Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
† sĕlēnītis, ĭdis, f., or sĕlēnītes, ae, m., = σεληνῖτις, crystalline gypsum, selenite. Form selenitis, Plin. 37, 10, 67, § 181; form selenites, Sol. 37, 31; Aug. Civ. Dei, 21, 5; Isid. 16, 4, 6.