FMF - Friends of Minerals Forum, discussion and message board

[John S. White]

I came into this discussion a little late so please forgive me if this is something that has already been disclosed. What confuses me is that I see no mention of the goniometry that enabled the writer to determine the angles between the various crystals. My inclination when I see satellite crystals growing upon a large crystal in many different angles is to believe that if one of them is at an angle very close to a recognized twin angle, that is merely an accident and not a twin. So first, my question is how were these angles measured?

[marco campos-venuti]

John, I think that when two crystals grow over a matrix, many different forces interact. In contrary, when a second crystal grows over a face of a first crystal, its orientation depends mainly by atomic lattice. The same happens in phenocrystals floating in magma.

[John S. White]

Not an answer to my question.,

[marco campos-venuti]

I mean that often it can be a twin instead of an accident.

[Pete Richards]

[marco campos-venuti]

I think auto-epitaxy is a synonym of twinning. Epitaxy is between two different minerals.
If a secondary xl (or a epitaxial film) grows over a main xl, its atoms will continue the lattice of the first. And they orientate following some laws. This is a twin.
The diagrams of reef look like epitaxy when he draws a small xl over a big one, but when he draws two xls of the same size they look more like twins. Isn't it?

[Reef]

[Frompard]

Thanks for your tips dear friend, like when getting a hand of help here!

[Reef]

Another law of twinning quartz under the working title "Ural". Theoretically, this law has already been predicted. Samples that confirm this law have not been found so far. It is an analogue of a twin by the law of Disentis, only with a different angle of rotation.

Description of this law:
The face of the rhombohedron (10-11) of one individual is parallel to the face of the prism of the other. One of them is a contact one. The angle of rotation relative to the normal to the prism m is 61.93 °. Vectors of the face zone [1-22] and [2-1-2] are parallel (r1b2q3 || r3q2b1 by C. Frondel). Most laws require instrumental confirmation. And the samples according to the laws of Disentis and Ural are even visually easy to diagnose (provided good visibility of parallel faces).

[Bob Morgan]

At the risk of adding confusion to this discussion, I prefer to call what Pete calls auto-epitaxy deposition twins. This is where a smaller crystal develops on an already established larger crystal and does so according to the twin law showing the same characteristic morphology of 'v' twins. This is quite common with Japan Law twinning. It is true of quite a lot of Greiserntal Law twins. In both cases there is more rapid growth in the twin notch, rapid widening of the smaller crystal, and markedly less growth in the supplemental angle on the other side of the smaller crystal. If the larger crystal is clear, the twin composition plane shows and is consistent with the twin law.
These depositions can be oriented in either of two directions 1) with the twin angle toward the termination of the larger crystal or 2) with the twin angle toward the base of the larger crystal. There are several instances where a larger crystal hosts separate twins of both orientations.


Now, this leads me to comment on the supposedly Reichenstein Law. There is no rational twin plane or twin axis for such a law. The photo's showing parallel pyramidal faces are therefore just the supplementary angle to the Greiseerntal Law twin that is oriented toward the larger crystal's base. Note too, that Japam Law twins have two sets of parallel pyramidal faces across the supplementary 96 degree angle, and that's not a separate twin law.

[Recovering - Academic]

[Recovering - Academic]

Interesting!!

[Recovering - Academic]