Phenocrysts are not inclusions, but may have inclusions. Clasts are inclusions
if they were very minor components incorporated with or after formation of the
whole rock, or "host" rock (e.g. a breccia is clastic, but it would make no
sense to call all clasts within that rock inclusions). This is arbitrary to
some degree, since percentages of total rock composition must be standardized
for the sake of practical classification, although formation mechanisms are
highlighted predictably by recognizing specific grades or proportions of
components of rock. Fuzziness abounds ... .
Xenoliths are not xenocrysts, but xenolithic rock may bear xenocrysts derived
from xenolithic material also incorporated.
Autoliths, xenoliths, and xenocrysts are clasts by definition, but their
presence does not necessarily make a rock clastic/brecciated.
Many individual crystals described in analyses of igneous meteoritic
material, like basaltic eucrites and many shergottites as "clasts" are actually
phenocrystic (or glomerocrystic).
Phenocrysts are not clasts.
Glomerocrysts are not clasts.
Clasts in breccias are individual fragments of either the same rock or
another rock within a matrix of melt, fused ash, fine-grained sediment
interlocked with clasts and precipitated minerals, or chemical precipitates. A
breccia may or may not be dominated by a single lithology, but always contains
macroscopic chaotic or partially sorted arrangements of clasts. Detrital
sedimentary rock may have homogeneous macroscopic grains, but are well-sorted,
with spaces between clasts proportionate. When heterogeneous, some detrital
sedimentary rocks are termed "microbreccias," such as greywackes and arkosic
sandstones, but must show bimodal grain size and/or poor sorting of
angular/sub-angular components (a result of local fragmentation), within a
detrital matrix.
Pyroclastic rocks are indistinct and bridge formation mechanism-defined
classes; materials like ash-fall tuff grade into "true," or detrital/clastic
sedimentary rock if deposited in wet environments (settling in/with
unconsolidated detrital material) or reworked in terrestrial environments, but
ignimbrites (welded tuffs) fuse upon formation, although they usually contain a
substantial percentage of xenolithic/xenocrystic
clasts, not to mention autoliths and phenocrysts. Autolithic materials in
pyroclastic rock are also termed "clasts."
Chondrites are petrogenetically similar to pyroclastic rock: accreted
chondrules solidify (quench/crystallize) and accrete within microclastic
accretionary matrix ... or did they?
Forgive my quibbling, but I can imagine that unspecific use of the term
"clast" contributes to misunderstanding.
If I need correction, I will accept it.
-Thaddeus
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