[meteorite-list] A problem with terminology

From: Thaddeus Besedin <endophasy_at_meteoritecentral.com>
Date: Sun, 17 Dec 2006 21:00:51 -0800 (PST)
Message-ID: <750344.11787.qm_at_web62515.mail.re1.yahoo.com>

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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Received on Mon 18 Dec 2006 12:00:51 AM PST