The chalk organisms were mostly photosynthetic heterokont or haptophyte protists related to living coccolithophores. If forced to call them between animal, vegetable, or mineral, I'd have to go for vegetable.
Yes, there will be some tiny animals in the chalk as well. But then there are tiny animals on the roots of the vegetables we eat.
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Ken Brown gives a clinching answer on CIX:
Date: 2006-03-15 01:49 pm (UTC)http://en.wikipedia.org/wiki/Heterokont
http://en.wikipedia.org/wiki/Haptophyte
http://en.wikipedia.org/wiki/Coccolithophore