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Identity HelpUnknown Thumbnail

6th Apr 2018 21:23 UTCDan Fountain

I know, I'm not providing enough information, but here goes:

This is an unknown thumbnail I got in a box of miscellaneous TNs. I don't know the SG - its glued onto the styrofoam. I don't know the hardness - I'm not gonna scratch the crystal faces with picks or try to scratch a too-hard test mineral with a termination. Ditto the streak. Luster is a judgement call, and I usually get it wrong. I don't know the crystal system - I haven't seen this shape in any of the examples I've looked at. It's like a stubby prism, square in cross-section, and terminated with a pyramid on each end. Think of an octahedron split at the base of the pyramids and a prismatic section inserted. Color is dark brown.

Photo attached.

Thanks!

Dan

6th Apr 2018 21:31 UTCAndrew Debnam 🌟

Zircon?

https://www.mindat.org/min-4421.html

see Zircon form 81 in the crystal atlas section

6th Apr 2018 21:35 UTCLouis Zulli

Based on your description and the image, this seems to be a crystal in the tetragonal system. Maybe zircon?

Edit: I agree with Andrew!

6th Apr 2018 21:39 UTCNick Gilly

Looks like a zircon to me.

6th Apr 2018 21:43 UTCDan Fountain

Thanks, Andrew et al. I checked & found it slightly radioactive. Nicest zircon I've got (others are microscopic cyrtolite).

Dan

8th Apr 2018 00:13 UTCPaul Brandes 🌟 Manager

Where is it from, Dan?

8th Apr 2018 15:39 UTCEd Clopton 🌟 Expert

Zircon may fluoresce yellow in SWUV, although if it is radioactive its structure may be been disrupted enough (i.e., become metamict) to disrupt the fluorescence, too. I tried to make it look like a cuboctahedral crystal (maybe limonite replacing pyrite), but the faces aren't right. If it's a zircon, which seems like a reasonable guess, then it was mounted with its c-axis horizontal, which gives me vertigo trying to reorient it mentally.

8th Apr 2018 17:13 UTCDonald B Peck Expert

Ed, I think that is exactly what was done. And looking at a photo, rather than a real crystal, it is almost unrecognizable.

Don

8th Apr 2018 19:17 UTCKevin Conroy Manager

Dan, would you post a photo looking at a termination?

9th Apr 2018 14:07 UTCAlfred L. Ostrander

Can anyone explain why the Miller indices for zircon, form 81, referenced above are for the second order bipyramid {101} and the second order prism {100}? Why weren't the faces shown as first order bipyramid {111} and first order prism{110}? There is a hierarchy in the Tetragonal System as to usage of first and second order nomenclature is there not? Here's a clue. The tetragonal first order dipyramid {111} is considered analogous to the octahedron {111} of the cubic system. The order of the prisms must be in accord with the dipyramid. In this case, as only two forms are present, one pyramid and one prism, the first order nomenclature is given priority by preferred orientation of the single dipyramid.

9th Apr 2018 14:39 UTCEd Clopton 🌟 Expert

Alfred, excellent question. I wonder about this sort of thing all the time (o.k., not constantly, but every now and then) and will watch for a reply by someone who actually understands this stuff!

9th Apr 2018 15:44 UTCDonald B Peck Expert

I think the answer is, it's a mistake.

Don

9th Apr 2018 16:56 UTCAlfred L. Ostrander

Ed,

I do understand this stuff. I am trained in classic crystallography and have studied and applied it for over 40 years. I am trying to get people to think through some of this stuff. I put this in the form of a question to perhaps encourage people to look some of this up and verify for themselves what I said about the hierarchy and position of first and second order forms. Please read what I wrote after I wrote "Here's a clue." I explained why I am of the opinion it is a mistake.

I believe Don is right. And I know Don understands. Don and I just recently had a discussion about this very problem. It's a mistake. If it is not a mistake, I want someone to explain why it isn't. But I don't think anyone can really explain the second order forms being used when first order forms are indicated. At least Mindat didn't call the form a pseudo-octahedron as another major site has done. If it is a false form, why not take the time to discover the true form or, as in certain particular cases why it is OK to use the pseudo- prefix. But maybe that is a topic for another discussion.

10th Apr 2018 06:54 UTCEd Clopton 🌟 Expert

Alfred, no offense meant--the rest of your post does indicate that you understand crystallography better than I probably ever will. I didn't read it carefully and replied hurriedly on the assumption that you were posting a question in search of an answer, not presenting "an exercise for the reader" to which you knew the answer all along.

8th Sep 2018 00:47 UTCMark Holtkamp

Sorry for my delayed answer, I just saw this post today.

The zircon form 81 is not a mistake, it is just a crystal that has developed these second order forms. They can easily be distinguished from the {111} and {110} forms, because the interfacial angles are completely different.

Mark.

