The Psephite Conundrum
Sunday August 10, 2008
An oddity of geology's history is that there are two sets of terms, one from Latin and one from Greek, for the particle sizes in sedimentary rocks:
- A rock made of clay size particles can be called either a lutite (Latin) or a pelite (Greek).
- Sandy stones can be arenites or psammites.
- Pebbly rocks can be rudites or psephites.
(Apparently A. W. Grabau is to blame, creating the Latin terms in 1904.)
Who uses them? My experience at meetings and in reading papers gives me the following impressions:
- The main users of lutite and rudite are limestone specialists, who also use arenite. In all cases they are purely size-related terms.
- Metamorphic petrologists use pelite and to a lesser extent psammite (actually "pelitic/psammitic") to signify a rock's origin, but there the terms have a geochemical sense, because after all clay-size particles are generally clay minerals and sand is mostly quartz.
- Sedimentary petrologists use arenite to denote clean sandstones, as opposed to impure sandstones which are wackes. They also talk about pelitic sediments, which again has a de-facto geochemical sense.
- Nobody at all talks about psephites. As a culture we appear to be antipsephitic.
So the older, Greek-based terms have wandered away from their original size-related meanings. I would take two lessons away—first, don't use words with silent P's, and second, hold the line with lutite-arenite-rudite. If they stop being useful, we'd have to start over by borrowing from a third language.
As a hypothesis, I will guess that the scientists in charge of studying the surface of other planets are sticking closely to the Latin terms. Anyone out there know?
Comments
Awesome! I love information like this. Cool post.
I’d never even heard of psephites - and probably wouldn’t be able to pronounce or spell the word right!
My sedimentology professor preferred the terms as lited above for use in the laboratory but rarely used them in the field. Andrew can you research the phenetic pronunciation of the Greek terms?
Matt, they aren’t really field terms–in the field you’d say shale/limestone/whatever, sandstone, conglomerate/breccia instead.
You pronounce the two problematic words “SAMite” and “SEF-ite.”
Trying to get as confusing as the medical field, huh.
Greek practice of using pebbles to vote, right?
The ‘p’ in ‘psi’ is not silent in ancient or modern Greek, so the letter sounds like the ‘ps’ in the words ‘lapse’ or ‘upset.’
I wonder how the ‘p’ came to be silent in scientific or biblical (as in ‘Psalms’) pronunciation.
Sam Milligan, Brooklyn, NY
As a UK based geologist the term psephite and psammite are well known. Psephites don’t attract much attention as their metamorphic contents are determined by the protolith (country rock) content. A metamorphosed conglomerate may contain clasts of metaquartzite, marble, hornfels etc.
Excellent — I figured the UK might preserve the older terms. But do you use psephite as a purely size related term or does it, like psammite, have a de facto geochemical significance?