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General~Flat~ Garnets in Mica (Muscovite)
15th Jul 2009 01:36 UTCP. Michael Hutchins
I guess the place I find them most is in the pegmatites of the Spruce Pine (N.C., USA) Pegmatite District.
One of the coolest things about them is that they're ... something like ... flattened.
That is, they have what looks like faces that are parallel, or sub-parallel, to the mica's cleavage planes..
..and their ~thickness~ (perpendicular to that) is relatively small.
But I've never figured out what makes them that way.
I've mostly let myself think that they're squished, by the metamorphism that makes the mica..
..but I'm not liking that any more. For one thing, the mica is intrusive plutonic, not metamorphic.
I guess we could call it competition for space, but...
I reckon for one thing, I need to look up exactly how the muscovite and garnets form in this environment.
They must be crystallizing from the magma/melt.
I would think that the garnets xlize before the mica, but I am guessing - and their shape is clearly affected by the mica, so that doesn't seem to square.
BTW, tourmaline in muscovite is - often(?) - similarly flattened - and I've never seen any w/ its c axis not in the mica cleavage plane.
So er why?
15th Jul 2009 08:06 UTCRay Hill Expert
In lieu of some kind of logical explanation, I just accepted that the mica's crystallization environment was somehow stronger than that of the garnet in the ongoing process of their crystallization and somehow forced it to conform to the limited space between sheets...otherwise it remained a mystery to me, as it apparently does to you too, as to what forces would be strong enough to actually mess with the normal crystallization rules that garnets everywhere else, are constrained by....
15th Jul 2009 11:20 UTCChris Stefano Expert
15th Jul 2009 12:14 UTCP. Michael Hutchins
The garnets are not interrupted in any way by the mica. Visually, they appear to be set into the mica, and leave clean little holes in the mica where they are absent
Nor do the garnets cleave the mica. The mica does, though, show stress lines, curves, fractures around the garnets (viewed perpendicular to the sheets), just as it does where it is abutting another mineral, such as quartz of feldspar.
Re the "limited space between sheets": although mica comes apart so easily into sheets, it certainly doesn't form that way - although it's sure easy to imagine that it does. Moreover, the actual space between "sheets" (2-D silica &c lattices) is on the order of atom widths.
I wonder whether there's some sort of ion migration involving the garnets that's differentially affected by the presence of surrounding mica - even that garnets and mica morph into one another. The former definitely happens during metamorphism, and both garnet and mica develop from - and into - other minerals.
15th Jul 2009 12:37 UTCJolyon Ralph Founder
Jolyon
15th Jul 2009 12:59 UTCsteven garza
good evidence; incorrectly interpreted. The "interrupted" doesn't mean "intergrown", which is what you are getting; "interupted", in this case, means you being pushed northward & a brick wall interrupts your ability to go that way. &, just like you being interrupted by that way, you are more likely to give & conform to its shape that the brick wall is.
Try to imagine a melt where 2 minerals are xlizing at about the same time. You noted that the mica is softer & easily peels into sheets; BUT, what you didn't notice is how STRONG it is, in the direction the sheet grew! this would apply a LOT of pressure to the growing garnet, in that direction, putting up a tremendous amount of preesure & resistance, in that direction. your own evidence of "The mica does, though, show stress lines, curves, fractures around the garnets (viewed perpendicular to the sheets), just as it does where it is abutting another mineral, such as quartz of feldspar." actually proves that point; I guess more interesting is that it's for the same reasons - limited room & the other minerals forming 1st! Stepping back a minute, I think the garnet was xlizing 1st, JUST ahead of the mica. Under normal conditions, garnet has a lower melting point, but, under pressure & w/water associated in a melt, it's mice that has a lower melting point; water, in that environment, is a flux, as well as active ingredient in the mica formation. The material to form the garnet had already been collected at that point, but, hadn't been fully xlized (more like a gel); so, when the mice began to form around it, it was smooshed (a HIGHLY technical term; feel free to use it! LOL) by the "walls of mica sheets" & allowed to grow between them.
In metamorphic rocks, conditions can be different, depending on how much water is available, how much pressure & heat are applied & how rapidly it builds, so, garnets in that invironment often are intergrown. An excellent place to collect such examples is Green's garnet mine, Roxbury, CT. Some have no intergrowth & some will, according to where you dig from the ledge; the composition of the rock gives best evidence as to which bed was "wetter" & the talc content of some of the rock tells how rapidly the heat & pressure built up.
Hope this helps.
Your friend, Steve
15th Jul 2009 13:07 UTCCharles Creekmur Expert
15th Jul 2009 13:14 UTCDon Swenson
15th Jul 2009 13:21 UTCAnonymous User
Lee
15th Jul 2009 14:37 UTCJared Freiburg
Jared
15th Jul 2009 14:56 UTCJohan Kjellman Expert
cheers
15th Jul 2009 15:32 UTCJasun D. McAvoy Expert
15th Jul 2009 15:36 UTCMark Heintzelman 🌟 Expert
Here's a quick pic of one, top view and side view, which I hope better illustrates what we're talking about.
