[Rockhounds] Scapolite

[Axel Emmermann]

Kreigh,

In the case of scapolite the crystal unit cell is a 3-D cage of
tetrahedrons, alternating AlO4 and SiO4 with increasing AlO4 content towards
the meionite end of the series. The cage can enclose a CO3 completely in 3
of its 4 possible position. In one position there is an oxygen atom sticking
out, apparently.
There's also a possible rotation of rings of tetrahedrons relative to each
other. This would break the symmetry of the tetrahedrons qua orientation.
Ideally you would see one of the tetrahedron pointing outwards of the cage
with its tip while the adjoining tetrahedrons point inwards. So if you
follow one direction of the network around the anion-cage, jumping from one
tetrahedron to the next, you would se the tip of the tetrahedrons sticking
in-out- in-out- in-out- in-out- in-out- in-out in any direction you take.
Also you see that each AlO4 is surrounded by 3 SiO4. 
When Ca2+ and CO3 concentrations increase while you move from the
theoretical marialite composition (with Cl- in the cage) to meionite there
(if I understood the paper I read ;-)  are two thing happening:
1) the in-out- in-out- in-out- in-out symmetry is broken and there are some
tetrahedrons pointing in the same direction as their neighbours. 
2) more adjoining Al-occupied tetrahedrons occur.
I can only guess what this does with the development of crystals... On the
other hand: if all crystals needed to be either perfect or gone, us
collectors would 
be left with empty shelves ;-)))

> Mashing two or more molecules together into a unit cell often
> introduces some distortions into the bondings. If you can hide the
> distortions inside a unit cell then large crystals become possible; the
> need to make unit cells makes any crystals less likely.

Yes there seems o be slight changes in distance between the atoms in some of
the Si-O bonds. In the order of a few picometers but I don't fully grasp the
significance of that... I think it means that the Si and Al are not in the
centre of the tetrahedrons and their "offset" changes a little with the
position in the marialite-meionite series (as do the angles between the Si-O
bonds. I don't know if that would influence the crystal shape because it's
only the angles between the tetrahedrons relative to each other that would
define the outer shape of the crystal (I think).

Hehe... much has been written about this 

Cheers
Axel

> -----Oorspronkelijk bericht-----
> Van: rockhounds-bounces at lists.drizzle.com
[mailto:rockhounds-bounces at lists.drizzle.com]
> Namens Kreigh Tomaszewski
> Verzonden: woensdag 24 september 2008 4:16
> Aan: Rockhounds at drizzle.com: A mailing list for rock and gem collectors
> Onderwerp: Re: [Rockhounds] Scapolite
> 
> Axel,
> 
> I think the key question is what is the unit cell for crystal
> formation? Quartz uses three molecules as a unit cell to provide a
> shape that stacks well to make crystals.
> 
> Single molecules often have some atom sticking out one edge that makes
> it difficult to stack them, but if you combine two or more into a unit,
> often introducing handedness, you end up with a regular shape that can
> stack.
> 
> Mashing two or more molecules together into a unit cell often
> introduces some distortions into the bondings. If you can hide the
> distortions inside a unit cell then large crystals become possible; the
> need to make unit cells makes any crystals less likely.
> 
> Kreigh
> 
> 
> 
> 
> On Monday, Sep 22, 2008, at 06:49 America/Detroit, Axel Emmermann wrote:
> 
> > Hi Kreigh,
> >
> >
> >
> > Thanks for the search but I'm afraid that the text you found only
> > states
> > that:
> >
> > " The other members of the group, meionite, mizzonite and marialite,
> > are
> > much rarer in occurrence. Their crystals are usually smaller and of
> > better
> > quality than those of wernerite. Meionite and missonite (misspelled
> > mizzonite?) are found in limestone blocks on Monte Somma."
> >
> >
> >
> > That's throwing in all the scapolite group ;-))) Mizzonite and
> > wernerite are
> > not recognized by the IMA.
> >
> > Mizzonite is still sometimes used to indicate a scapolite with
> > composition
> > closer to that of marialite. The article describes wernerite as
> > somewhere in
> > the mid-section between marialite and meionite but somewhat closer to
> > the
> > meionite-end.
> >
> > My question came up after reading:
> > http://www.minsocam.org/ammin/AM51/AM51_1014.pdf
> >
> > It's on page 9, last paragraph.
> >
> > It's much more complex than I like my reading but it got me thinking.
> > conflicting symmetries that disturb electron densities could lead to
> > disturbed growth, could it not?
> >
> >
> >
> > Cheers
> >
> > Axel
> >
> >
> >
> >
> >
> >> -----Oorspronkelijk bericht-----
> >
> >> Van: rockhounds-bounces at lists.drizzle.com
> > [mailto:rockhounds-bounces at lists.drizzle.com]
> >
> >> Namens Kreigh Tomaszewski
> >
> >> Verzonden: maandag 22 september 2008 2:59
> >
> >> Aan: Rockhounds at drizzle.com: A mailing list for rock and gem
> >> collectors
> >
> >> Onderwerp: Re: [Rockhounds] Scapolite
> >
> >>
> >
> >> Meionite is less likely to be found as large crystals. See
> >
> >> http://www.miningbasics.com/html/scapolite_group_-_wernerite_co.php
> >
> >> from my "I'm Feeling Lucky" Google search.
> >
> >>
> >
> >> Kreigh
> >
> >>
> >
> >>
> >
> >>
> >
> >>
> >
> >>
> >
> >> On Sunday, Sep 21, 2008, at 18:11 America/Detroit, Axel Emmermann
> >> wrote:
> >
> >>
> >
> >>> Hi list,
> >
> >>>
> >
> >>>
> >
> >>>
> >
> >>> I'm doing some pondering on scapolite.
> >
> >>>
> >
> >>> Does anybody know whether the Ca and CO3-rich end-member, meionite,
> >>> is
> >
> >>> less
> >
> >>> likely to be found well formed, large crystals than the Na and Cl
> >
> >>> end-member
> >
> >>> marialite?
> >
> >>>
> >
> >>> It should have something to do with crystal-chemical hindering by the
> >
> >>> carbonate group sticking through one of the boundary planes of the
> >>> unit
> >
> >>> cell.
> >
> >>>
> >
> >>>
> >
> >>>
> >
> >>> Anybody who knows?
> >
> >>>
> >
> >>>
> >
> >>>
> >
> >>> Cheers
> >
> >>>
> >
> >>>
> >
> >>>
> >
> >>> Axel
> >
> >>>
> >
> >>>
> >
> >>>
> >
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> >>
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