[Rockhounds] Scapolite

[Axel Emmermann]

Hi Brother Don,

I stumbled upon an issue with scaoplites in which rings of SiO4 and AlO4
tetrahedrons form a cage in which an anion is "encapsulated".  
It appears that Cl- fits in nicely but CO3 can have four  possible
orientations within the cage. The CO3 anion is unable to rotate at low
temperatures and in one instance of the possible positions it is believed to
actually stick out of the cage some.
I would, as a complete layman looking at this complex matter, infer from
this that 1 in 4 unit cells of theoretically pure meionite (Ca, CO3 end
member) would suffer from crystallo-chemical deviation. The electron density
in that face of the unit cell would be different from the others.
Moving towards the marialite end of the series would decrease the number of
these "faults?".

That is why I would like to know whether there is a noticeable discrepancy
between the occurrence of large, gemmy crystals of both species or that it's
apparently not a big deal ;-)

It seems also that the ratio of Al-occupied/Si-occupied tetrahedrons is much
higher in meionite than in marialite. Something in the order of 10 to 15 %
more Al than Si.  Does this mean that Fe3+ substituting for Al could also
occur more frequently in meionite than in marialite?

Greetings from Belgium where the weather is fair but chilly....

Axel

> -----Oorspronkelijk bericht-----
> Van: rockhounds-bounces at lists.drizzle.com
[mailto:rockhounds-bounces at lists.drizzle.com]
> Namens DonH
> Verzonden: maandag 22 september 2008 22:51
> Aan: Rockhounds at drizzle.com: A mailing list for rock and gem collectors
> Onderwerp: Re: [Rockhounds] Scapolite
> 
> 
> Brother Axel,
> 
> I would imagine that crystallization of regular inorganic solids is more
> dependent on the conditions of formation.  If it has a well-defined
> structure, then under the right conditions it would tend to form
> crystals as large and well-shaped as any other.  This usually involves
> slow cooling and, of course, an abundance of the necessary elements or
> polyanions necessary to form the mineral.  Minerals formed from
> low-temperature alteration would tend not to form large and wel-defined
> crystals, though of course there may be exceptions to this.
> 
> To mineralogists, for example, gypsum brings to mind large crystals from
>   4 cm to huge crystals in the Cave of Swords.  To a soil scientist,
> gypsum usually brings thoughts of white crusts and nodules.
> 
> Best,
> Don
> 
> 
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