FMF - Minerals Forum, discussion and message board

[bugrock]

I am wondering if there is a reliable physical method short of formal analysis to distinguishing groutite from the more common manganese minerals manganite and pyrolusite. All three vary in habit.

If the question is manganite vs pyrolusite the different color of streak seems to work (brown vs dark gray/blank) but it appears the color of the streak for groutite varies and may overlap the colors seen with the other two minerals.

As groutite is less common and specimens often have a higher price tag it would be very useful to be confident of identity.

Thanks for your help.

[Peter Megaw]

This is a tough call...mostly because few care that much about manganese oxides...historically soft Mn-ox got called pyrolusite and hard Mn-ox got called psilomelane. Having worked extensively with Mn-oxides I gave up on visual identification unless very well crystallized and stuck to XRD. I have been pleasantly surprised by recent interest in some Mn-ox species, like the recent romancheites from Chihuahua. it took Raman to ID it though...

[bugrock]

Peter,

Thanks. Have been told the 'psilomelane" minerals are particularly difficult in that you need both elemental analysis and a spectrum to pin a name on them.

George

[Montanpark]

George,
Peter has made a very valid point. An example of my experience: since i have got quite a lot of specimens a few years ago from the collection of the late Paul Hanses from the Bülten-Adenstedt mine, THE locality for Groutite and Ramsdellite in good specimens in Germany (and maybe Europe) i was very pleased (but also quite puzzled) seeing the all black and lustrous crystallized (but with quite differing habit) manganese oxides from there in the lot. So i decided to do an XRD on all visually different specimens (with one example for each "habit group") and was surprised that all gave groutite patterns. Some also had manganite and very few ramsdellite matches so intergrowth and pseudomorphs are possible (and well known from there). Of course i can not guarantee that ALL of the approx 50 specimens are groutite but i have a good probability from the exemplary XRD analyses. Visually i would say it is very hard to determine but the "classic" groutite habit from this mine is very characteristic. For any other mine from where i do not know the specimens from own experience i wouldn´t be able to visually identify groutite without doubt so any phase analysis would be necessary.

Regarding the "psilomelane" stuff .. no chance without chemical and other analytical methods to nail down exactly.

cheers
Roger

[Luiz Menezes]

Inside cavities in granitic pegmatites the most common form of manganese oxyde is cryptomelane; for me it is impossible to guess without a qualitative chemical analysis, done through micro-probe, that will detect the presence of K, and further X-ray diffraction has confirmed that it is cryptomelane; some few times K cannot be detected, and then the X-ray normally shows that the mineral is pyrolusite; it is totally impossible to identify them just by the habit.

Luiz