Dear Dr. Nolze,

Silicification of riebeckite in deposits of the banded iron formation (BIF) leads to dark and bright brown mineral aggregates which by gemologist are called tiger’s-eye or an equivalent gemstones as hawk’s-eye owing to its bluish tint. Quartz containing oriented fibers of crocidolite or riebeckite displays a special luster. Tiger’s eye is displaying chatoyancy (a vertical luminescent band like that of a cat's eye). The most important mine for tiger ´s eye is in Griqualand West, South Africa (Dryer and Söhnge 1992).

Dryer, C.J., Robinson, H.A. 1981. Occurrence and exploitation of amphibole asbestos in South Africa. In: Riordon, P.H. (ed.) Geology of asbestos deposits, 25-44, AIMA, New York

Dryer, C.J. and Söhnge, A.P.G. 1992 .The crocidolite and amosite deposits of the Republic of South Africa and Bophuthatswana. Handbook 12, Geological Survey, Pretoria, 126 pp.

Below you find a rough stone which has not yet been been polished.

Glückauf !

Harald G. Dill

Dear Gert,

The quartz variety "tigers'eye" formed metamorphic-hydrothermal from "hawk Eye", whose Magnesioriebeckite ('crocidolite') fibers (blue gray) oxidize during or after silicification to iron hydroxides (golden yellow to brown color). The Magnesiorieebeckite fibers are not replaced by quartz, but impregnated with quartz and completely embedded in quartz!!

Best regards,

Guenter

Dear Guenter, many thanks for your answer. Is this our current believe, or are there specific proofs for this interpretation like experiments connected with texture measurements? If riebeckite is replaced by quartz (which is certainly the case but the question is how it happened), would this a) not either cause a total loss of microstructure (i.e. the former fiber boundaries), or b) automatically mean that there shouldn't be a fibre texture of quartz? What does the term "impregnation" covers? The SiO4 still remains and everything else disappears? If not, why SiO4  of riebeckite (or any other amphibole...I am not an expert and don't want argue about the correct phase involved) should be removed and then it is required again for the formation of quartz? Sorry for my very basic questions...

Dear Dr. Nolze,

I give my answer to your question  this way.

The primary effect I have mentioned already in my answer. It is the Cat´s eye effect. It is an oriented intergrowth of needles or fibers of the so-called "blue asbestos" group of minerals belonging to the amphibole group (in this case Na-enriched amphibole) with a host mineral, here quartz. . A different story is told by the "white asbestos" (serpentine-group phyllosilicates). Amphibole is an inosilicate with double unbranched chains. The terms replacement is a neutral term describing the substitution of a pre-existing mineral by another one. Elements such as Na from the pre-existing silicate cannot be accommodated into the newly formed mineral quartz and hence get released from the system. If the morphology of the pre-existing minerals is preserved it results in a pseudomorphosis. The peculiar intergrowth of fibers and quartz gives rise to the afore-mentioned cat´s eye effect, which can also be found in diabasic magmatic rocks in the Central European Variscides (e.g. Hof). In this case it is an intergrowth of a more actinolite amphibole with quartz. The replacement of the Fe-Na amphibole by quartz is not running to full completeness but keeps some of the fibers well preserved. If it were a complete replacement of amphibole by quartz which can be observed in some of the diabases, the special mineral-optical effect no longer would occur. Both minerals present side-by-side maintain their own crystal structure. It is the intergrowth that matters with fibres well oriented.

Best regards

Harald G. Dill

Tiger eye is a mineral mixture of brown and yellowish colors. It is composed of quartz as a main component, limonite and riebeckite. Quartz adopts the fibrous structure by  pseudomorph  (substitution preserving the shape ) of crocidolite, blue variety and fibrous of riebeckite (amphibole of the group of inosilicates) .

Dear Gert,

On your question whether the magnesioriebeckite is partly or completely replaced by quartz, I can only confirm what Professor Dill has just responded.

The magnesioriebeckite crystals (needles) remain stable during the quartz-crystallization and are cemented by quartz only. If in other cases the Magnesioriebeckit crystals are partly altered  and/or completely replaced by Fe-hydroxides for example, the Chatoyance-effect is lost.

