Discussions

💬 Home 🔎 Search 📅 Latest

╳Discussions

💬 Home 🔎 Search 📅 Latest

Groups

Recent Images in Discussions

EducationCrystal system of autunite group minerals

28th Oct 2017 18:12 UTCNoah Horwitz

Was looking at some of the "uranium mica" minerals (autunite, torbernite, etc.) and I am rather confused about the crystal systems of these minerals. All of these minerals have similar formulas and form similarly-shaped crystals (to my untrained eye). But some of them are tetragonal (e.g. torbernite), some orthorhombic (autunite), and some monoclinic (saléeite). Even more confusing, orthorhombic autunite is apparently isostructural with monoclinic saléeite. How does all this work out? Thanks!

28th Oct 2017 20:31 UTCTravis Olds Expert

Each mineral of the autunite group is built from the same type of sheet containing uranium, oxygen, and phosphorous (or arsenic) atoms.

This is what the sheet looks like:

http://eurjmin.geoscienceworld.org/content/gseurjmin/27/4/589/F1.large.jpg

These sheets are stacked like a sandwich and held together by water molecules and cations between the sheets. Bonds holding sheets together are weak, and so break easily along these planes, hence the "mica" like cleavage. The major difference is within the interlayer, different cations have different coordination shapes, and can bind to different amounts of water. This results in symmetry differences, depending on the preferred packing of the interlayer cation, and also depending on its charge and coordination type.

One of the interesting minerals from the group is uranospathite, it has the largest distance between sheets of ~15 Å. It's the so-called "dagwood" sandwich of the autunite group.

Many other uranium minerals are built from sheets, and you can also think of them as different sandwiches with unique types of bread (the sheets) plus meat, cheese, or other fixings (cation + water) in between.

28th Oct 2017 21:36 UTCRalph S Bottrill 🌟 Manager

Autunite and saleelite are pseudotetragonal, the differences in structure and symmetry are relatively subtle and require single crystal XRD to be seen.

30th Oct 2017 20:27 UTCNoah Horwitz

Thanks Travis and Ralph for the good answers! Makes a bit more sense now.

31st Oct 2017 23:33 UTCŁukasz Kruszewski Expert

If the space groups or - at least - point symmetry groups do not match, then maybe a group division / redefinition is necessary?

2nd Nov 2017 14:05 UTCUwe Kolitsch Manager

Don't think so - the overall topology counts. Otherwise you would have to split up the comparable mica group as well.

Mindat.org is an outreach project of the Hudson Institute of Mineralogy, a 501(c)(3) not-for-profit organization.
Copyright © mindat.org and the Hudson Institute of Mineralogy 1993-2024, except where stated. Most political location boundaries are © OpenStreetMap contributors. Mindat.org relies on the contributions of thousands of members and supporters. Founded in 2000 by Jolyon Ralph.
Privacy Policy - Terms & Conditions - Contact Us / DMCA issues - Report a bug/vulnerability Current server date and time: May 5, 2024 14:42:01

Go to top of page