Scratches on lunar meteorite slice

[bapowellphys]

Hi all,
I just acquired a gorgeous lunar meteorite slice (NWA 15373) and I notice it has a multiple fine scratches to its surface. On one side these are parallel (but following a curved path) suggesting they were the result of the cutting process? (There is also uneven shine on this face).

On the other side, the lines are more randomly arranged, though tend to follow a similar direction:

Since I spent a decent chunk of change on this specimen, I'm mostly wondering whether this is something that can be expected or whether this is suggestive of poor preparation, care, etc.

Secondly, is there anything that I can do to polish out these scratches? I'm an amateur with no specialized knowledge or tools.

Thanks for your help!

Brian

 

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[Red-Coat]

That’s a good-sized lunar specimen.

For sure the parallel curved marks in the first picture will be from a circular lapidary saw. Usually these marks arise from the blade slowing or stalling if too much pressure is applied, such that the cut is uneven. The blade needs to be allowed to do the work in a slow and steady manner

The marks in the other picture don’t look like saw marks. I would guess that the cut was going well until either the blade or the specimen slipped, or the last part of the specimen gave way before the blade reached it. That can result in a ‘snap’ which leaves a very uneven surface on that part… and it looks like an attempt has been made to flatten it with something abrasive.

Regarding removal of these marks, the questions are: can you?; and should you?

Certainly you can, if you are careful, but it’s a laborious process… especially if the marks are deep. You would need sandpaper in a range of grit sizes. Place the sandpaper on a had flat surface and rub the specimen on it, face down, pressing firmly and using a circular motion. Start with 100 grit if the marks are deep and the specimen is robust. Otherwise, start at 200 grit. Work progressively through to 600 grit in small steps. Don’t press too hard if the specimen is fragile, or not generally flat (check if it ‘rocks’ up and down on a flat surface), since it could break or chip where there are impact shock veins.

But… should you? The consequence of polishing out the marks is that you will lose mass and, the deeper the marks, the greater the loss. Without care and experience, there is also a risk of the specimen breaking, and the risk increases if the specimen is very thin or the shock veins are weak. At the end of the day it has to be your decision, but it’s still a nice specimen either way. The decision may also be influenced by where you are on the spectrum running from “I’ve got a piece of the moon”… to “serious study of the petrology under a microscope”.

From a personal perspective, if it were mine, I would polish out on one side only… the side with the shallow scratches, if the surface is otherwise relatively flat.

 

[bapowellphys]

Great answer, thanks for taking the time Red-Coat. Yes, it's a very attractive specimen all things considered; the shallow scratches really don't show unless viewed at the right angle. On the reverse side, with the more obvious saw marks, there is also an uneven-ness to the shine: is it safe to try and clean this with some isopropyl alcohol? I appreciate the help!

 

[Red-Coat]

Great answer, thanks for taking the time Red-Coat. Yes, it's a very attractive specimen all things considered; the shallow scratches really don't show unless viewed at the right angle. On the reverse side, with the more obvious saw marks, there is also an uneven-ness to the shine: is it safe to try and clean this with some isopropyl alcohol? I appreciate the help!

You're welcome. IPA won't do any harm, but I think it's unlikely to make any difference to uneven shine. You would still need to resort to an abrasive, but probably 800 grit or finer would do the job without too much effort. Again, the resullt will depend on how flat the surface is overall, unless you use an abrasive paste applied with a cloth rather than abrasive paper. Pastes are messy, need careful application, and washing with perhaps a soft brushing to remove any residue. If you use anything wet, use the IPA to force out any residual moisture, followed by drying in a warm place.

 

[galenrog]

Any good lapidary should be able to smooth the surface and, if desired, polish to state desired.

Time for more coffee.

 

[Emil W]

I would leave it as is. You really don't want to lose any of it's mass. Even though it would be a very minimal amount you'd lose, giving up even the slightest amount of the weight would be considered significant with such a rare specimen.

 

[Red-Coat]

Any good lapidary should be able to smooth the surface and, if desired, polish to state desired.

Time for more coffee.