Yes, these are very flat, like a little tablet. In the side view remember almost all of this xtl is sticking out of the mica.
MRH
15th Jul 2009 15:36 UTCJasun D. McAvoy Expert
15th Jul 2009 16:38 UTCsteven garza
I don't know if anyone noticed, but, most of the garnets that are trapped in the mica, tend to be VERY gemmy! That's why I love finding them there! My most beautiful garnet xls come from them!
Your friend, Steve
15th Jul 2009 16:43 UTCsteven garza
I wonder if xlizing in the mica, which is an excellent insulator (used as windows on old wood & coal burning stoves), allows the garnet solution to xlie more perfectly, allowing it to be gemmier? Inclusions would have more time to drop out & structural deformities would have time to correct themselves.
Your friend, Steve
15th Jul 2009 17:17 UTCEverett Harrington Expert
One of the best places in Spruce Pine district to find these lil beauties (garnets in mica) is the Abernathy mine seen here Abernathy mine
Now the fun stuff from the Spruce Pine District is the schorl in mica.(Ray Mica mine) See attached photos!!
IMO with the Schorl in mica, they also have crystalized pretty much at the same time. They are bent to the forms of the mica.
KOR
E
15th Jul 2009 17:25 UTCAnonymous User
Jasun, how do you know that your spec is almandine? (analysis?)
Philippe.
15th Jul 2009 19:37 UTCJyrki Autio Expert
tourmaline in muscovite
Jyrki Autio
15th Jul 2009 20:24 UTCP. Michael Hutchins
Maybe later... :-(
Jolyon, any way to avoid this?
(w/o, obviously, my having to remember, in the heat of composition, that the space that I'm typing into is silently volatile)
16th Jul 2009 00:40 UTCsteven garza
I used to have the same problem, but, my solution was simple; just open a personal letter page & copy & paste what you've written onto it. I suggest reducing to a bottom tab, before launching a search engine or such; then, there's no chance of accidently closing your letter. Some people can open a new window, to the same address, to use the search or visit another article, then, return without losing a thing. I believe you can do that on Foxfire. After I've posted, I kill the letter window & it's done!
Hope this helps.
Your friend, Steve
16th Jul 2009 08:14 UTCRay Hill Expert
really thin and very gemmy plates between the sheets , with pressure lines visible beautifully under crossed nicols, but still between and not
penetrating more than the thinnest of single layers..they were absolutley phenomenal under microscopic examination.
16th Jul 2009 14:42 UTCP. Michael Hutchins
As it happens - sadly - I don't have any equipment that can take pix of such small things. I desperately wish I did, but just haven't been able to decide on what and spring for the $$.
OTOH, we've had a pretty good discussion here, we've gotten to see good pix of many peoples' specimens..
..and I've been doing some Web research on the subject.
As I wrote ydy..
(which ended up in the black hole of insufficiently state maintaining web pages)
..my intent is to summarize what I'm finding, after I've done as well as I think I can at finding info on this.
(So far, I haven't found anything particularly conclusive, just a lot of educated hypothesizing. One hyp. cites differential ion mobility across the mica xl lattice.)
(& steven, yes, that is a good way to avoid such pain & suffering - and I used it for this; just wish I didn't have to (& know it needn't be the case, being a s/w engineer) )
Finally, I did try to reply ydy to Mr. Freiburg, who wondered about the relationship of my localities to the large "Alaskite" body found in Spruce Pine, & eclogite...: I (or...) will have to research that; although I'm from there (Asheville), I haven't llived there for 43 years, and get back only once a year or so. I'm not familiar w/ eclogite, but will definitely check out the Bakersville area - and know of a large-granet mine somewhere in the area (from the MAGMA club, qv).
16th Jul 2009 17:04 UTCP. Michael Hutchins
(Fig.s for Mathews are attached.)
ref. whose text wasn't located: Flattened garnets in mica at Spruce Pine, North Carolina: Jour. Tenn. Acad,emy Sei.,8,
no. 3, 268-272, 2 figures (1933)
From Rocks, Gems and Minerals
By Herbert Spencer Zim, Paul R. Shaffer, Raymond Perlman, Jonathan P. Latimer
:
"Crystals are common and may include flattened garnets, quartz, or tourmaline."
(In my recent examples, it looks to me like the quartz is not flattened.)
from http://www.archive.org/stream/circularj121johnuoft/circularj121johnuoft_djvu.txt:
Notes on some Flattened Garnets from North Caro-
lina. By Edwakd B. Mathews.
