Hort,re 119, Good observation at reply ...

Hort,
re 119, Good observation at reply 130, it has to be a possibility. Fibrous flourescent minerals? Hydrated Calcium Sulphate for a start (fibrous gypsum), some of the asbestos group. Erionite. Illite which is an alteration product of feldspar and others.
Any soil which includes weathered fibrous minerals will of necessity include fibres. Magnetic attraction can also give the appearance of fibres. But mineral fibres in soil are not a good thing.