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Identity HelpIlmenite or ferberite?

2nd Sep 2017 01:05 UTCNorman King 🌟 Expert

The photo below shows an opaque, black grain in a Mesoproterozoic conglomerate from Bahia, Brazil (right side of photo). It seems to have a tabular habit with conspicuous striations. I guess ilmenite or ferberite. Is there any way to favor one of those over the other? "Wolframite" might be the better term than ferberite since I do not have an analysis. Or maybe this view suggests something else besides either of those. The grain is about 2 mm across. Any suggestions or comments would be welcome. Thank you.

Norm King

2nd Sep 2017 01:24 UTCDon Saathoff Expert

Hello Norman,

Ilmenite is usually slightly magnetic. If you magnetize a small sewing needle and hang it on a length of thread, under the 'scope you can see any deflection when brought to the xl.

Don

2nd Sep 2017 02:37 UTCAlfredo Petrov Manager

Wolframite is not impossible in sediments, but quite uncommon. Being very brittle and having perfect cleavage, it tends not to survive mechanical transport very well.

Hard to tell luster from a photo, but I wouldn't eliminate rock-forming silicates from consideration yet either.

2nd Sep 2017 22:14 UTCRalph S Bottrill 🌟 Manager

I would agree with Alfedo, wolframite is very rare in sedimentary rocks compared to ilmenite, being chemically and physically unstable. Most ilmenite in conglomerates is partly to largely altered to "leucoxene", so may not be very magnetic. But some content would help - eg. is the onglomerate mineralised, what rocktypes are preent?

2nd Sep 2017 23:49 UTCAlfredo Petrov Manager

If this is one of the specimens of diamond-bearing conglomerate that have been marketed as being from Río Formoso, Bahia, then watch out for andalusite, kyanite and staurolite, which are sometimes present as crystal fragments or whole crystals with rounded edges. Am not saying your dark crystal is one of those, but just something to look out for in that material. The ones I studied, I don't recall seeing much else other than quartz and limonite, and sporadic bits of those aluminosilicates.

4th Sep 2017 12:21 UTCHarold Moritz 🌟 Expert

I would expect neither brittle mineral to survive in the high energy and corrasive depositional environment of a conglomerate. It is probably staurolite or an amphibole fragment.

4th Sep 2017 23:25 UTCNorman King 🌟 Expert

Thank you all for the comments and suggestions. I have seen reference to wolframite in heavy mineral sands of Brazil, but not often. Ilmenite is mentioned in almost all reports. So, of those two possibilities, ilmenite seems to be the better guess.

All of the grains are completely opaque, so I wasn't thinking feldpsar, amphibole, or pyroxene groups or the andalusite-kyanite-staurolite group. Only a few show the crystal habit like the one I pictured, though, and it may be there is more than one black, opaque mineral. Here is a statement, unfortunately not well written, but the information was taken from other reports, and seems to be typical of what I have seen in the literature so far for Minas Gerais and Bahia (e.g. Serra do Chapada--a possible locale for this sample). The last comment is puzzling, however, as I also have transparent, colorless, cleaved crystals (but somewhat rounded) that I guessed are a feldspar. Did I say that the large crystal at upper left is a diamond? Quartz is the most common large clast in this conglomerate. It appears to be vein quartz, not rock crystal as mentioned below.

"The stones are found in sands and gravels in the watercourses, and are accompanied by the minerals, which constitute the most important associations of diamond at Diamantina, namely, the oxides of titanium and of iron, tourmaline, and quartz (rock-crystal). In addition to these are a few others, which do not occur in Minas Geraes [sic]. In a sample of diamond-sand from the Serra da Chapada, Damour determined the following minerals: pebbles of rock-crystal, crystals of zircon, tourmaline, hydro-phosphates, yttrium phosphates, diaspore, rutile, brookite, anatase, ilmenite, magnetite, cassiterite, feldspar, cinnabar, and gold. Garnet and staurolite have also been observed here and recently euclase, but the last only as a rarity. Of these minerals, cassiterite, feldspar, and cinnabar have never been found in Minas Geraes in association with diamond." (http://www.minelinks.com/alluvial/diamondGeology16.html)

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