Through the kindness of Mr. Geo. F. Kunz the writer has had an oppor-
tunity to study some curiously flattened garnets which had been collected
by Mr. Kunz during one of his trips to the mineral localities of North
Carolina. The locality whence these garnets come and tlie exact geological
occurrence were not given, but we know that tliey are found as an accessory
constituent of tlie pegmatite dykes and that they are intimately associated
with the mica between lamellae of wbicli they were developed.
This inauspicious habitat has caused a marked distortion of the crystals
during their growth, and we have as a result small plates of garnet substance
in which the diameter of the larger face's is from 10 to 50 times that normal
to those faces.
From a consideration of its color, its place of occurrence, and especially
its specific gravity which ranges from 4.199 to 4.258 it is probable that this
is a member of the spessartine group.
When the crystals were studied goniometrically ' it became evident that
the forms present are those ordinarily developed; the rhombic dodeca-
hedron ooo and the icositetrahedron 202. Some of the faces belonging to
these forms are usually absent but the characteristic angles are so frequently
noted that there is little doubt as to the correctness of determination. It is
found that the larger, flatter faces are of two sorts : tiie perfect ones //coO
and the vicinal, curved ones produced by the oscillatory combination of
such faces and the adjoining faces of the icositetrahedron. This oscillatory
combination of two forms does not give an irregular serrated surface, as is
seen frequently in quartz and other minerals, but, instead, we seem to have
a curved face of small convexity which has a perfectly smooth surface,
probably produced through the influence of the smootli mica lamellae as
they were pushed apart.
Fig. 1.
Fig. 2.
Fig. 1 shows the two forms ooO and 202 in the normal development, while Fig. 2 shows
the modification.s a.s found in the garnets. On the edge of the crystal we find the charai-
teri-slic grouping of the icositctrahedral faces about the diamond shaped dodecahcdral
face, the only difference arising from their elongation in a single direction (a feature
developed in the faces of both forms whenever their zonal axes lie parallel to the broader
faces.
Optically, also, these crystals show interesting phenomena as might be
expected from their unusual habit. The crystals are of a rich reddish
brown to carmine color in incident light, and of the same in lighter tones
when viewed in transmitted light. The centres are usually darker and,
with the slight variations of color along the lines of growth, give the
appearance macroscopically of a section of a minute tree. Microscopically
this black centre is seen to be composed, for the most part, of small pores
and a few liquid or gaseous inclusions which frequently show some arrange-
ment parallel to the crystal faces or the plane of the diagonals connecting
their edges. A few of these inclusions influence polarized light and may
be individualized tliough indeterminate minerals.
In size the clearest and thinnest individuals range from J-} cm. by 1-2
mm. The darker centres range in width from one-eighth to seven-eighths
of the diameter, the darkest portions always remaining towards the interior,
the lighter forming an outer rim.
In order to study the .anisotropism of these garnets' they were immersed
in a dilute solution of methylene iodide to counterbalance the high index
of refraction. The cross-polarized light showed that the outer zone is
doubly refracting, though but feebly.
With the aid of the gypsum plate
the eflect of the double refraction
was increased and the following
features were noted : —
1. The outer rim is made up of
a series of zones of growth varying
in size and frequency.
2. These zones or lamellae of
growth are parallel to the exterior
faces. In this particular instance,
from the modification due to flat-
tening, the area of growth is a fibre
or prism whose long axis is paral-
lel to the erystallographic edge of
the rhombic dodecahedron, while
the shorter axes are parallel to
the octahedral edge and erystallo-
graphic c.
3. The whole crystal breaks up into four sections whose dividing lines
pass inward towards the corners of the interior isotropic core (see Fig. 3),
)'. e., parallel to the octahedral edge.s.
4. The directions of ehisticity in the lamellae are parallel and normal to
the longer directions, as is seen by the extinction parallel to the faces of
growth.
5. These lamellae are negative, their longer directions being the direc-
tion of less elasticity as shown by the mica plate.
6. The mineral lamellae seem to be biaxial, if we may judge from the
wavy character of the brush shown by the broader zones in converged light.
The above facts may be explained by various sorts of optical orientation
and, though no direct proof can be given, the writer is inclined to consider
the orientation to be : German b parallel to the dodecahedral edge, German
a to the octahedral edge, and Germ.an c normal to these two and approxi-
mately normal to the plane of flattening.
All the features observeil seem to show that the optical anomalies are
the result of irregularities in growth, as held by Klein and his followers;
and are not due to irregularities in pressure arising from the external
conditions. The circumstances of the growth produced an accentuation of
the zonal growth and so indirectly increased the anisotropism, but from the
facts at hand we are not justified in asserting that there is any direct in-
crease in the double refraction on account of the flattening of the crystals.
