The possible existence of silica-rich rocks on Mars requires consideration of all of the candidate forms of silica that might exist there. We synthesized coesite and cristobalite, and obtained natural samples of tridymite, opal-A, and opal-CT. The thermal emission spectra of these silica phases were analyzed and related to their crystal structure and crystallinity. Structural differences between the polymorphs result in unique spectral character and the degree and type of crystal ordering in opal-CT results in cristobalite- or tridymite-like spectral features. These spectra will be useful for analysis of Thermal Emission Spectrometer (TES) data from Mars. While silica polymorphs are not modeled in deconvolution of the two main TES surface-type spectra, this does not preclude the possible detection of local abundances of these minerals on Mars. Spectral detection of silica phases on Mars would have implications for the volcanic and alteration history of the planet.