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Collection of Dontgogreen
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dontgogreen




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PostPosted: Dec 15, 2017 08:59    Post subject: Re: Collection of 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.


golconda5_1.jpg
 Mineral: Rutile (variety struverite)
 Locality:
Golconda pegmatite, Governador Valadares, Vale do Rio Doce, Minas Gerais, Brazil
 Dimensions: ~4.5 cm
 Description:
 Viewed:  1779 Time(s)

golconda5_1.jpg



capao de lana_bedogne_ 5pt5cm_2.jpg
 Mineral: Rutile
 Dimensions: 5.5 cm
 Description:
 Viewed:  1785 Time(s)

capao de lana_bedogne_ 5pt5cm_2.jpg


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dontgogreen




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PostPosted: Dec 22, 2017 09:26    Post subject: Re: Collection of 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!



cavradi1.jpeg
 Mineral: Hematite with Rutile
 Locality:
Cavradi, Curnera Valley, Tujetsch (Tavetsch), Vorderrhein Valley, Grischun (Grisons; Graubünden), Switzerland
 Dimensions: ~4 cm
 Description:
 Viewed:  1666 Time(s)

cavradi1.jpeg


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Philippe Durand




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PostPosted: Dec 22, 2017 10:23    Post subject: Re: Collection of Dontgogreen  

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




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PostPosted: Dec 22, 2017 13:35    Post subject: Re: Collection of Dontgogreen  

dontgogreen wrote:
Hematite with rutile
Cavradi, Curnera Valley, Tujetsch (Tavetsch), Vorderrhein Valley, Grischun (Grisons; Graubünden), Switzerland
~4 cm!
Great Cavradi specimen!!!
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Jordi Fabre
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PostPosted: Dec 23, 2017 16:52    Post subject: Re: Collection of Dontgogreen  

dontgogreen wrote:
...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)...

Before the text mentioned it was this note related with the first analysis of the Zambian material wrote in December 2007 by the late Dr. Joan Viñals :

Comments on the possible pseudomorph of rutile and hematite from ilmenite crystals:

Commonly, rutile occurs as oriented crystals on hematite crystals. This may be due as an epitactic growth or from an exsolution process. In both cases, hematite appears as single crystals or as aggregates of a limited number of single crystals. Microscopic examination of material from Jordi Fabre shows that rutile forms a delicate intergrowth of rutile and hematite. However, hematite occurs as very fine grained masses which form the interstices between the rutile needles. This could be consistent with an hydrothermal (oxidizing) alteration process of a former ilmenite crystals. Textures of this type were reported by Ramdohr (The ore minerals and their intergrowths. Pergamon Press, Oxford.1980. Vol 2. p.1008). In this case, rutile forms trigonal network in the basal sections of the former ilmenite, while fine grained hematite filled the interstices.



Hematite and Rutile - Mwinilunga_Zambia.jpg
 Description:
The original analysis and report written by the missed Dr. Joan Viñals
 Viewed:  1537 Time(s)

Hematite and Rutile - Mwinilunga_Zambia.jpg


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dontgogreen




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PostPosted: Jan 12, 2018 23:38    Post subject: Re: Collection of Dontgogreen  

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


hiddenite_melanson_1.jpg
 Mineral: Rutile
 Locality:
Hiddenite, Alexander County, North Carolina, USA
 Dimensions: 2.7 cm
 Description:
 Viewed:  1361 Time(s)

hiddenite_melanson_1.jpg



hiddenite_melanson_2.jpg
 Mineral: Rutile
 Locality:
Hiddenite, Alexander County, North Carolina, USA
 Dimensions: 2.7 cm
 Description:
 Viewed:  1350 Time(s)

hiddenite_melanson_2.jpg


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dontgogreen




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PostPosted: Jan 13, 2018 23:28    Post subject: Re: Collection of Dontgogreen  

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


graves mtn cyclic thumbnail.jpg
 Mineral: Rutile
 Locality:
Graves Mountain, Lincoln County, Georgia, USA
 Dimensions: 2.5 cm
 Description:
 Viewed:  1274 Time(s)

graves mtn cyclic thumbnail.jpg


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dontgogreen




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PostPosted: May 03, 2018 15:44    Post subject: Re: Collection of 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.



twinning1.jpg
 Mineral: Rutile
 Locality:
Zagi Mountain, Hameed Abad Kafoor Dheri, Peshawar, Khyber Pakhtunkhwa, Pakistan
 Dimensions: FOV ~ 1.5cm
 Description:
 Viewed:  679 Time(s)

twinning1.jpg


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dontgogreen




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PostPosted: May 04, 2018 09:18    Post subject: Re: Collection of 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.


zagi mtn2.jpg
 Mineral: Rutile
 Locality:
Zagi Mountain, Hameed Abad Kafoor Dheri, Peshawar, Khyber Pakhtunkhwa, Pakistan
 Dimensions: 4.3 cm
 Description:
 Viewed:  600 Time(s)

zagi mtn2.jpg


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dontgogreen




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PostPosted: May 23, 2018 10:24    Post subject: Re: Collection of 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!


gmbtn1.jpg
 Mineral: Rutile
 Locality:
Graves Mountain, Lincoln County, Georgia, USA
 Dimensions: 2.5 cm
 Description:
 Viewed:  342 Time(s)

gmbtn1.jpg


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