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17th Nov 2018 14:57 UTCMark Casto

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17th Nov 2018 15:47 UTCPavel Kartashov Manager

The first time listen about "titanian karelianite". A lot of Ti-V complex oxides are known, but karelianite itself usually contains admixtures of Cr, Fe or Al rather than Ti.

What is locality of your sample? Outokumpu?

17th Nov 2018 15:51 UTCMark Casto

Locality: New England, found in float ore close to hydrothermal vein deposits of silver, lead, iron...etc.

17th Nov 2018 19:01 UTCFrank K. Mazdab 🌟 Manager

"karelianite titanian, syn" (with even a specific mixed formula, mind you) came up in the search because that compound had been added to the database that your analyst was searching. Notice the bulk of its peaks nearly overlap with your goethite. There are a few peaks that don't, but I wonder if those aren't actually Fe2O3 (hematite), a mineral likely to be admixed with goethite. Hematite and karelianite (V2O3) belong to the same mineral family and have the same crystal structure, and would be expected to have similar XRD patterns, differing only due to the size difference between Fe3+ and V3+. Natural karelianite has a very limited geologic occurrence... for example at a place like Outukumpu as Pavel suggested, or at other V-enriched localities like Merelani. It is a rare mineral.


The problem here is that the analyst gave you a printout that appears to say, "this is the match", instead of giving you a printout saying, "these are the possible matches". So minerals that might be more likely and geologically reasonable, but whose measured peaks are perhaps slightly offset from the database spectrum, get incorrectly eliminated from consideration.


I'd like to see an overlap of the hematite XRD pattern here to see how defensible karelianite really is.


edit: let me add that the "syn" at the end means that the database pattern that came up in this match was measured on someone's lab-created material, and that researcher had the pattern added to the XRD database. Karelianite a natural mineral, but this particular material, with added 0.6 apfu Ti, is not. I bet the actual pattern for natural karelianite, which is not typically Ti-enriched, would not be such a great match for your errant peaks (at least no better or worse than hematite).

17th Nov 2018 19:20 UTCFrank Keutsch Expert

Often the ID programs of the XRD software has a large number of possible matches. Just picking the top one is dangerous, really all the top ones should be listed. Not sure whether this happened here, but I can see this happening (I have fallen for this myself a few times).


Frank is correct: The lines identified as karelianite also are a really good match for hematite. I will not comment in detail but I have previously encountered some similar problems. False analysis in a way is worse than none as then one can be convinced of the wrong thing...


So very, very likely this is hematite with goethite...


Frank

17th Nov 2018 20:25 UTCReiner Mielke Expert

While on this subject of false ID's it amazes me that commercial companies do bulk powder XRD ( containing as many as a dozen minerals) and then come up with a list of minerals and their %. Is there something I don't know about modern XRD? When I last worked on trying to quantify a two phase system based on peak areas I had a difficult time. Have things improved so much that you can do it reliably with a 12 phase system?
 
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