FMF - Friends of Minerals Forum, discussion and message board

[Rei]

I recently got one of those "round tuits" and setup my sandblaster after acquiring some various blasting agents so I could clean mineral specimens better. I've had some bad results, some great results, etc... but also some unexpected results. And one of those is stilbite.

I've got a lot of stilbite-Ca specimens, some nice quality, most not so much. Trying glass blasting led to amazing results with all stilbite that I tried it on - removing any ugly brownish surface deposits and leaving it with a bright sheen without any visible erosion (if there is erosion, it's quite slow). In general, the blasting was a big "quality upgrade" on all of my specimens. Yet glass is Mohs hardness 5. Stilbite is hardness 3,5 to 4. Shouldn't the glass be scratching the surface, leaving it dull and eating it away?

I'll add that I've tested it on species that I sent in for analysis and came back as stilbite-Ca, so there's no question that it's really stilbite that I'm trying it on.

I've also had the occasional bit of weirdness with the dulling (sometimes practically eliminating) of features on botryoidal what-I-thought-was-chalcedony. Not sure what to make of that.... my main guess is that there might have been some non-quartz surface coating that was enhancing the underlying texture. Yet some of my transparent pieces seemed to get a bit scratched up too. I'd break these into two categories:

1) Those which I'm just baffled by because they otherwise really do seem to be quartz-based.

2) Those that I had assumed were quartz because I thought my only calcite was massive, but on second glance might actually be calcite. Vinegar testing was unclear. I may try to dig them out of their substrate a bit better and maybe see if I can get some birefringence. :)

But this is all a side topic. The main question is, why would stilbite be resistant to glass blasting?

[alfredo]

Mineral hardnesses are usually given using the Mohs scale, which compares resistance to scratching. But the damage caused to a mineral surface by impacting grains does not necessarily correlate to the Mohs hardness. There are different kinds of hardnesses.

I was once studying the effects of abrasion by wind-blown desert sand. Tourmaline got abraded faster than quartz, even though on the Mohs scale tourmaline is slightly harder than quartz. Mica, which is very much softer than quartz on the Mohs scale, resisted desert wind abrasion remarkably well - I suspect that's because of its elasticity - wind-blown sand grains just bounce off.

So you can't necessarily use the recorded hardnesses of minerals to predict which ones will or will not survive attempts to clean them by glass bead "air abrasion" equipment.

[Rei]

Thanks, that would help explain it - I have to admit, I was rather confused ;) I can actually picture that, stilbite seems to have a little bit of "give" to it, at least compared to quartz, while quartz is very rigid, so if there's any entrained crystalline silica in the glass at all (I've been recycling used blasting material) it could scratch it up. Like mica stilbite forms as a series of of tabular crystals (albeit elongated), giving the possibility to yield between the crystal planes, while quartz has a continuous 3d structure. So that makes sense.

[GneissWare]

[Mike Wood]

Hi Rei, I've never seen a glass-blasted stilbite before, and I was curious to see what they looked like!
I don't suppose you have a before and after photo you could post for us?

Cheers, Mike

[Rei]

[Mike Wood]

Hi Rei, thank you for taking the time to blast some more samples and do some photography. The results are quite impressive for removing that brown crud on the outside of the specimen, which can take a long time to wash and brush off (my usual method).
Also the familiar-looking matted needles are worth blasting away if they are full of dirt and stained brown, especially if there are more interesting crystals underneath. On your specimen the little cauliflorous crystals could be either stilbite or heulandite but more likely stilbite. It looks like towards the left on this picture, there are a few individual crystals that look rather like heulandite.
If the needle-like crystals are mordenite then this could be a bit dangerous to dry blast, maybe more so than mesolite/scolecite; it is wise to do this outdoors, you dont want this kind of dust hanging around the house...

I've seen plenty of this kind of material on Skye where I go collecting, and normally find that blasting the specimen with water quickly removes most of the mesolite (or mordenite) which goes down the drain.
Of course, if I found a nice clean specimen with upstanding mesolite/ scolecite etc. crystals then I would treat it very carefully, if at all. Maybe a gentle water spray to remove dust and tiny black particles from between the needles (holding the specimen upside-down).
I guess water in Iceland could be a bit awkward to use in the winter, with the very low temperature outside - the drains could freeze the waste water ?

Thanks again for your report :-)

Mike