Having qualities or a structure somewhat similar to amiant; resembling amiant in form but not necessarily in exact composition.
From 'amianthoid' plus the suffix '-al' (from Latin, creating adjectives). This is a doubly-suffixed form: 'amiant' + '-oid' (resembling) + '-al' (relating to).
Stacking suffixes like this created increasingly specific mineralogical terms—'amianthoidal' means 'somewhat like something that resembles amiant,' which shows how scientists need precise language to describe nature's subtle variations.
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