[Rockhounds] Super-deep more perovskite

[pmodreski at aol.com]

Hi, Dennis,


I think you understand this, mostly.  What you said in your last sentence is exactly correct:


"The event of note is that we have a high pressure 
mineral incapable of existing at normal pressures contained within a 
mineral that is stable at a crustal pressure?"

A mineral has a name, and it has a certain crystal structure.


Thus, calcite is CaCO3, with a certain crystal structure.
Aragonite is also CaCO3, with a different crystal structure, which we would refer to as "the aragonite structure".  And as it happens, aragonite stable at higher pressures, thus, calcite can transform into aragonite, at high pressure.  In this case, both have the same chemical composition.


Spinel is a mineral that is stable at more-or-less ordinary pressures (or at least, in metamorphic rocks and "moderate" pressures); and the chemical composition of "ordinary" spinel is MgAl2O4.
And, olivine is magnesium silicate, Mg2SiO4, and it also is stable at low pressure (as in Hawaiian lavas).  The crystal structure of spinel is considerably different than that of olivine; thus, we can talk about other minerals with the "olivine structure" or the "spinel structure".


But a really high pressures, olivine, composed of its Mg and Si and O ions, can transform into a spinel-type crystal structure, which still has the same chemical composition, Mg2SiO4, but now, the 2 Mg + Si ions take the crystal lattice positions that the Mg + 2 Al ions occupy in spinel, and it turns into a very dense, high-pressure mineral, ringwoodite, which is "the spinel-structure polymorph of olivine".


Likewise, "ordinary" perovskite, which is found (though rarely) in some igneous rocks in the earth's crust, formed at relatively low pressures, has the chemical formula CaTiO3, calcium titanium oxide.
But other combinations of metals can also exist with the perovskite crystal structure; and one of these is calcium silicon oxide, CaSiO3, which would normally (at lower pressures) form the mineral, wollastonite, but at extremely high pressures--in the lower mantle--it can exist with the perovskite crystal structure, and that is what these people discovered, within diamond.


This doesn't necessarily mean that the Ca and Si that formed this mineral, actually came from grains of wollastonite.  That didn't need to happen; the Ca and Si can come from a lot of other minerals that exist in the mantle, such as for example diopside, CaMgSi2O6, with the Ca+Si forming the new dense perovskite-structure mineral, and the leftover Mg going into other minerals.


That's my short (?) answer to that!


Pete


-----Original Message-----
From: Dennis Buffenmyer <buff1 at ptd.net>
To: pmodreski <pmodreski at aol.com>; Rockhounds at drizzle.com: A mailing list for rock and gem collectors <rockhounds at rockhounds.drizzle.com>
Sent: Fri, Mar 9, 2018 5:14 pm
Subject: Re: [Rockhounds] Super-deep more perovskite



On 3/9/2018 12:26 PM, pmodreski at aol.com wrote:
> Glenn, I think the simple answer is that these transformations back to ordinary low-pressure mineral forms occur very quickly, and that it is only under exceptional situations, such as a crystal being enclosed within a diamond, that the high-pressure minerals are SOMETIMES (very rarely, sometimes never) preserved.
>
>
> Pete
> Very interesting.
>
> I would like to learn how, why, and what these minerals would morph into in "normal" surface conditions and an estimated time frame for the morphing changes to complete.
>
> Thanks all for sharing!
>
> Glenn Wimpee
>
>
>
> The spinel-structure polymorph of olivine, ringwoodite,
Ok, here comes my question. at least we have a new name for a structural 
difference resulting in a new mineral name. It is confusing enough that 
the naming of minerals seems broad in some areas, and others are tightly 
confined. Am I to assume they have given a mineral name to a particular 
structure?? With perovskite as a mineral being Calcium titanium oxide, 
the usage of calcium silicon oxide as perovskite is confusing.
And if it only exists in a fluid phase or at least a "toothpaste" phase 
then what is the relevance of "crystal" symmetry?
I realize this is all probably a geology 101 question, or possibly an 
igneous/metamorphic rocks 101 question, but I hope I am not belaboring 
the point here. Bottom line, in the example the silicon does not 
magically transform into Titanium as the mixture can not exist in a 
crustal atmosphere? The event of note is that we have a high pressure 
mineral incapable of existing at normal pressures contained within a 
mineral that is stable at a crustal pressure?
Sorry folks
Dennis Buffenmyer