[Rockhounds] Scapolite

Fri Sep 26 03:28:30 PDT 2008

Hi Kreigh,

It does and doesn't ;-)))
At First sight, I mean. I'm going to read it carefully and take my time for
it.... 

Thanks, this looks promising (If I can understand what it says ;-)))

A xel

> -----Oorspronkelijk bericht-----
> Van: rockhounds-bounces at lists.drizzle.com
[mailto:rockhounds-bounces at lists.drizzle.com]
> Namens Kreigh Tomaszewski
> Verzonden: vrijdag 26 september 2008 2:54
> Aan: Rockhounds at drizzle.com: A mailing list for rock and gem collectors
> Onderwerp: Re: [Rockhounds] Scapolite
> 
> Axel,
> 
> I think the paper below may shed some light on the crystal structure of
> the Scapolite series and move you closer to an answer to your
> questions. Recommended reading material if you are losing sleep over
> Scapolite crystal structures.
> 
> Kreigh
> 
> 
> ====
> 
> http://www.minsocam.org/ammin/AM70/AM70_134.pdf
> 
> 
> Scapolite: alkali atom configurations, antiphase domains, and
> compositional Yariations
> 
> C. Pece CHerasenLeIN, Jnuet  A.  Docre,  Jeprnr,v E.  Posr, nNo Cnenres
>   W.  BunNneu
> Department of  Geological Sciences
> Harvard  University, Cambridge, Massachusetts 02138
> 
> 
> Abstract
> 
> Electrostatic energy calculations on an intermediate scapolite indicate
> that Na+ should prefer to  be adjacent to  Cl-  anions and Ca2* to  be
> adjacent to  CO3- radicals. The calculations also indicate that
> short-range ordering of  NarCl  and CaaCO3 clusters is energetically
> favored and might give rise to antiphase domains. Electrical neutrality
> considerations on the anion/radical site in scapolite suggest that the
> unusual compositional variations observed in the marialite-meionite
> series are the result of crystallographic constraints on  the  two
> possible independent exchange reactions in scapolite: NaCICaCO-3 and
> NaSie6-Ai. The compositions of naturally occurring scapolites may be
> explained in terms of the scapolite composition that represents local
> charge balance between Ca2*, Na* cations and Cl-,  COI- anions.
> 
> 
> 
> On Thursday, Sep 25, 2008, at 14:23 America/Detroit, Axel Emmermann
> wrote:
> 
> > 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
> >>>
> >>>>>
> >>>
> >>>>>
> >>>
> >>>>>
> >>>
> >>>>> --- StripMime Report -- processed MIME parts ---
> >>>
> >>>>> multipart/alternative
> >>>
> >>>>>   text/plain (text body -- kept)
> >>>
> >>>>>   text/html
> >>>
> >>>>> ---
> >>>
> >>>>> --
> >>>
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> >>>>>
> >>>
> >>>>
> >>>
> >>>>
> >>>
> >>>> --
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