Yes... but would they accept liability in the event of breakage (which would greatly reduce its value)?. Especially on a specimen which has obvious shock veins?

 

[galenrog]

Yes... but would they accept liability in the event of breakage (which would greatly reduce its value)?. Especially on a specimen which has obvious shock veins?

A knowledgeable lapidary would not. When I did lapidary, decades ago, my standard agreement for modifying any stone held clauses that allowed for stones to break. This was even for a simple cut of any kind.

The decision to increase the eye appeal is, in the end, with the owner.

Time for more coffee.

 

[pepperj]

That’s a good-sized lunar specimen.

For sure the parallel curved marks in the first picture will be from a circular lapidary saw. Usually these marks arise from the blade slowing or stalling if too much pressure is applied, such that the cut is uneven. The blade needs to be allowed to do the work in a slow and steady manner

The marks in the other picture don’t look like saw marks. I would guess that the cut was going well until either the blade or the specimen slipped, or the last part of the specimen gave way before the blade reached it. That can result in a ‘snap’ which leaves a very uneven surface on that part… and it looks like an attempt has been made to flatten it with something abrasive.

Regarding removal of these marks, the questions are: can you?; and should you?

Certainly you can, if you are careful, but it’s a laborious process… especially if the marks are deep. You would need sandpaper in a range of grit sizes. Place the sandpaper on a had flat surface and rub the specimen on it, face down, pressing firmly and using a circular motion. Start with 100 grit if the marks are deep and the specimen is robust. Otherwise, start at 200 grit. Work progressively through to 600 grit in small steps. Don’t press too hard if the specimen is fragile, or not generally flat (check if it ‘rocks’ up and down on a flat surface), since it could break or chip where there are impact shock veins.

But… should you? The consequence of polishing out the marks is that you will lose mass and, the deeper the marks, the greater the loss. Without care and experience, there is also a risk of the specimen breaking, and the risk increases if the specimen is very thin or the shock veins are weak. At the end of the day it has to be your decision, but it’s still a nice specimen either way. The decision may also be influenced by where you are on the spectrum running from “I’ve got a piece of the moon”… to “serious study of the petrology under a microscope”.

From a personal perspective, if it were mine, I would polish out on one side only… the side with the shallow scratches, if the surface is otherwise relatively flat.

Just wondering something and the use of sandpaper vs emery cloth vs crocus cloth for the final polishing.

 

[bapowellphys]

Thanks everyone for the feedback! I do have one additional somewhat unrelated question: what are people's thoughts on displaying a specimen like this? I mean, it's gorgeous and I would hate to lock it away (I've got an acrylic display box, not airtight though I do have a desiccant in there which seems to keep the humidity at 50%). But, I also don't want to be foolish and ruin this thing over time via moisture exposure.

 

[Red-Coat]

Thanks everyone for the feedback! I do have one additional somewhat unrelated question: what are people's thoughts on displaying a specimen like this? I mean, it's gorgeous and I would hate to lock it away (I've got an acrylic display box, not airtight though I do have a desiccant in there which seems to keep the humidity at 50%). But, I also don't want to be foolish and ruin this thing over time via moisture exposure.

I wouldn’t worry too much. Lunar meteorites are generally stable. They typically have low porosity and so aren’t much at risk of deterioration from oils and sweat arising via handling. Your particular specimen (NWA 15373) is an impact breccia and so will contain at least some asteroidal debris in addition to components from the lunar crust. That means there will be some tiny inclusions of metal (iron with low nickel content) but they will be very much a minor phase to the extent that rust-spotting shouldn’t be a problem… even for long-term storage. Including a sachet of silica gel might afford some protection (obviously less so in a display box that isn’t airtight or if the sachet isn’t regularly refreshed) but probably isn’t really necessary.

I tend to keep stable specimens in (non-airtight) Riker mounts without any silica gel, but I also sometimes use “membrane boxes”. The latter have the advantage that both sides of a specimen can be seen without removing it from the container, while leaving it safely sealed between the two membranes which are pretty effective at excluding air.

I would say that (especially if the specimen is thin) it’s probably much more at risk from breakage if roughly handled than it is from any kind of chemical deterioration.