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Questions about microcline
  
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Josele




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PostPosted: Nov 18, 2017 14:19    Post subject: Questions about microcline  

I have understood that microcline and orthoclase can be distinguished at naked eye only when the crystal is very well developed and is not too small. Now I have a good microcline sample with flat shiny faces and sharp edges but I'm not able to find any characteristic that allows differentiate from orthoclase. Theoretically gamma angle is 89º in microcline and 90º in orthoclase but in my sample the angle between (010) face and the edge between (001) and (-101) looks a right angle. My goniometer is not very precise but enough to note a one degree difference.
I'm doing something wrong? Can someone explain how to differentiate one K-feldspar from another by naked eye?



P1200000.jpg
 Mineral: Microcline
 Locality:
Hachupa, Shigar Valley, Skardu District, Baltistan, Gilgit-Baltistan (Northern Areas), Pakistan
 Dimensions: Main crystal size: 10 x 6 x 6 cm
 Description:
Front view
 Viewed:  588 Time(s)

P1200000.jpg



P1200001.jpg
 Mineral: Microcline
 Locality:
Hachupa, Shigar Valley, Skardu District, Baltistan, Gilgit-Baltistan (Northern Areas), Pakistan
 Dimensions: Main crystal size: 10 x 6 x 6 cm
 Description:
Side view
 Viewed:  587 Time(s)

P1200001.jpg



P1200002.jpg
 Mineral: Microcline
 Locality:
Hachupa, Shigar Valley, Skardu District, Baltistan, Gilgit-Baltistan (Northern Areas), Pakistan
 Dimensions: Main crystal size: 10 x 6 x 6 cm
 Description:
Upper side view
 Viewed:  590 Time(s)

P1200002.jpg



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alfredo
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PostPosted: Nov 18, 2017 20:49    Post subject: Re: Questions about microcline  

Sorry, Josele, I can't help with your question, but I would like to point out to other collectors that the difference between orthoclase and microcline would not, under modern species definition rules, be sufficient for them to be classified as two different species, and the species distinction is kept mainly because of historical inertia. At some time in future it is likely that one will be "discredited" and they will officially be a single species. So in my own collection I just label them "K-feldspar" and don't bother trying to differentiate them.

I know a lot of field geologists just name the K-feldspars one or the other based on environment of formation: "orthoclase" from granites, "microcline" from pegmatites, "adularia" from hydrothermal veins or alpine-type veins. I doubt this is always accurate.

Josele, are you using a contact goniometer or an optical instrument?
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Pete Richards
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PostPosted: Nov 18, 2017 21:03    Post subject: Re: Questions about microcline  

Joselle,

Can you indicate which Miller indices you are assigning to the faces on your crystal? Without this information I am not sure how to try to interpret your question.

I would also remind you that in the triclinic system a primary pinacoidal face - i.e. (100), (010), (001) is not perpendicular to the axis with the same indices [100], [010], or [001]. Thus making measurements between a line and a face risks not measuring what you want to know.

As for telling microcline and orthoclase apart, microcline almost always has polysynthic twinning in two directions parallel to [001], which creates a characteristic cross-hatch pattern in thin section, while orthoclase almost never has polysynthetic twinning. (This according to my old optical mineralogy texts). If you can observe this twinning while examining the intact crystal (as opposed to a thin section), you've got microcline. If not, you cannot be sure....

Following up Alfredo's question, if you are using a contact goniometer or a less precise (e.g. photographic) method, I would not think a distinction of a degree could be made reliably.

Alfredo, I'd like to hear more about the distinction or lack of it between orthoclase and microcline proper (leaving aside adularia and maybe some others for the moment). Is it uncertain whether there is a true orthorhombic K-spar that we call orthoclase? This is different from assuming that all granitic K-spar is orthoclase, of course.