8th Sep 2018 01:14 UTCMark Holtkamp

Just for the record, the miller indices of the forms on the drawing are based on measurements on a zircon crystal from the Vesuvius. The drawing was published in 1876 and can be found in Goldschmidt's Atlas der Krystallformen. Dana's System of MIneralogy 6 has a comparable picture.

Mark.

8th Sep 2018 05:15 UTCKeith Compton 🌟 Manager

Dan

As for checking hardness - your specimen is glued onto a styrofoam base - simply peal it off and check the hardness there as that is probably where there may be a contact point so you wouldn't deface the specimen.

8th Sep 2018 16:24 UTCAlfred L. Ostrander

Mark,

Just for the record.

What has changed the shape of the zircon from the first order prism {110} and the second order prism {100} and the corresponding first order bipyramid {111} and the second order bipyramid {101}? Nothing in the shape has changed so how can the interfacial angles have changed? All that has changed is the position relative to the c axis, that is, a rotation. How has that changed the interfacial angles? Did the axial ratio change? Dana addressed this by indicating no difference between the first and second order prisms can be made unles certain characteristics such as striations are present or more forms than the prism and corresponding bipyramid are present to indicate the proper orientation. Are any of those characteristics seen here? So how does that change the form or shape of the corresponding bipyramids? You have to change the shape of something to change the interfacial angles. This is not the only situation in crystal morphology that presents this problem. Think of the positive and negative rhombohedrons in quartz and calcite.

As to the position of the p {111} and the m {110} forms, see Dana's Textbook of Mineralogy 4th Edition, 22nd printing, 1966 by W. E. Ford page 610 under the descriptive discussion of zircon. The same can be said for Mineralogy by Kraus Hunt and Ramsdell, 5th Edition 1959. See pages 63-69 for the discussion of the tetragonal system and forms of the ditetragonal bipyramidal class. A brief discussion for zircon follows on pages 366-367 for zircon. You can also check Crystallography and Practical Crystal Measurement Vol1, 1922 by A. E. H. Tutton pages 192-198. Tutton agrees the forms do not differ in shape, only in position. Therefore, how can the interfacial angles be different unless a modified bipyramid appears. The modified bipyramid cannot be a {111} or {101} form.

Lastly, consider the parametral plane (111). Following more recent convention than Goldschmidt, the parametral plane (111) of the 1st order bipyramid is given precedence as it sits in the same position as the parametral plane of the octahedron {111}. See Kraus Hunt Ramsdell page 64. Preferred orientaion should then follow placing a dominant bipyramid {111} first. See my earlier comment. Zircon no 81 and zircon no 133 as featured on Mindat do not appear to follow this convention.

Goldschmidt is great but sometimes he called the equal development of the positive and negative rhombohedron in quartz dipyramids with no reference to high or low quartz. Many of his drawings show no consistency in any orientation, clinometric or isometric. Perspective varied. This was not his intent He showed the forms and habits from many perspectives.

I will freely acknowledge that the drawings of zircon on Mindat are noted as after Goldschmidt. That notation is fitting and proper. Conventions have changed since 1876.

8th Sep 2018 23:17 UTCMark Holtkamp

Quote: "What has changed the shape of the zircon from the first order prism {110} and the second order prism {100} and the corresponding first order bipyramid {111} and the second order bipyramid {101}? Nothing in the shape has changed so how can the interfacial angles have changed?"

MH: The {111} bipyramid and the {110} bipyramid have different steepness. That is how you can distinguish them. Your assumption that the {111} and {101} bipyramids are geometrically indentical, is wrong. That's pretty basic, and you can easily check this by calculating the interfacial angles or by using a crystal drawing program. If you want I can help you with that.

Quote: "Dana addressed this by indicating no difference between the first and second order prisms can be made unles certain characteristics such as striations are present .."

MH: For the general case, Dana is right. But we are comparing {111} and {110} bypiramids for a specific mineral. The difference in steepness is the 'certain characteristrics' you need.

"Quote": " Think of the positive and negative rhombohedrons in quartz and calcite. "

MH: That doesn't apply here, these are correlate forms, the bipyramids are not.

About the parametral plane: Before X-ray diffraction analysis was available, the unit cell choice for zircon indeed was based on the parametral plane. The modern cell we use nowadays is different. Compared with the modern cell, the old cell is rotated 45 degrees around the c-axis, and has a different axial ratio. With the modern unit cell, the miller indices of the common bipyramid are {101}. In the old unit cell, they are {111}. Of course both systems refer to the same planes within the crystal structure.

According to Goldschmidt, zircon 81 has the first order bipyramid {111}. I converted the miller indices to the modern cell, and created the 3D drawing. As an additional check I compared the interfacial angles in my 3D drawing whith those of one of the original authors ( Dana's System of MIneralogy 6). Since I used the modern cell, zircon 81 has the modern orientation and miller indices. You'll find the same orientation and indices in modern handbooks. See the 21th edition of Dana's Manual of Mineralogy for instance for a crystal with the same forms as my zircon 81. I am sure that your handbooks use {111} for the common bipyramid in combination with the old cell, or {101} in combination with the modern cell.