1 The writer is indebted to Mr. A, C. Ppencer for his careful study of Iho forms prcscnl
on the crystals.
from Mountain & Kent (http://www.minersoc.org/pages/Archive-MM/Volume_25/25-162-125.pdf):
The garnet occurs in two fairly distinct ways which may be conveniently
referred to as rounded and flattened respectively. The rounded
type occurs only at the margin of the mica books. That portion or those
specimens which occur just within the mica are invariably idiomorphic
crystals up to 2 cm. diameter consisting of the icositetrahedron modified
by the rhombic-dodecahedron; while those just outside the mica and
thus within the pegmatite are generally in the form of irregular masses,
sometimes weighing several pounds and completely lacking in crystal
facets. These rounded forms are usually of a deep reddish-brown colour
and practically opaque in the hand-specimen except for the smaller
idiomorphic specimens. They are, moreover, largely altered to a limonitic
product. Tested chemically they are found to contain a high percentage
of manganese.
The flattened garnets, on the other hand, occur exclusively within the
mica books, the plane of flattening being invariably parallel to the
cleavage-plane of the mica. They appear to be limited in their distribution
to certain layers within the mica and are more prevalent towards
the outer fringes of the books. In addition, they arc more frequently
encountered in the larger books. In thickness they vary from 0-I to
15 mm., being on the average 1-2 ram., while in diameter they range
from that of a pin's head up to 2 cm. The flattest garnet encountered
was 12 Inm. in diameter and only 0.25 ram. in thickness. According to
Mr. F. Butch, a former manager of the mine, they are thicker on the
average in the ' edge-waste' of the books than within the larger leaves.
They vary in colour from a pale brownish-pink to a deep ruby-red.
These flattened garnets occur sometimes as isolated crystals and sometimes
in strings, in which case they tend to be nearer to the outer margin
of the books. Such strings of garnet (figs. 1 and 3) consist usually of
tiny crystals up to 2 mm. across and 0"3 ram. in thickness. They are
found in a feathery type of muscovite in which the large plates are
divided up into areas, each characterized by a sort of fluting or foliation
parallel to the rays of the pressure-figure of the mica respectively. Now
each of these foliated areas contains only strings of garnet crystals
perpendicular to this foliation, and thus the strings are arranged parallel
to the traces of the unit prism and clinopinakoid, or, in other words,
parallel to the rays of the percussion-figure (fig. 4). The strings are
spaced irregularly, and parallel to them are also other lines of disturbance
without the presence of garnets. Within the strings the tiny
crystals are themselves spaced at irregular intervals, but generally the
larger the crystals the greater the space between them. Occasionally
such strings penetrate a considerable thickness of the mica boo>~, the
garnets then lying in a plane roughly perpendicular to the mica cleavage.
Thirty crystals, both of the isolated type and of those in the strings,
were detached and examined carefully on the *wo-circle goniometer,
being mounted on the large face ; but before removal the trace of the
optic axial plane of the muscovite was ruled on one of the large faces
of each crystal. They sometimes possess a fairly regular six-sided outline
(fig. 2) and may be distinctly elongated parallel to a pair of these
edges, but generally possess a somewhat rounded or polygonal outline.
They also occur in parallel growths and in groups of two or three irregularly
associated. The large face rarely gives a good image owing to
appreciable convexity, and it frequently carries a pattern of growth-pits
which, however, proved useless for orientation purposes. The edges of
the tablet are usually moclified by half a dozen or so reoogmzable faces,
but the rest of the margin is usually indistinct and fluted. In spite of
this, however, a number of reflections were obtained on the goniometer
from ill-defilmd faces. These showed that the crystals are nearly always
tabular near a face of the rhombic-dodecahedron and th~at other reflec-
tions all belong to either the rhombic-dodecahedron or the icositetrahedron
(211). Faces of the former usually give the better reflections,
while the latter tend to be striated parallel to their intersection with
the former.
Where a dodecahedral face is located at anything over p ~ 5 ° or so,
it appears distinctly as a sort of vieinal face modifying the large face
and recognizable in the hand-specimen. In this case the large face gives
a number of images, both isolated and in strings, and of varying degrees
of definition within a radius of 5 °, corresponding to various vicinal faces
and zones of faces. In those cases where the pole of the plane of flattening
departs appreciably from that of a dodecahedral face it is found to
lie roughly on one of the principal zones passing through a dodecahedral
face, but in only onc case did it lie between the dodecahedron and cube.
The poles were plotted on a gnomonic projection on a (110) plane as
shown in the diagram (fig. 5), only those falling outside the 3 ~ radius
being indicated; 18 poles out of the 30 thus fell within 3 ~ of a dodecahedral
face. It will be noticed that flattening does not occur anywhere
near a cubic face.
With regard to the orientation of the garnet individuals relative to
the trace of the optic axial plane of the muscovite, very little can be
stated. There is, however, a strong tendency for the optic axial plane
to lie parallel to either the longer or the shorter diagonals of the dodecahedral
face on which flattening occurs, while other cases occur where
it lies definitely parallel or perpendicular to a side of the face. The
results of the goniometric measurements show conclusively that there
is no common orientation.
