[Rockhounds] Super-deep more perovskite

[Peter Richards]

I think it should be added that transformations from high pressure and/or high temperature forms to “Earth surface” forms do not all happen rapidly.  The glaring example here is the diamond itself, which should be graphite at the Earth’s surface, but its structure is so strong and resistant to change that it persists and protects the perovskite-structure mineral we are discussing. In order to convert diamond to graphite, one has to break the very strong carbon-carbon bonds and construct new ones with a different geometry.  At surface conditions (but elevated temperature) diamond will burn and become carbon dioxide rather than form graphite.

Quartz, on the other hand, exists as a high-temperature form with full hexagonal symmetry.  When it cools past the critical temperature (~573°C), it converts immediately and easily to low-quartz, because no bonds need to be broken - they just have to twist a bit.

Most if not all of the minerals in igneous and metamorphic minerals are not stable at surface conditions, but persist anyway because their transformations are sluggish.  Many of them eventually meet their demise not by converting to a different structure with complete preservation of the chemical makeup, but by being attacked chemically, with more soluble components being lost from the local system while other components re-combine, for example to form clay minerals and iron oxides.

Pete
___________________________
R. Peter Richards, Ph.D.
rpr at heidelberg.edu
Morphological Crystallographer

> On Mar 9, 2018, at 7:43 PM, pmodreski at aol.com wrote:
> 
> 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
> 
> 
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