Sedimentary Minerals for Dummies

Fossiliferous limestone

It's bad enough to our staff here at the Antisocial Network when a bumbling freelancer does a halfwit job in the alleged "research" and transfer of information, but they're even more disgusted when they run across people who are supposed to know better doing a crappy job. A case in point: our staff geologist turned up a contributor to suite.io,¹ Alexandra Matiella Novak, who pretty much undermined the validity of the PhD she claims to have in geology with her poor research. Well, it seems Alexandra's back again, and this time she's giving short shrift to "Minerals in Sedimentary Rocks."

The problem we find with Novak's (Matiella Novak's?) contribution is not that she got the facts wrong. Most, if not all, she says in her post is true – though somewhat simplified in a vague attempt to "dumb it down." For instance, she claims that

"The composition of the source rock determines what the composition of the sedimentary rock will be..."

...a gross overstatement and, in fact, one that is only applicable to detrital sedimentary rocks. Alexandra is obviously hung up on igneous source rocks, and goes into some detail about which mineral species have greater or lesser resistance to chemical weathering. Sadly, she neglects any mention of physical weathering: yes, feldspars are susceptible to chemical alteration and quartz isn't, but feldspars are also susceptible to physical weathering because of their cleavage; an attribute quartz lacks.

Novak also tries to tell her readers that "Elements within the water react to the olivine and biotite to create clay. These clay particles are transported downstream where they are deposited. Several thousands of years of compaction and cementation lead to the creation of a claystone formation." Another vast oversimplification, not to mention that a far greater amount of clay results from alteration of feldspars than that of olivine. Oh, yeah, and claystone (or shale) isn't usually "cemented."

There is, of course more (there's always more): after a primer – mostly accurate but demonstrably incomplete – on the resistance of certain mineral species to chemical weathering, Alexandra goes on to discuss textures in sedimentary rocks, by which she apparently means only grain size (only one aspect of texture) of – again – detrital rocks. Her explanation includes this cogent statement:

"As are silt grains to from siltstone and clay particles to from claystone, mudstone or shale..."

...that no one in the team was able to decipher. Be that as it may, Alexandra's most profound dumbassery in this context is that she completely ignores sedimentary minerals: the very first sentence reads

"Sedimentary rocks form from the compaction and cementation of sediments that have been eroded from other rock formations."

It's a statement that's only partially true (not to mention slightly circular): there are a number of minerals that form in the surface environment rather than being the result of erosion and transportation.

Alexandra's post ignores not just the not uncommon evaporite minerals – gypsum and salts – found scattered throughout the sedimentary record, it also completely ignores some 10% of the world's sedimentary rocks. By that we mean the carbonates, limestone and dolomite, which form by precipitation from water, usually by biological processes. Neither of these rocks is "eroded from other rock formations", though both are quite important. We think that at least some mention of these rock types was warranted -- apparently Alexandra didn't. As far as we're concerned, that means she deserves to be the Dumbass of the Day. Why? because – at least in theory – she knows better.

¹ Sadly (NOT), suite101.com changed its name to suite.io, and then disappeared. Oh, well, no loss...

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