A few of the thicker crystals possess marked parting-planes parallel
to faces of the rhombic-dodecahcdron, especially developed parallel to
the large face of flattening. The specific gravity determined in Clerici
solution was found to vary slightly, being about 4"10. The pale brown
crystals have a slightly lower figure than the deeper-coloured ones.
Under the microscope, the crystals are mostly quite isotrop~c and their
refractive index is about 1"805-1.810, but no variation was observed.
A few of the crystals show well-defined colour zoning, the inner zones
being polygonal but not necessarily of the same shape as the crystal
outline.
A clear reddish tabular crystal weighing
0"3 gram was crushed and a rough analysis
was made of the iron and manganese.
The iron was determined by titration
with titanous chloride and the manganese
colorimetrically after oxidation with
sodium bismuthate, giving total iron (as
FeO) 16 ~o and MnO 24 ~ which correspond
to Alm4oSps o. A number of other
crystals were tested with the sodium carbonate
bead and found to be rich in
manganese, so that all the garnets are
assumed to consist predominantly of the spessartine molecule.
The garnet crystals are in general not situated on the surface of the
muscovite books but are very definitely enclosed deep within them.
They have, however, influenced the adjacent cleavage-layers and leave
a slight hollow where the immediate layers have been in contact with
the crystals, an effect which is noticed through a different thickness of
mica in different cases. The mica layers in a continuous plane with the
crystal tablet stop abruptly against the edge of the tablet usually without
modification except for the existence of a system of radial cracks in the
mica presumably due to pressure, like expansion cracks commonly
found round olivine crystals altered to serpentine. The pressure exerted
by the garnet on the surrounding muscovite is not regarded necessarily
as an effect of the original cystallization of the garnet, but may possibly
be due to subsequent alteration of the garnet crystals caused through
oxidation of ferrous iron. The garnet crystals themselves, especially the
larger ones, are commonly riddled with cracks as though they had been
under strain.
In' Ore deposits of the Western States' (p. 144 et seq.), 1 W. T. Schaller
1 Amer. Inst. Mining and Metall. Engineers, Lindgren volume, 1933.
states that irrespective of whether the original pyrogenic rock of the
pegmatites consisted essentially of potash-felspar or of graphic granite
or both, the outstanding fact of pegmatitic development is that the
aggregates of albite, micas, garnets, and many other minerals are formed
by later hydrothermal replacement processes acting on and replacing
the first formed potash-felspar rock. As stated previously, it would
appear that the parent rock in the present case was essentially albitequartz-
pegmatite, though some of the albite is almost certainly secondary.
Of the other minerals, Schaller gives the following order: albite,
muscovite, and the general group including tourmaline, garnet, and
beryl. Where the garnet occurs in the albite or micropegmatite, the
appearance of the garnet and its mode of occurrence suggest that it was
formed at the expense of the felspar, but where the garnet ocours in
muscovite, the case for replacement and a later age for the garnet is not
so' clear. It is di~cult to imagine, perhaps, some of the larger garnets
up to 2 cm. across developing in a book of muscovite by replacement,
but such a process may nevertheless be possible.
The principal features of the garnet are the habit of the flattened
crystals and the arrangement in strings. In Dana's 'System of Mineralogy'
(6th ed., 1892, p. 619) appears the following statement: 'Muscovite
often encloses flattened crystals of garnet, tourmaline, also quartz in thin
plates between the sheets; further, not infrequently magnetite in
dendrite-like forms following in part the directions of the percussion
figure, also those of the pressure figure.' It might be added that white
chalcedony occurs in films up to 8 ram. thick, interleaved with the mica
now described, and is associated with films and even small crystals of
quartz. The thicker films ccnsist sometimes of alternating bands of dull
white chalcedony and clear quartz and resemble extremely flattened
agates in structure. These, however, appear to be almost entirely confined
to the upper 40 feet of the workings and are therefore limited to
the zone of oxidation. They show that on weathering the muscovite
has become invaded by siliceous solutions.
Even under stress conditions garnet crystals usually h~ve an equidimensional
development, thrusting aside other minerals by their power.
of crystallization in such roc~s as garnetiferous mica-schist, and conse-
Quently there seems to be no doubt that the tabular habit is due not
to mechanical forces, but to some influence of the muscovite on the
molecular aggregation of the garnet crystals. Dr. R. Brauns in his book
'The Mineral Kingdom'1 writes: 'Enclosures of other minerals in musco-
1 English translation by L. J. Spencer, 1912, p. 331.
VOL. 25-162-4
rite are frequent and are of special interest. Growing along with the
mica between its planes of lamination these foreign minerals have been
constrained to take a thin, flattened form. Thus crystals of garnet,
which are so characteristically rounded or grain-like in form, when found
embedded in sheets of mica have the form of thin plates with an almost
circular outline . . . . That mica is capable of exerting some influence
over the crystallization of other substances is shown by the following
simple experiment. An aqueous solution of potassium iodide when
allowed to crystallize on a glass plate produces a crop of thick cubes.
