FMF - Friends of Minerals Forum, discussion and message board

[prcantos]

Hi. Antonio Reolid, a member of the Spanish-side FMF Forum ask for some help with the crystallography of this sulphur micro crystals. Can you help him?

The questions are:

1) Are there two different crystallization phases?
2) What crystallographic forms can you recognize here?

Thank you.

[Pete Richards]

As we have said many times in this forum, interpreting morphology from photographs/images has considerable potential for error, but my interpretation of the first image is that it shows two pinacoids, two prisms, and a bipyramid. Further, I would guess that the crystal is elongate along the c-axis (this is just a guess, but it is informed somewhat by the steepness of the bipyramid). In this setting, the forms might be {100}, {010}, {110}, {210}?, and {111}. These forms were used to make the drawing below.

The crystal is divided into two parts by the diagonal irregular line seen in the SEM image, the surface manifestation of a healed fracture, and the part nearest the termination on the c axis (at bottom right) is offset from the rest.

The second crystal has a quite different habit - equant rather than elongated. The topmost face appears to have edges that make angles other than 90° with each other (a parallelogram not a rectangle), but I suspect this is due to the angle at which the image was made. I suspect that the angles are actually 90°. My interpretation is that the top or uppermost face, and the four faces that are nearly vertical in the image, belong to the three pinacoids that are perpendicular to the three orthorhombic axes. Then there are two different prisms between the pinacoids, and one bipyramid. An additional thin beveling face on the left side belongs to another prism.

This interpretation is consistent with the morphology shown in the image, but it is not the only possible interpretation. Verification of the morphology of either crystal would require goniometric measurement, which obviously would not be possible with these crystals.

The difference in habit between the two crystals might indicate that they formed at different times and under somewhat different conditions. However, it is not possible to say whether the difference reflects "different phases" of crystallization, or just some unknown differences at the specific sites of crystallization. Having observations on a number of crystals would help. If they all crystallized in the same phase, one might expect a gradation of habit between these two examples, but if they represent crystallographically-meaningful "different phases", one might expect to find two distinct habits, each represented by multiple, similar-looking crystals.

[prcantos]

Thank you, Peter. I'm sure this information will be useful for Antonio.