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Unknown mineral - Virgem da Lapa
  
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dontgogreen




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PostPosted: Jun 12, 2017 15:45    Post subject: Unknown mineral - Virgem da Lapa  

Here is a specimen obtained in Brazil near Virgem da Lapa, locality given as Virgem da Lapa. It's an equant, roughly 6cm, dense specimen which was given to me as a rutile. I removed a small fragment from the back of the specimen to use for powder XRD. I have attached photomicrographs of the fragment and the area from which it was taken, which exhibits muscovite and iron oxides. Indeed, much of the specimen is coated with iron oxides. The XRD results are shown below.

I am fairly confident the fragment I used for XRD is representative of the specimen. The pattern appears to be an excellent match for goethite (some goethite is expected), but there was no trace of rutile (no 100% peak at 27.4 2theta). There is some possibility that hematite is present (100% peak is at ~33.3 2theta) however the weaker lines are not great matches.

Other fragments of this nature were examined by SEM-EDS. No trace of titanium was found, nor tin. In fact, the fragments, which exhibited some of the complex growth lines seen in these photos, gave off only Fe and O x-rays.

I have also attached some photos that illustrate some of the morphological features. There are four flat faces exhibiting triangular etch pits. The angle between these faces is about 110 degrees. Another set of faces , somewhat diamond-shaped, exhibits striations. There appear to be two faces that are parallel or very nearly so, but as the measurements were made by hand on imperfect surfaces I cannot be sure.

Can anyone identify the crystal system of this specimen? I'm not savvy enough to tell whether this is tetragonal or trigonal, or maybe something else.

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IMG_20170612_145657346_HDR.jpg
 Mineral: Unknown
 Locality:
Virgem da Lapa, Jequitinhonha, Minas Gerais, Brazil
 Dimensions: 6 cm
 Description:
 Viewed:  1510 Time(s)

IMG_20170612_145657346_HDR.jpg



IMG_20170612_150250501_HDR.jpg
 Mineral: Unknown
 Locality:
Virgem da Lapa, Jequitinhonha, Minas Gerais, Brazil
 Dimensions: 6 cm
 Description:
 Viewed:  1503 Time(s)

IMG_20170612_150250501_HDR.jpg



IMG_20170612_150402162_HDR.jpg
 Mineral: Unknown
 Locality:
Virgem da Lapa, Jequitinhonha, Minas Gerais, Brazil
 Dimensions: 6 cm
 Description:
 Viewed:  1514 Time(s)

IMG_20170612_150402162_HDR.jpg



IMG_20170612_153931173_HDR.jpg
 Mineral: Unknown
 Locality:
Virgem da Lapa, Jequitinhonha, Minas Gerais, Brazil
 Dimensions: 6 cm
 Description:
 Viewed:  1512 Time(s)

IMG_20170612_153931173_HDR.jpg



vdl1.jpg
 Description:
 Viewed:  1512 Time(s)

vdl1.jpg



vdl2.jpg
 Description:
 Viewed:  1512 Time(s)

vdl2.jpg



vdl3.jpg
 Description:
 Viewed:  1514 Time(s)

vdl3.jpg



vdl4.jpg
 Description:
 Viewed:  1511 Time(s)

vdl4.jpg



goethitepattern1.jpg
 Description:
 Viewed:  1511 Time(s)

goethitepattern1.jpg



hematitepattern1.jpg
 Description:
 Viewed:  1516 Time(s)

hematitepattern1.jpg



rutilepattern1.jpg
 Description:
 Viewed:  1509 Time(s)

rutilepattern1.jpg


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marco campos-venuti




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PostPosted: Jun 12, 2017 16:35    Post subject: Re: Unknown mineral - Virgem da Lapa  

I suggest this is a pyrite crystal, replaced by limonite. The shape is typical.
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Pete Richards
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PostPosted: Jun 12, 2017 16:46    Post subject: Re: Unknown mineral - Virgem da Lapa  

marco campos-venuti wrote:
I suggest this is a pyrite crystal, replaced by limonite. The shape is typical.


I agree totally. Note that the diamond-shaped faces which the etched faces surround are deeply striated in one direction. On each of these faces the direction of the striations changes, just like it does on a pyrite cube. These are (or were) cube faces, and the ones with the triangular etch pits are (or were) octahedral faces, showing the three-fold symmetry component of pyrite. Even the etch pits show that three-fold symmetry. Even the change in direction of the striations from cube face to cube face reflects the three-fold symmetry.

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dontgogreen




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PostPosted: Jun 12, 2017 16:48    Post subject: Re: Unknown mineral - Virgem da Lapa  

I should add that I was able to remove the iron oxides that coat parts of the specimen with a steel implement, but it did not scratch the crystal. I believe it is harder than 5.5
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dontgogreen




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PostPosted: Jun 12, 2017 17:00    Post subject: Re: Unknown mineral - Virgem da Lapa  

Regardless of how hard it is, I agree with both of you, having examined it with your input. All the pieces of info add up. I have to admit that I'm admonished by the fact that some basic crystallography texts might have served better than a million dollars worth of machinery.
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Pete Richards
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PostPosted: Jun 12, 2017 17:01    Post subject: Re: Unknown mineral - Virgem da Lapa  

Given your x-ray results, at least the outer part of the crystal IS now iron oxide. The "limonite" that replaced the crystal may well be more coherent than other iron oxide that may have been deposited on the surface. But it will probably not be as hard as the original pyrite.
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PostPosted: Jun 12, 2017 17:15    Post subject: Re: Unknown mineral - Virgem da Lapa  

dontgogreen wrote:
Regardless of how hard it is, I agree with both of you, having examined it with your input. All the pieces of info add up. I have to admit that I'm admonished by the fact that some basic crystallography texts might have served better than a million dollars worth of machinery.


I can't resist.... Nothing wrong with a million dollars worth of machinery, and it's very useful indeed. Morphology is not always useful, but in the right circumstances, it is a powerful tool, and it is often not used to its potential.

To my mind, there are three major tools in mineralogy: x-ray and other forms of crystallography which identify the internal structure of a mineral, various probes (SEM-EDS, microprobe, Raman and IR spectroscopy) which identify the chemistry, and morphology which can both guide the use of the other two and catch them up when complex situations arise, e.g. the pseudomorphy in this case. Any of them, used with expertise, can provide correct answers much of the time, but the wise use of all three covers the weaknesses of each and provides the most reliable answer!

From an unabashed champion of morphological crystallography....

Now someone can write in support of optical mineralogy, and I won't disagree. All tools are good.

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marco campos-venuti




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PostPosted: Jun 12, 2017 17:16    Post subject: Re: Unknown mineral - Virgem da Lapa  

Striated faces are the sum of 1 cube face (6 faces) and 2 pyritohedron faces (12). For that reason they result curved.
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