scratch test

Meteorites do not contain any substantial amount of quartz, whereas quartz is common in many terrestrial rocks. Quartz is harder than any of the common minerals in meteorites. Quartz is so hard that it will easily make a deep scratch in glass. Even if you press hard, a meteorite will at best only make a weak scratch mark. (Use a sharp edge of the rock.)

In the photo above, the top scratch was made with a quartz crystal. The next mark was made with a piece of chert, which is another form of quartz. The two scratches at the bottom were made with ordinary chondrites (click on image for enlargement of bottom portion). Ordinary chondrite 1 left some rusty material along the weak scratch mark.

Bottom Line:

If you have a rock that bites into glass and makes a scratch mark like the two at the top of the photo above, then it contains quartz and it is not a meteorite.

If your rock does not make a scratch, then the rock is still probably not a meteorite because many terrestrial rocks do not contain quartz.

Do Lunar Meteorites Scratch Glass?

First, let's define terms. By glass, I mean window glass or bottle glass. Some glasses are harder than others.

Do lunar meteorites scratch glass? I've tried it with a couple. They don't scratch glass any better than the ordinary chondrites above. Quartz bites into glass; you can feel it and you can hear it. Chondrites and lunar meteorites slide across the top. If you press hard, the meteorite breaks and maybe a weak scratch happens in the glass.

But, an opinionated correspondent thinks I'm wrong. Below are excerpts from four e-mails he sent me in one day after I told him that it looked to me, from a photograph he sent, that his rocks contained quartz. I suggested that he try the scratch test.

"I think your wrong. Lunar meteorites are different than those cheap iron meteorites. Also, how on Earth can you think a hard rock that survived through falling through the atmosphere and landing on the ground can't even scratch glass. Also, most meteorites contain metal. Metal EASILY scratches glass. Your so called test refers to whether it scratches glass one way or scratches glass another way so either way even your test states that it will at least make some kind of scratch.

Also, all meteorites are NOT even close to being the same so when I'm obviously talking about a lunar meteorite and you keep referring it as a regular meteorite test then you're universal way of testing it instead of focused direction is doomed to fail.

I think this is yet another so called expert that go by the book so much that he doesn't realize that a scratch test was only designed to be a minor test for only some meteorites and certainly not a test to even consider ruling it out as not even being a meteorite."

I think he's wrong. First, he confuses mineral hardness, an intrinsic property of minerals, with rock coherency. Lunar meteorites are coherent, but the three main minerals of which they are composed, plagioclase feldspar, pyroxene, and olivine are not as hard enough to scratch glass.

Also, metal in meteorites is not hard. I've also tried scratching glass with the sharp edge of Gibeon, and iron meteorite that I have here in my office. No scratch. Some steels may be harder than glass, but the iron-nickel metal in meteorites is not. Besides, lunar meteorites contain at most only a few percent iron-nickel metal.

"The scratch test is the worst test that I've ever heard of regarding meteorites.
Not only do meteorites contain contain harder than glass substances like iron metal, but some have even been found to contain HARDER THAN DIAMOND crystal in them:"

Yes, some chondrites (but, I thought we were talking about lunar meteorites?) contain really hard stuff like silicon carbide, diamonds, and boron carbide. But these rare minerals are present as microscopic grains in minute quantities, not enough have any effect on the meteorite's abrasiveness.

"Your own website describes and admits that lunar granite contains more than 60% SiO2(quartz). Quartz scratches glass. Therefore, lunar meteorites CAN scratch glass. Stop misguiding people with lunar meteorites!"

Yes, there are some rare granites in the Apollo collection. They're small. I'm not even going to think about trying to scratch glass with one (and I've got several here). But, there still hasn't been a lunar meteorite that is a granite. Silica minerals (quartz, tridymite, cristobalite) do occur at trace levels (<1%) in some lunar meteorites but, again, not in high enough levels to have affect the rocks abrasiveness.

"I found out that lunar meteorites that are Feldspathic Breccias all are rich in Plagioclase Feldspar. Plagioclase Feldspar is about 6 in hardness. Therefore lunar meteorites could easily scratch glass because glass is only about 5.5 in hardness"

This is better (and read this). But, try it! The reality is that plagioclase feldspar doesn't bite into glass. Plagioclase is brittle. If you press hard, it breaks. Quartz doesn't do that.

So, I stick with everything I say above. If you can make a scratch in glass like the quartz or chert examples above, then your rock isn't a meteorite.

Prepared by:

Randy L. Korotev

Department of Earth and Planetary Sciences
Washington University in St. Louis

Please don't contact me about the meteorite you think you’ve found until you read this and this.