Best regards,

Guenter

Thanks, Guenter. HOW quartz replaces /substitutes/cements the fiber microstructure of magnesioriebeckite? Is there any relationship reported how the SiO4 groups re-arrange during formation of quartz? 

only for visualization... The EBSD map displays the crystal orientation of the reference direction Y which is  nearly parallel to the former fiber axes of the magnesioriebeckite. The pole figure does not show a strong but still recognizable fiber texture which could be "blurred" by the re-arrangement of the SiO4 tetrahedra during transformation but also is blurred by the visible bending of the fibers.

BTW: The tiger's eye I investigated contained mainly magnetite arranged in layers perpendicular to the fibers. No other Fe-oxide or hydroxide has been discovered. 

The greyscale image is an orientation contrast image using backscattered electrons.

Dear Gert,

Thank you for the interesting images and informations,

Let me continue our discussion on the genesis of the quartz variety 'tiger eye' with the following observations and facts:

Four stage genesis:

1) The magnesioriebeckite fibers (blue gray) are secondarily cemented by quartz. The texture (SiO4-position) of the grain boundaries between quartz and Magnesioriebeckite remains unknown.

2) The two ends of the magnesioriebeckite fibers are altered by oxidizing solutions

3) magnesioriebeckite deteriotates and is removed from the system.

4) The open capillaries in the porous quartz cement are colored by golden yellow to brown adhering Fe-hydroxides.

The grayscale image clearly shows a polished quartz surface with numerous capillary openings. This represents an advanced stage of the alteration.

Fine-grained to powdered magnetite (absolutely opaque) colors the embedding mineral or material black (by no means golden yellow!).

The golden-yellow to yellow-brown color of Tiger eye  is caused by various Fe-hydroxides (but not by magnetite!) that are left in traces in the capillaries. It should be remembered that these extremely fine-grained traces of Fe-oxides are very likely to be washed out of the capillaries during sample preparation (for example, during the ultrasonic cleaning). Maybe, that is why you can not find and demonstrate Fe-hydroxides at the sample surface.

What do you think?

Best regards,

Guenter

Dear Guenter,

at least with the Fe hydroxydes you are certainly right. These parts of a microstructure are too small in order to find them, even if they would exist. The magnification is not sufficient. For me they are also not important. I simply mentioned the existence of magnetite in order to give the readers a glue which kind of tiger's eye I perhaps investigated. I even don't find hematite as transition between magnetite and goethite or any other Fe-containing hydroxide.

Regarding the formation of quartz I am not sure whether this is a well-accepted model (current believe :-) ) or a definite statement. I still wouldn't exclude a "transformation" of riebeckite to quartz. From my point of view the still existing fiber walls are an indicator. Maybe I am wrong with the term pseudomorphosis, but I always assumed that there really the former mineral disappeared and the cavity has been filled by another mineral but keeping the shape of the former one. In case of fibers this wouldn't be possible since the walls also do not exist anymore. What could be also an indicator is the texture of quartz. Why it should form this kind of preferred orientation within independently "grown" or formed quartz individuals? Again, I am not specialized in quartz or minerals formation. I only see it from a very specific and simply crystallographic point of view. :-)

Thanks in advance for reading and answering.

Dear Gert,

May I present you a graphic that can help to explain the parallel structure of the quartz palisades. It is clear that the tiger eye mineralization is a vein filling type.

The attached graphic from the book by Ron H. Vernon (2004) 'A practical guide to Rock Microstructure', Cambridge University Press (ISBN: 0 521 89133 7) shows very clearly how the fibers are bent during deformation (shear) and crystallization. It is precisely this structure, that is at least partially visible in your image, where the upper ends of the quartz palisades are clearly bent to the right.

The wave-like character of the 'tiger eye' mineralization is produced by the simultaneous crystallization and deformation of Magnesioriebeckite fibers and quartz palisades (falcon eye).

Best regards,

Guenter

I will check the explanation in the book you referred to. Thanks!

