File:Jiddat al Harasis 091 L6.jpg

English: The Triangular Cleavage of the Jiddat al Harasis 091 L6 Chondrite: These meteorites are made of thousands of little spheres called chondrules, molten droplets of silicates that accreted together with some free metals to form the first asteroids 4.55 billion years ago. They are the earliest condensates from the swirling gas disc of our solar system, and preserved in the asteroid belt, having never been absorbed into the molten mass of the planets. They are pristine time capsules from before the planets formed.

But looking at them, I was struck with a curiosity: What makes some chondrites have a rough triangular cleavage?

I am relatively new to collecting, so perhaps this is understood, but I have not seen it mentioned in any of the meteorite books I have read. All five of these stones are from the same find in Oman, an L5 chondrite called Jiddat al Harasis 091 in the Met Bull. Each of them has triangular forms visible to the eye, some more rounded than others. All five here are whole stones, from 374g to 5.75kg with a beautiful dark reddish-brown sand-polished patina.

From my intuition from terrestrial geology, one would expect sharp cleavage in homogenous crystals (from cut diamonds to the cubic cleavage of salt) and quite the opposite in a fine-grained conglomerate. I would think that chondrites are conglomerates from space.

As an L5, it has been “metamorphosed under conditions sufficient to homogenize olivine and pyroxene, convert all low-Ca pyroxene to orthopyroxene, cause the growth of various secondary minerals, and blur chondrule outlines” as defined in the Met Bull. Does this process commonly cause bulk material cleavage like this? If it were a homogenized mineral like an achondrite stone, I might expect to see bulk effects like this, but not from a fine-grained composite rock, with some impact-induced blurring of outlines. Each grain would presumably have a random orientation versus its neighbors, if it had any orientation at all.

And lastly, if this is a common phenomenon, could it be used to judge the degree of metamorphosis from visual inspection at the time of discovery — a rounded stone being closer to an L3 as a possibility and an angular form almost certainly an L5, L6 or L7 (assuming it is an L chondrite, but the same generalization could hold for H and LL I would think)?