I got four private replies to this question, two that were informative and two that wanted me to forward them any info I received off list. So let me take this opportunity to relate my updated understanding of this, bcc: Bernd Pauli with a formal invitation to post his thoughts. (RSVP - Bernd, please come back. If anyone starts up with the disrespect that caused Bernd to remove himself from the list, I'll threaten them to be recipient of my well intentioned but WORDY emails. Ask Anne, she might expand on what you're in for).
OK, here's the thoughts hopefully to be constructively critiqued:
Inclusion is the more general term, for which a clast is a special case with arguable limits of application. An inclusion which is not called a clast is a chance introduction of material into the matrix from an external source which is generally not characteristic of a concretion type matrix.
All clasts are inclusions which additionally are common in the concretion type matrix or in its case, derived from it. In practical terms, this means most reasonably sized samples from the overall matrix have a high probability or definable composition proportion of the clast material.
Size doesn't matter. Composition and frequency of inclusion does. Thus, it is inconsistent to restrict or exclusively suggest the term "inclusions" for larger elements and "clasts" for smaller ones of the same type in the same type of matrix. Opinion: If something is a clast, it should be referred to as so, and not interchangably in the same writing with the more general term inclusion. If the writer doesn't wish to commit whether it is a clast of not - and this requires knowledge of the martrix being studied -, only the general word inclusion should be used throughout.
It would be nice if a general agreement on the limits were had since science seems to depend somewhat on precision.. This sort of reminds me of the terminology issue in the difference between Howardites and Eucrites. The latter is based on diogenite composition and frequency as I understand. Yet for a ridiculous tweak in the composition, it appears that one can sell for triple the price of the other to a collector.
Finally, special thanks to Mark Ferguson, for his helpful comments re: application in geology (Mark didn't critique this yet, though.)
Saludos
Doug