Hi Guenter, in principle I do not have any problem with understanding the bending or the vein-like shape of quarz since this is already explainable by the occurence of the former asbestos fibers. Your interpretation of the grayscale image "The grayscale image clearly shows a polished quartz surface with numerous capillary openings. This represents an advanced stage of the alteration." is one option. I see it more as result of the hardness anisotropy of the crystal. At least these very strongly visible effects are the result of the acquisition condions of the image: 70 degrees tiltes sample surface, i.e. any topography will be increased by a factor of 3. the locally different backscatter  coefficient (surface curvature) causes the strong contrast. Another imaging option shows that there is no different material at all. The sample simply reacts like wood slightly tilted cut and polished against the woodfiber. The bending really existed already before the formation of quartz since there is no remarkable orientation variation or subgrain formation in quartz. Quartz simply seems to replace the existing shape of riebeckite. 

Dear Gert,

There is absolutely no hardness anisotropy in quartz crystals (except the variety chalcedony). Quartz is a tectosilicate. The hardness in quartz is the same in all directions. The fracture is conchoidal. Only between the quartz grain boundary discontinuities are observed, where relief differences can be seen.

Please find attached some photo micrographs, I have just taken with my bright field / dark field microscope from a tiger eye cabochon.

The first two photos show big difference between cross section and diameter of a single crystal palisade marked by black spots. The diameter of a single needle of the original magnesioriebeckits (now capillary) is only a few microns.

The openings of the altered capillaries are characterized by cavities partially filled with Fe-hydroxides.

The extremely fine needles (originally Magnesioriebeckite) now capillaries occur locally in bundles.

I think your hypothesis from a complete silicification is not understandable.

Best regards,

Guenter

No anisotropy of quartz? As crystallographer this would surprize me (although I am not spezialized in this field). But maybe these guys know more about it?

Guzzo, P.L.

Effect of crystal orientation on lapping and polishing processes of natural quartz.

Ultrasonics, Ferroelectrics, and Frequency Control, IEEE Transactions on (Volume:47 , Issue: 5 )

Abstract: Lapping and polishing processes of natural quartz are investigated in relation to crystallographic orientation and applied normal stress. Weight loss measurements and surface profilometry were carried out in X-, Y-, Z-, and AT-cut samples. The relationship found between material removal rate and stress depends on specimen orientation. Based on indentation fracture mechanics, this behavior is discussed in relation to fracture toughness and scratch hardness anisotropy of quartz crystals. Scanning electron microscopy (SEM) shows that brittle microcracking is the primary mechanism involved with material removal in the lapping process. Plastic deformation mechanisms begin to operate on lapped and polished surfaces above a certain value of stress. Surface profiles and SEM micrographs show that the roughness of lapped surfaces decreases with increasing normal stress, but an opposite behavior is observed in polished surfaces.

Dear Gert,

The hardness anisotropy which you mention is in the micrometer range as a result of stress exposure during grinding and polishing, I completely agree. However, this does not explain the cm-long quartz-grain structure of your photos. We really need to take into account the range size (Bereichsgröße) in our discussion.

We must consider not only the surface effects but also the three-dimensionality of the material, and in detail the relationship between crystallization and deformation during the growth of the quartz and magnesioriebeckite crystals.

However, I miss your comment on my photomicrographs.

Best regards,

Guenter

Dear all. It looks like a Liesegang rings effect. In some mollusc shells the effect is very beautiful. The accepted explanation is a fenomenon of self-organization, based on the non-concordance of the velocity of reaction and the velocity of transport of the reactants to the place of reaction. Perhaps the emeritus Prof. Dr. Peter J. Plath may give us much more light about this effect because he is a prominent german especialist in synergetics and catalysis.

Hello Guenter, the (scanned) grain structure is a few mm long, not cm....(only in case you misinterpreted some scale bar, cf. the electron image). I totally agree about the 3D effects. Therefore I wonder whether there is a relationship between the former strong textured riebeckite and the still textured quartz showing the same fiber axis. This is a strong argument for a relationship, but can also be a fake like pseudomorphosis. 