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Bob Carnein




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PostPosted: Nov 18, 2017 21:56    Post subject: Re: Questions about microcline  

At the risk of seeming old fashioned, I like the distinction between microcline and orthoclase (and definitely won't drop it in the time I have left on Earth). It conveys useful information about crystallography and, to some extent, associations. Should we drop sanidine, too? (By the way, I also prefer to use the term "K-feldspar", if I don't have thin-section or other information available)

I think some scientists are a bit too eager to throw out terms that some expert has called obsolete. Another example is the admittedly arbitrary subdivision of the plagioclase feldspars. Calling them all albite or anorthite doesn't provide much useful information about composition; in fact, it's misleading to anyone familiar with the 6-fold classification I learned when I was in school. (I think it's just as arbitrary to set the albite/anorthite boundary at 50%.)When a geologist writes that a particular rock contains labradorite, it conveys useful information about what other minerals are likely to occur in association--the first step in mineral identification, at least in the field. Admittedly, "labradorite" isn't a mineral, in the formal sense, but I think we all know that. But, if you say the rock contains anorthite, that doesn't really imply much, unless you use it in the "old" sense, rather than the sense that the plagioclase contains >50%Ca. Besides, there are many thousands of articles and books out there that use the "old" classification; why make this older literature more inaccessible to young people trying to break into the science? The alternative, of course, is to give the actual Ab:An ratio, but that may not be readily available. OK, so shoot me and put me out of my misery??
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Josele




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PostPosted: Nov 19, 2017 14:24    Post subject: Questions about microcline  

First of all, thank you very much for your enlightening answers.
As a goniometer, I'm using a small carpenter's square (sorry Alfredo, I know this not very scientific!). As I'm looking for an angle very close to 90º, I was thinking it is enough to detect it. Maybe not, especially because one of the edges where I measure is only 3 cm long.
About the angular difference between axes and perpendiculars to faces in the triclinic system noted by Pete, this is just what I'm looking for, trying to estimate if the angle between the edge between (001) and (-101), which I suppose is parallel to b axis, and (010) face measured in a line parallel to lateral edges of this face, which I suppose are parallel to c axis, is a right angle or not. Please see following pictures and correct me if this approach is wrong.

Let me explain another thing in this crystal that is causing difficulty with the measurements in the aforementioned edge. Although it looks like a single crystal, I think this sample is a twinned crystal which twin operation is a symmetry by [001]. Then the edge where I try to detect a difference of 90º is in fact an addition of two edges belonging to one and another crystals of the twin (as can be seen in pictures) that should conform an angle of 178º but that I see absolutely straight.

Second question about this piece: Is it a recognized twin? The contact plane is parallel to (010), like in an albite twin. We used to talk about "albite twins" when polysynthetic twinning occurs but, is it correct to name it "albite twin microcline" in this case? If not, which would be the proper name?



P1200013 copia.jpg
 Mineral: Microcline
 Locality:
Hachupa, Shigar Valley, Skardu District, Baltistan, Gilgit-Baltistan (Northern Areas), Pakistan
 Dimensions: Main crystal size: 10 x 6 x 6 cm
 Description:
This is the angle that I'm trying to measure (between green lines). It should be 89º but I'm unable to see any difference with a right angle (theoretically the orange line but in practice both lines coincide). With 1 degree and 30 mm of edge, deviation from a right angle should be about 0.5 mm, enough to be detected with my carpenter's square.
 Viewed:  399 Time(s)

P1200013 copia.jpg



P1200007 copia.jpg
 Mineral: Microcline
 Locality:
Hachupa, Shigar Valley, Skardu District, Baltistan, Gilgit-Baltistan (Northern Areas), Pakistan
 Dimensions: Main crystal size: 10 x 6 x 6 cm
 Description:
(001) face at reflex.
Can see some reflections of inner cleavage planes at the half left
 Viewed:  400 Time(s)

