Learning Center What is a mineral?The most common minerals on earth Information for Educators Mindat Articles The Elements The Rock H. Currier Digital Library Geologic Time

Minerals by Properties Minerals by Chemistry Advanced Locality Search Random Mineral Random Locality Search by minID Localities Near Me Search Articles Search Glossary More Search Options

The Mindat Manual Add a New Photo Rate Photos Locality Edit Report Coordinate Completion Report Add Glossary Item

Mining Companies Statistics Users Mineral Museums Clubs & Organizations Mineral Shows & Events The Mindat Directory Device Settings The Mineral Quiz

Photo Search Photo Galleries Search by Color New Photos Today New Photos Yesterday Members' Photo Galleries Past Photo of the Day Gallery Photography

Discussions

💬 Home 🔎 Search 📅 Latest

╳Discussions

💬 Home 🔎 Search 📅 Latest

Groups

Recent Images in Discussions

Improving Mindat.orgSafflorite formula

6th Nov 2014 01:57 UTCReiner Mielke Expert

http://www.mindat.org/min-3500.html I think the safflorite formula cannot be as written since the structure cannot contain more than 80% cobalt without becoming clinosafflorite. In other words there is no such mineral with that formula. See: Acta Crystallographica E64 (2008), i62

6th Nov 2014 03:08 UTCRob Woodside 🌟 Manager

Does the paper say what the 20% is that stabilizes safflorite against a clinosafflorite transition? Such an element X (Ni or Fe) required for the safflorite structure would suggest (Co,X)As₂

6th Nov 2014 14:07 UTCUwe Kolitsch Manager

The paper says:

"Chemical analyses of various natural and synthetic samples reveal that Pnnm safflorite always contains some amounts of Fe and Ni, whereas materials with 80–100% (mole) CoAs2 crystallize in monoclinic P21/c symmetry (Holmes, 1947; Swanson et al., 1966; Radcliffe & Berry, 1971)."

Sounds convincing although the cited refs. are a bit old and p/T conditions of crystallisation are not addressed (but maybe in the cited literature?).

The IMA list uses the (simplified) formula CoAs2.

6th Nov 2014 14:11 UTCReiner Mielke Expert

Hello Uwe,

Where on the IMA site does it give the compositional limits of each species? Can you give me a link to this?

6th Nov 2014 14:17 UTCUwe Kolitsch Manager

"Where on the IMA site does it give the compositional limits of each species?"

The IMA list of approved species (http://pubsites.uws.edu.au/ima-cnmnc/IMA_Master_List_(2014-09).pdf) gives sometimes formulae containing an x (e.g. A1-xBx..., with x = ...) or an n (e.g. ... .nH2O, with n = ... or n ~...-...).

6th Nov 2014 14:30 UTCReiner Mielke Expert

This list obviously contains mistakes. For example alloclasite is shown as CoAsS which is the formula for cobaltite.

6th Nov 2014 15:25 UTCUwe Kolitsch Manager

"For example alloclasite is shown as CoAsS which is the formula for cobaltite."

Have you ever heard of dimorphism...?

6th Nov 2014 17:20 UTCReiner Mielke Expert

Well then the formula given in Mindat is wrong because it is not the one given by the IMA.

6th Nov 2014 21:07 UTCAlfredo Petrov Manager

Formulae can be written in somewhat different ways for different purposes, Reiner.

Contrary to what several contributors to Mindat think, there is no such thing as an "official formula".

7th Nov 2014 01:03 UTCReiner Mielke Expert

Well I didn't know we could pick and choose what we wanted from the IMA.

7th Nov 2014 01:19 UTCAlfredo Petrov Manager

The IMA makes official decisions about mineral names and mineral species validity. There is no committee to choose an "official formula".

You can write a formula that reflects just a theoretical end-member composition, or you can write one that also includes common substitutions. Some writers simplify formulae so that they merely show the relative numbers of different atoms, others try to use the formula to more fully represent the structure of the mineral. Such structural formulae can be more or less complicated, but are never adequate - You can't fully represent a 3-dimensional structure with a 1-dimensional formula. And there are even differences in the way Americans and Europeans write formulae. We really all need to exercise a bit more mental flexibility and stop making a big deal about the exact format for mineral formulae (...and I'm not referring specifically to you, Reiner; there have been several cases recently of users bitching about a Mindat formula not being written exactly the same way as in some other reference).

Sometimes of course there are real errors and it is great to get them corrected. But a variation is not necessarily an error.

7th Nov 2014 02:02 UTCReiner Mielke Expert

I agree with you, I like the formula for alloclasite that mindat uses.

11th Feb 2015 17:56 UTCReiner Mielke Expert

The formula should be changed as recommended by Hexiong Yang,* Robert T. Downs and Carla Eichler 2008.

11th Feb 2015 23:51 UTCRob Woodside 🌟 Manager

Iwas waiting to find time to put a reference in to .this paper. When i get back from Tucson, I'll add the ref and change the formula, if no one does it first

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: April 25, 2024 11:11:55

Go to top of page