If, on the other hand, the solution is crystallized on a clean, fresh cleavage
sheet of mica, the crystals of potassium iodide take the form of
flattened octahedra; and not only this, but the tiny crystals are all
regularly oriented in the same direction on the sheet of mica, one of their
triangular edges being parallel to the optic axial plane of the mica.'
From these considerations it seems certain that in our case the muscovite
is not merely moulded on earlier formed garnet crystals, but that already
crystallized muscovite has influenced the habit of the garnets. This, of
course, will explain why the flattened crystals are found within the
mica books, while the rounded type is found only at the margin where
the crystal structure of the muscovite has had no influence on their
habit.
In two recent papers in the 'American Mineralogist '1 there are
described oriented inclusions of tourmaline, magnetite, and haematite
in muscovite ; and the authors incidentally referred to flattened garnet
crystals, though without any details. In the case of magnetite the
flattening is perpendicular to a threefold axis, and the authors showed
that a close similarity exists between the spacing of the oxygen atoms
parallel to the basal plane of muscovite and those of an octahedral
plane of magnetite. In the case of garnet the crystals are flattened
parallel to a rhombic-dodecahedral face and not an octahedral face.
From data kindly supplied by the Royal Institution, the arrangement
of the oxygen atoms in this plane has been plotted, but no appreciable
amount of coincidence with those of the basal plane of muscovite could
be recognized. The silicon atoms were also plotted, in both cases with
similar negative results. In the cases mentioned here, where the trace
of the optic axial plane of the muscovite is parallel to one of the diagonals
of the dodecahedral face of flattening of the garnet, the plane of' symmetry
of the muscovite corresponds to a plane of symmetry of the
x C. lerondel, 1936, vol. 21, p. 777; C. Frondel and G. E. Ashby, 1937, vol. 22,
p. 104.
garnet, but apart from these two directions at right angles there are
no other coincidences among planes of low indices.
The existence of strings of garnets in lines parallel to what would
presumably during growth be the equatorial faces of a crystal of muscovite
suggests further that they have developed along growth-planes, and
thus simultaneously with the muscovite. On the other hand, growthplanes
are likely to be, and definitely are, planes of weakness, and it is
possible that hydrothermal solutions could have followed such lines and
introduced the material of the garnets subsequently.
Evidence has already been given to show that the development of
garnet within the muscovite led ~o strains being developed in the mica,
but the layers of muscovite are far from being entirely bent round the
inclusions 'augen'-fashion. In fact, the muscovite appears to be only
slightly disturbed in contact with the edges of the garnet tablets. Small
rhomb-shaped highly birefringent strips of muscovite sometimes occur
within the books near the garnet crystals and at first it was thougl~t that
they were fractured pieces which had been introduced subsequently, but
in one or two cases these fragments were embedded in a ferruginous
matrix.
In two cases of very flat crystals there is a narrow green zone along
part of the margin of what appeared to be the garnet crystal, while the
surrounding muscovite was also a little deeper green tl~n the rest. This
green marginal zone in the garnet was found to be soft and when the
powder was examined under the microscope it was seen to consist of
massive mica with an optic axial angle somewhat Smaller than the
ordinary muscovite, but simi>ar in other respects and Showing aggregate
polsrization. The garnet portion consists of a parallel growth of crystals
pa~ing into isolated crystals in the green marginal matrix, themselves
arranged in parallel positions. This kind of crystalline habit does not
~eem to support any suggestion that garnet crystals have been produced
by replacement within the large muscovite plates. Moreover, the occurfence
of euhedral zoned garnet crystals within the muscovite is surely
not reconcilable with a replacement origin so far as replacing muscovite
is concerned.
In conclusion, then, garnet inclusions flattened more or less parallel to
the rhombic-dodecahedron occur in muscovite books parallel to the mica
cleavage without any other well-defined orientation of the crystals
themselves, but sometimes occurring in rows parallel to growth-planes
of the muscovite. The evidence indicates that the mica cannot have
developed round these garnets, and that probably the crystallization
of garnet and muscovite was more or less simultaneous, the partially
crystallized mica influencing the habit of the garnet inclusion by molecular
forces.
The authors are indebted, to Messrs. F. Butch and T. H. Stamp of
Mica for information and specimens.
from Gresens (http://www.minsocam.org/ammin/AM51/AM51_524.pdf):
THE AMERICAN MINERALOGIST, VOL. 51, MARCH-APRIL' 1966
DIMENSIONAL AND COMPOSITIONAL CONTROL OF GARNET
GROWTH BY MINERALOGICAL ENVIRONMENT
ReNn'lrr- L. Gnrscwsr, Geology Department, Flori,da State
fl niaers ity, T allahass e e,F lorida.