P1200007 copia.jpg



P1200008 copia.jpg
 Mineral: Microcline
 Locality:
Hachupa, Shigar Valley, Skardu District, Baltistan, Gilgit-Baltistan (Northern Areas), Pakistan
 Dimensions: Main crystal size: 10 x 6 x 6 cm
 Description:
At reflex, (-101) face of the other crystal of the twin
 Viewed:  399 Time(s)

P1200008 copia.jpg



P1200008&MI copia.jpg
 Mineral: Microcline
 Locality:
Hachupa, Shigar Valley, Skardu District, Baltistan, Gilgit-Baltistan (Northern Areas), Pakistan
 Dimensions: Main crystal size: 10 x 6 x 6 cm
 Description:
Miller indices and boundary between crystals
 Viewed:  400 Time(s)

P1200008&MI copia.jpg



P1200010 copia.jpg
 Mineral: Microcline
 Locality:
Hachupa, Shigar Valley, Skardu District, Baltistan, Gilgit-Baltistan (Northern Areas), Pakistan
 Dimensions: Main crystal size: 10 x 6 x 6 cm
 Description:
Backside with reentrant angle
 Viewed:  399 Time(s)

P1200010 copia.jpg



P1200003 copia.jpg
 Mineral: Microcline
 Locality:
Hachupa, Shigar Valley, Skardu District, Baltistan, Gilgit-Baltistan (Northern Areas), Pakistan
 Dimensions: Main crystal size: 10 x 6 x 6 cm
 Description:
Zenithal view
 Viewed:  399 Time(s)

P1200003 copia.jpg



P1200011 copia.jpg
 Mineral: Microcline
 Locality:
Hachupa, Shigar Valley, Skardu District, Baltistan, Gilgit-Baltistan (Northern Areas), Pakistan
 Dimensions: Main crystal size: 10 x 6 x 6 cm
 Description:
As Alfredo likes to say, broken part of the crystal is often the most interesting
 Viewed:  399 Time(s)

P1200011 copia.jpg



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PostPosted: Nov 19, 2017 17:42    Post subject: Re: Questions about microcline  

It appears to me that you have a Carlsbad twin. This twin law places (001) of one individual and (-101) of the other in near co-planar orientation, but they deviate from each other by a bit more than 2°. This leads to the scaly steps your photo shows - on (-101) I believe - as crystal growth "attempts" to bring the two planes into coincidence.

Given both the twinning and the compensation growth on the (-101) face, I'm not sure you can trust the measurement you described....

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PostPosted: Nov 20, 2017 17:35    Post subject: Re: Questions about microcline  

Carlsbad twin? What a surprise! One expect the typical "horns" in a carlsbad twin. After thinking a bit about it, I see that the twin operation is the same but in my specimen termination is conformed by (001) and (-101), whose after the twin operation they stay almost aligned, and in the typical Carlsbad twin terminations are conformed by (001) and (-201), whose form the horns after the rotation; that's why they look so different. On the other hand, boundary between the two individuals in my sample is a plane with a little step (as it is usually in a contact twin) and in a typical Carlsbad twin (theoretically a penetration twin) is more complex.
Effectively, in my sample (001) face of one crystal and (-101) of the other are not exactly in the same plane, there is a small deviation between them that con be detected moving the piece at light reflex.
Well, although a little shocking to me, clarified matter, thanks for your help.



P1100148.jpg
 Mineral: Microcline
 Locality:
Hachupa, Shigar Valley, Skardu District, Baltistan, Gilgit-Baltistan (Northern Areas), Pakistan
 Dimensions: 9 x 7 x 5 cm
 Description:
This kind of twin should not be uncommon, at least in this locality, here another example:
 Viewed:  271 Time(s)

P1100148.jpg



P1200054.jpg
 Mineral: Microcline
 Locality:
Hachupa, Shigar Valley, Skardu District, Baltistan, Gilgit-Baltistan (Northern Areas), Pakistan
 Dimensions: 9 x 7 x 5 cm
 Description:
Miller indices and boundary
 Viewed:  253 Time(s)

P1200054.jpg



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