals of the olivine group with different iron contents: olivine, fayalite, and forsterite. Fayalite is the iron endmember and forsterite the magnesium endmember of the olivine group ( $\rm (Mg,Fe)_2SiO_4$).

Optical constants of minerals of the olivine group have already been published (Steyer 1974 (Mg-rich olivine); Hofmeister 1997 (fayalite); Servoin & Piriou 1973 (forsterite); Iishi 1978 (forsterite)). For forsterite and an Mg-rich olivine, we improve the quality of the data by a broader wavelength coverage and the higher spectral resolution of our measurements, respectively. We identify certain additional vibrational modes in the spectra. The optical constants of fayalite are reconsidered on the basis of our new reflection measurements. We find deviations from the data published by Hofmeister (1997) in terms of band strengths.

In contrast to amorphous silicates, crystalline silicates have a lot of diagnostic bands due to metal-oxygen vibrations. Emission features of crystalline silicates have been observed in numerous astrophysical environments. Evidence for the presence of magnesium-rich crystalline olivines and pyroxenes have been found in circumstellar regions (envelopes/disks) around young and evolved stars, IDPs, comets, and in meteorites (see, e.g., for AGB stars: Molster et al. 1999; planetary nebulae: Waters et al. 1996, Beintema 1998, Cohen et al. 1999, Molster 2000, Lim et al. 2001; Orion nebula and the HII region M17-SW: Cox & Roelfsema 1999, Cesarsky et al. 2000, Molster 2000; circumstellar disks surrounding Herbig Ae/Be stars: Malfait et al. 1998, Malfait et al. 1999 (HD 100546), Waelkens et al. 2000 (HD142527), Meeus et al. 2001; IDPs: Sandford & Walker 1985, Sandford & Walker 1985, Bradley 1995, Bradley et al. 2000; comets: Hanner 1999, Crovisier et al. 1997, Wooden et al. 1999). Fayalite has not been observed in circumstellar shells so far. However, fayalite has been found in the matrix of a chondritic meteorite indicating a vapour growth prior to parent body accretion (Weisberg et al. 1997).

Silicates are assumed to condense in the ejecta of evolved stars mainly in an amorphous state. However, at high temperature, a certain percentage can also condense as manifoldly shaped nanocrystals as was shown by condensation experiments (Tsuchiyama 1998). Initially amorphous silicates can also transform into an ordered crystalline structure by the process of thermal annealing at sufficiently high temperature (Gail & Sedlmayr 1998). Annealing experiments have been performed by Hallenbeck et al. (1998), Brucato et al. (1999), and Fabian et al. (2000).

The aim of this paper is to provide optical constants of crystalline silicates of olivine composition ranging from forsterite to fayalite. The composition and homogeneity of the materials were characterized by scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX) (see Sect. 2). Section 3 presents the reflection spectra and the derived optical data. In Sect. 4, morphological effects as well as the influence of the matrix on band profiles and absolute absorption efficiencies will be outlined. Finally, the results of this study are used to discuss the recent band identifications of crystalline dust features in ISO spectra.