Relating to crystallography; of or pertaining to the study and analysis of crystal structures and their properties; same meaning as crystallographic but less commonly used.
From crystallography with the adjectival suffix -ical, an older variant formation alongside crystallographic. Both forms coexist in scientific literature, though crystallographic has become more standard in modern usage.
The existence of both 'crystallographic' and 'crystallographical' shows how English gives scientists multiple ways to describe the same thing—physicists often prefer the shorter form because precision is already built into the technical context.
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