FMF - Friends of Minerals Forum, discussion and message board

[dontgogreen]

A genticulated rutile twin from the collection of Paul Tucker circa 1970.

[dontgogreen]

A few more photos...

[Tobi]

I really like that gorgeous gold specimen from Serra do Caldeirão, great one!

[dontgogreen]

Thanks Tobi, I figured I had to get at least one since they were coming out very recently. Gold collecting is a little rich for my blood though.

[Nunzio]

Sweet Adamite!

[dontgogreen]

After spending a little time collecting worldwide minerals without any focus, I have narrowed my interest to rutile crystals. I think their beauty is a bit more subtle than many species, and they are generally far more affordable. Here are two Brazilian specimens that I like quite a bit.

[dontgogreen]

Hematite and rutile both posses a hexagonal close-packed oxygen sublattice, which enables simple transitions among them by exsolution, epitaxy, and intergrowth (Daneu et al. [2014] and references therein).

Other examples of hematite and rutile intergrowth are the speicmens from Novo Horizonte, Brazil, and those of Mwinilunga, Zambia. A study of the Zambian material indicates that the source of Ti for rutile growth is the hematite crystal itself. Diffusion of Ti ions lead to topotaxial nucleation and growth of rutile on structurally compatible lattice planes (Rečnik et al., 2015). I am not sure whether or not this also holds true for specimens from Cavradi, as I have not been able to find a similar study. If anyone is aware of one, please direct me to it. Additionally, if you have Mwinilunga specimens, I would gladly trade for them!

[Philippe Durand]

Ha ! I love this hematite; my favorite mineral.
It is a marvelous specimen.

[Tobi]

[Jordi Fabre]

[dontgogreen]

The hoppered termination lets the red color come through nicely at the top. These striated prisms are some of my favorites.

[dontgogreen]

A multiply twinned equant crystal from Graves Mountain with excellent luster.

[dontgogreen]

Here is an interesting multiply-twinned crystal reported to be from Zagi Mountain. I will produce photos of the full specimen at a later date, but for now I am interested to explore the twin boundaries. Daneu et al. (2014) studied the structure and formation of (101) rutile twins from Diamantina and in this paper, which I do not purport to fully comprehend, the authors describe a complex process by which these twins form. They propose that the formation of these twins is chemically induced, and a result of topotaxial replacement of an Al-rich oxyhydroxide precursor. They find corundumn precipitates segregated on the (101) twin boundary, suggesting that they are remnants of past transient processes involving the dehydration of a parent Al-rich hydroxlian pseudorutile phase, which served as a precursor for rutile crystallization at elevated temperatures. During the late stages of dehydration, the transformation of aforementioned corundumn particles from diaspore leads to a volume decrease, causing tensions in the surrounding rutile which are compensated by the formation of parallel {101} twin boundaries. This is really quite a fascinating paper and I suggest that you read it, as I am sure that I have missed quite a bit of the story here.

Obviously this crystal is not from Diamantina, but I wonder if the same chemical and temperature-related mechanisms operate to produce the same type of twin? One feature that I note in the photograph is that some of the smaller twins in the center of the photo do not extend along the length of the crystal face, but rather taper off about halfway. This seems to be what one might expect if they are parallel twin boundaries created by a dehydration-induced tension in the crystal structure.

Surely there must be some crystallographers who have something to add?

The citation for the paper I referenced is:
Daneu, N., Recnik, A., and Mader, W., 2014. Atomic structure and formation mechanism of (101) rutile twins from Diamantina (Brazil). American Mineralogist, Volume 99, pages 612-624.

[dontgogreen]

Here is the entire specimen. Unfortunately not entirely free from damage, but there are rather few references to large, good quality rutiles from this locality in the literature.

[dontgogreen]

Here is another thumbnail from Graves Mountain; it is similar in character to the thumbnail posted previously but lacking matrix. It is multiply twinned and the re-entrant angle may be seen at the top of the crystal. The forms are almost identical to those of the crystal posted above, but exhibit slightly different degree of development. The price from the 1970's label is $27.50!