A noun in the Quendi and Eldar essay from 1959-60 glossed “frost” (WJ/417). It also appeared in some etymological notes from around 1959 as a variant of niquis “ice-flake or snowflake” under the root √NIK(W) (PE17/168).
Conceptual Development: The word for “frost” was ᴹQ. helor in the Declension of Nouns of the early 1930s and ᴹQ. helle in The Etymologies of the mid-to-late 1930s, both based on the short root ᴹ√KHEL “freeze” (Ety/KHEL). In The Etymologies Tolkien deleted this short root and its derivatives, retaining only longer ᴹ√KHELEK, and in later writings Tolkien seems to have decided “frost” was based on √NIK(W) instead.
Neo-Quenya: For purposes of Neo-Quenya, I’d limit nixë to “frost” and use niquis for “snowflake” to help differentiate the two words.
nixë noun "frost" (WJ:417); previously described as a synonym of niquis "ice-flake or snowflake", q.v. (PE17:168)