Your micrographs look nice. I will see whether I can also generate a few from my samples in order to compare them. Unfortunately I am no specialist for such kind of mineralogical samples which might be a pity but sometimes it can be also beneficial since we have a lot of experience in other materials. From this point of view I would not totally exclude that the capillars are preparation artifacts. The preparation we are usually apply are quite rough (even for EBSD where the preparation is extremely gentle in comparison to typical metallographic or mineralogic preparation). For a brittle material Iike quartz existing in agglomerations of millions of bended fibers which are perhaps seperated by a thin film of Fe-hydroxide or oxides I wouldn't exclude the formation of microcracks along the interface but also perpendicular, especially when the fiber axis is only slightly slanted to the preparation surface. Without any doubt we can assume that the exchange of riebeckite by quartz happens with keeping the former microstructure of riebeckite. The question is only: did the entire riebeckite left this microstructure (what I do not believe since then the interface structure wouldn't exist anymore), or has only Mg, Ca etc been removed and SiO4 rearranged to quartz inheriting some former structure property (like partially crystal orientation, and texture) from riebeckite?

Do you know and have this paper?

 Heaney, P. J. & Fisher, D. M.

New interpretation of the origin of tiger's-eye

Geology, 2003, 31, 323-326

Abstract: Tiger's-eye is an attractive and popular gemstone that is ubiquitous in stores that cater to rock and mineral collectors. For more than a century, textbooks and museum displays have identified the material as an archetype of pseudomorphism, i.e., the replacement of one mineral by another with the retention of the earlier mineral's shape. Our study has revealed that the textures responsible for the shimmer of tiger's-eye do not represent pseudomorphic substitution of quartz after preexisting crocidolite asbestos. Rather, we argue that tiger's-eye classically exemplifies synchronous mineral growth through a crack-seal vein-filling process.

Best greetings

Gert

Hello Gert,

No, I did not know this paper. But now, this paper confirms exactly that what I tried to explain:

Simultaneous growth and deformation of Magnesioriebeckite and quartz as the sealing of an opening vein fissure. The extremely thin euhedral crystals of Magnesioriebeckite (needle-like crystal form, average diameter: 1 micron, average length: a few mm) were surrounded and sealed by thick anhedral quartz crystals (cauliform grain shape, average diameter: 200 microns, average length: a few mm).

Best regards,

Guenter

This means that tiger's eye is not a transformation product of "asbestos" but has its own genesis?

In case riebeckite and quartz growing together, how do you explain the weak texture of quartz? Do you expect an epitaxial growth of quartz on riebeckite? Do you expect that riebeckite is still there...and if not, what happened with the Mg and all the other cations? If it is still there, is it possible to prove it with SEM? 1µm is not that small :-). 

Dear colleagues,

there are many good contributions mainly focusing on the textural part and spiraling a little bit out of control. I would like to see the discussion does not completely ignore the chemical composition. In other words, which minerals are involved, what is their chemical composition and how are these minerals intergrown with each other so that the reaction can be described. Is the chemical composition of the various minerals checked and proved by EMPA? In my opinion it is combination a textural and compositional processes, both of which are overlapping each other. In the course of mineralization there are no cut-and-fill structures in these lit-par-lit system.

Best regards

H.G.Dill

@ H.G. Dill: You are certainly right if the formation of tiger's eye would be the focus of my question. In so far the short title of this thread is a bit irritating. My major interest is the explanation/understanding of the discovered fiber texture incl. microstructure for quartz. No doubt, that the consideration of all factors like chemistry, environmental conditions etc. are extremely important for the formation of tiger's eye. If riebeckite is still existing it is clear that the chemistry will to be confirmed although the diffraction patterns should be quite different to quartz so that a reliable phase assignment seems to be no problem. It is also my opinion that the textural and compositional processes are related to each other. The problem is only that we didn't found (yet) riebeckite. But this is not unusual, even in materials science, that phases were important for the formation of a microstructure, but finally only the inherited properties of another phase are able to investigate. 

Best greetings

G. Nolze

Hi Gert,

Have fun with the attached work of Takashi Miyano and Cornelis Klein (1983), I've just found it!

It answers a lot of questions!

Best regards,

Guenter