OccunnpNcB AND CRYSTAL MoRPHoLoGY oF GARNET
During the course of a geochemical study of the Kiawa pegmatite
group, Las Tablas Quadrangle, Rio Arriba County, New Mexico
(Gresens1, 964),a ttention was calledt o the dual nature of the occurrence
of spessartiteg arnet in the pegmatites.L arge irregular anhedrals pessartite
masses (10 cm or more in diameter) are present in albite concentrations.
These masses are weak and commonly crumble when handled.
Smaller (1 cm or less) clear euhedral tablets are found within large
muscovite books. These crystals are "flattened" in the c-axis direction
of the muscovite crystals and are hard with no tendency to crumble'
Jahns (19,16) noted these relationships in an earlier study. The "flattened"
garnets show dodecahedral faces. These faces are commonly
distorted, that is, the polygonal outline of the face does not have the
perfect bilateral symmetry of the polygonal face of a perfect dodecahedron.
The large "flattened" face is not necessarilya dodecahedrafl ace'
(The angle between the "flattened" face and an adjacent face is usually
not the dodecahedraal ngle') rt seemsi nstead to be merell' the termination
of the garnet against the muscovite layer structure, truncating the
dodecahedrafl aces.
l Present address: Geology Department, Universityof Washington, Seattle, Washing
The occurrence of the "flattened" crystals in particurar argues against
growth of the surrounding mineral around pre-existing garnet. rf the
garnet had been formed prior to the muscovite, the enclosed garnet.
should be more equidimensional. Also, the "flattened" garnets are found
only in muscovite books. The control exerted by the muscovite crystal
lattice on the dimensional growth of the garnet impries that the two
minerals either grew simultaneously or that the "flattened" garnets grew
within pre-existing muscovite crystals.
The ager elationshipb etweena lbite and the rargeg arnet massesis not
as clear. As pointed out by Jahns (1946), the albite is a late replacement
feature in the pegmatite. The garnet courd have been formed either
before or after the albite. rrowever, even if the albite is later than the
garnet' the garnet would probably have been originally surrounded by
perthite or quartz rather than muscovite before the replacement of these
neously with or later than the albite. For example, Jahns (1946), con_
sidered that the association of most of the mica with albite indicated
The exact age relationship is not necessary for the discussion that
follows. What is important is that the medium (albite, perthite, quartz,
or fluid) surrounding the large garnet masses during growth was different
from the medium (muscovite) surrounding the "flattened" garnets during
their growth.
Devore (1959)d iscussedm inimum interfacial free energy as a control
on certain features of mineral assemblages,in cluding grain form. The
"flattened" garnets may be an example of this kind of control. The
"flattened" form presents a large garnet surface area to the (001) face
of the surrounding muscovite and a minimum interfacial garnet surface
to other planes in the muscovite crystal. This suggests that the interface
between garnet and the (001) plane in muscovite is the interface of lowest
free energy.
rn addition to the dimensional control exerted by the mineralogical
environment, a compositional control may also have been present. Suppose
that the garnets represent growth simultaneous with that of the
enclosing minerals. Because growth takes place by additions to the surface,
the surface energy difference between the feldspar (?)-garnet inter
face and the muscovite-garnet interfac! could lead to compositional differencesb
etweent he garnetso f the two environments.A dditions to the
mineral are controlled in part by the state of the mineral surface. When
two mineral surfaces are in contact, there are mutual interactions between
their force fields. Mutual polarization of the surface atoms can
occur. The surface of a garnet in contact with muscovite is thus energetically
different from the surface of the same garnet in contact with
feldspar, qrrartz, or even a fluid. The differences, however small, could
Iead to discrimination with respect to certain atoms during crystal
growth.
If the "flattened" garnets grew by replacement within a pre-existing
muscovite crystal, the above argument still holds, but an additional factor
may be considered. At least some of the elements composing the
garnet must then have reached the site of growth by intracrystalline
diffusion through the muscovite. The muscovite could have a "sieve"
effect with respect to ionic diffusion, allowing some elements to difiuse
more freely than others. This could also result in compositional differences.
S imi lar ly,i f the largeg arnetsa re later than the feldspar( microcline
or albite) or quartz, growth must have taken place by difiusion
along feldsparfqtartz grain boundaries or through feldspar/quartz crystals.
A "sieve" effect could also be present in this situation.
Compositional differences could also result from incorporation of some
of the components of the host mineral in the guest mineral during replacement.
This would also be a compositional control because of mineralogical
environment.
The postulated reasons for the compositional differences between the
two garnets have been of a very general nature and somewhat speculative.
An attempt to be more specific would be even more speculative.