Edit: removed an earlier explanation that missed the point

Edit 2: fixed a few typos

9th Sep 2018 02:39 UTCAlfred L. Ostrander

Mark,

So you did switch out Goldschmidt and didn't make note of it. Cute trick. I wondered what was happening and now I know.

9th Sep 2018 06:02 UTCAlfred L. Ostrander

Mark,

I have been considering how to best demonstrate the congruous nature of the tetragonal prism and pyramid. Consider the 1st order prism {111} on the crystallographic axis. Terminate it with the {111} first order bipyramid. Now consider rotating only the prism to the {100} position of the second order prism. The prisms as agreed are geometrically equivalent and as such are congruent. Both are parallel to the c axis and square in cross section parallel to the lateral axes. Now rotate the bipyramid to match up with the prism now in the second order position. That pyramid {111} will sit exactly on the new prism now in the second order position and become a second order bipyramid. By a counter clockwise rotation the (111) face will become the (011) face and the (110) prism face becomes the (010) face. The intercept of the bipyramid on the c axis has not changed. The steepness or attitude of the face of the pyramid has not changed. The bottom edge of the bipyramid intersects the lateral axes at a unit length in both positions. The original 1st order form, as constructed has been rotated into the 2nd order position. The combined forms are geometrically congruous. This is about as simple, pragmatic and applied an explanation as I can come up with. Applied. For me, end of discussion.

I do not need a computer program to draw crystals, thank you. I can draw accurate projections quite readily by the graphic method devised by Naumann. I just did it while working out a way to explain the rotation of the first order prism and bipyramid to the second order position and maintain the integrity of the unit forms. I can very easily change the steepness of the faces of the bipyramid but then it becomes a modified first order pyramid {hhl} or a modified second order bipyramid {h0l). I have seen too many people who do not understand a lot about crystallography place two enantiomorphic forms on a projection and wonder why it can't happen in nature if their computer lets them draw it.

You completely missed the point about the rhombohedrons. Think about it. Consider the statement in context regarding other characteristics, not correlate forms. Think striations, etc. Think how it is sometimes possible to determine which form is present by other physical characteristics or the presence of more forms. Again, correlate forms are not the issue. This does apply to the first and second order forms and positive and negative forms. It is not at all about correlate forms.

And if you did change the indices assigned by Goldschmidt, for any reason, you should not list them as Goldschmidt or attribute them to Goldschmidt. You don't change a thing when citing an authors work. You changed his work. New parameters or not. And as you yourself said, for the general case Dana was right. I will look into the new parameters. Putting this explanation into the drawings you converted would have been more than helpful, and maintained the integrity of Goldschmidt's work.

9th Sep 2018 10:20 UTCMark Holtkamp

Quote: "The bottom edge of the bipyramid intersects the lateral axes at a unit length in both positions. "

MH: No it doesn't. Your rotated bipyramid face (111) now intersects the b-axis at a length of the reciprocal of the square root of 2, approximately 0.7. The miller indices of this face are not (101). If you do this rotation in a projection of the axial cross on (001), your mistake is easy to spot.

Or calculate the steepness, if we take this as the angle between the face normal and the c-axis, calculate it for both (111) and (101). Use a lattice where the axes have equal lengths, so we don't have to convert to a cartesian system. Then the cosines of the angles are the reciprocal of the square root of 2, and the reciprocal of the square root of 3.

Dana and Goldschmidt don't agree with your statement that both bipyramids are congruous. They give different angles for the bipyramids {111} and {101}. For instance the 6th edition of Dana's System of Mineralogy, page 483, different interfacial angles for the bipyramids {111} and {101}. Goldschmidt's Kristallographische Winkeltabellen (a copy is online somewhere), page 379, different goniometric angles.

Quote: "I just did it while working out a way to explain the rotation of the first order prism and bipyramid to the second order position and maintain the integrity of the unit forms. "

MH: I guess you also implicitely rotated the underlying lattice without realizing it.

Quote: "I have seen too many people who do not understand a lot about crystallography place two enantiomorphic forms on a projection and wonder why it can't happen in nature if their computer lets them draw it. "

MH: If it can't happen in nature then why did you put one of those crystals in your Mindat articles on crystallography? Are you one of these people? But you are in good company: both Dana and Goldschmidt have drawings of crystals like these.

Qoute: "It is not at all about correlate forms."

MH: Yes, that's what I said, after you brought them up.

Quote: "And if you did change the indices assigned by Goldschmidt, for any reason, you should not list them as Goldschmidt or attribute them to Goldschmidt. You don't change a thing when citing an authors work. You changed his work. "

MH: I didn't change his work, I used it and tried to build on it. Nor did I change his indices, I just converted them to the modern unit cell.

Edit 1, 2, 3, 4: fixed a few typo's

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