VNIR spectral characteristics of terrestrial igneous effusive rocks: mineralogical composition and the influence of texture

Visible and Near-Infrared (VNIR) reflectance spectroscopy is an important technique with which to map mineralogy and mineralogical variations across planetary surfaces using remotely sensed data. Absorption bands in this spectral range are due to electronic or molecular processes directly related to mineral families or specific compositions. Effusive igneous rocks are widely recognized materials distributed on the surfaces of terrestrial planets, and are formed by primary minerals that can be discriminated by electronic absorptions (e.g. crystal field absorption). In this paper, we review the current knowledge of effusive rock compositions obtained by crystal field absorption in VNIR reflectance spectroscopy, and consider how different petrographical characteristics influence the mineralogical interpretation of such rock compositions. We show that: (1) the dominant mineralogy can be clearly recognized for crystalline material, especially with relatively large crystal dimension groundmass or high porphyritic index; (2) both grain and crystal size are important factors that influence the spectra of effusive rocks where groundmass is generally characterized by microscopic crystals; and (3) glassy dark components in the groundmass reduce or hide the crystal field absorption of mafic minerals or plagioclase otherwise expected to be present.