For example, the higher iron content of the "flattened" garnet could be
attributed to at least three mechanisms:
(1) Iron was available in about equal amounts at both sites, but the sutface conditions
controlling the crystal growth resulted in a higher iron content in the "flattened" garnets.
(2) The muscovite crystal lattice exerted a "sieve" effect, allowing iron to difiuse more
freely than manganese (Fe2+ has a smaller ionic radius than Mn2.+). (3) The higher iron
content of the "flattened" garnet is simply due to the higher iron content of the muscovite
(usually about 4.50/6 Fe) as opposed to that of feldspar (only a trace). The excess iron is
therefore iron incorporated by the garnet from the host muscovite during replacement.
Another example is the higher yttrium and beryllium content of the
"flattened" garnet. It might be argued that this means that yttrium and
beryllium could diffuse freely through the muscovite crystal lattice. This
is highly speculative to begin with, but is even further complicated by
the fact that there is little similarity between yttrium and beryllium
with respect to ionic size, charge, polarizing power, etc. Therefore, to
assigns pec'if,cd ifferencesi n composition to a single mechanism is difficult.
In the more generais ense,e ither of the three mechanismsd iscussed
above in relation to iron content are ultimately due to differences in the
mineralogical environments of the two garnets.
Another possibility is that the "flattened" garnets formed b1' exsolution
within the muscovite. However, as pointed out by DeVore (1964,
pers. comm.), if the garnet grew by exsolution, almost all of the muscovite
crystals should have a few. In the Kiawa deposits most mica books do
not contain garnet inclusions.
It is also possiblet hat the two occurrenceso f garnet were formed at
different times from different chemical environments. Although the two
analysed garnets were only 8 meters apart in the same pegmatite body,
this remains a very real possibility. It does not, however, invalidate the
possibility that the mineralogical environment also exerted a control on
the cr)rstal composition. The fact that one of the garnets is in a distinctly
special mineralogical environment (a muscovite book) and that the
environment exerted a definite dimensional control during crystal growth
lends support to the possibility that the mineralogical environment also
exerted a compositional control during crystal growth.
The author is grateful to Dr. G. W. DeVore for his critical appraisal
of the manuscript.
Rnrcr-oNcBs
DrVonr, G. W. (1959) Role of minimum interfacial free energy in determining the macroscopic
features of mineral assemblagesL. The model. J our. Geol.6 7, 211-227.
Gnesnxs, R. L. (1964) A geochemical and structural study of metasomatic formation of
certain pegmatites. Unpubl. Ph.D. thesis, Florida State University.
Jarrws, R. H. (1946) Mica deposits of the Petaca District, Rio Arriba County, New
Mexico. N. Mer. Bur Mines Mi.neral Res. Bull.25
16th Jul 2009 17:08 UTCEverett Harrington Expert
http://mcrocks.com/ftr08/StreeterFeb08.html
KOR
E
11th Apr 2014 02:09 UTCDaniel Etelman
We have been finding garnet muscovite schists and garnet graphite muscovite schists. We find schists with flat garnets, and schists with round garnets; never mixed.
First, garnets are metamorphic minerals and thus, do not crystallize out of igneous melts. Your pegmatitic Muscovites have undergone deformation which has resulted in the formation of garnet porphyroblasts.
So I have asked the same question: how have these garnets become flattened?
My original theory was the most obvious and rudimentary: they were flattened by the same forces that formed the Muscovite schists' foliation.
However, the temperatures and pressures required to simultaneously flatten a garnet while not breaking it would have caused the muscotive schists to alter into a different rock ENTIRELY.
Ok so...why are they flat?? Could growth space be an issue? We checked to see if we were finding the flat garnets near intrusive quartz dikes where garnet-forming minerals would be absent and the garnet crystals would grow flat along the boundary with the quartz vein. As suspected, we did not.
So, we are still looking for an answer. My theory is that it is caused by a combination of two factors: volatility and the retardation of fluidity perpendicular to the mica's foliation. I believe that the materials necessary for forming a garnet would flow more quickly parallel to foliation rather than perpendicular. Thus, the garnets are growing faster parallel to foliation, and slower perpendicular to foliation.
But then, why are there round garnets? Perhaps an increase in volatiles (water, CO2, etc), which allows for faster movement of ions, is speeding up the movement perpendicular to the foliation. Perhaps, due to limited surface area, an increase in fluidity can only speed growth up so much which may mean that growth parallel to foliation reaches a cap, thus, allowing perpendicular growth to "catch up".
So:
Flat garnets form in areas of lower volatility, and round garnets form in areas of higher volatility.
More research is needed by more qualified geologists than myself :/
-Daniel
11th Apr 2014 03:04 UTCDoug Daniels
Garnets are not solely a metamorphic species - they also form in igneous rocks (pegmatites, kimberlites and rhyolites come to mind.
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