Storage and Weathering
Over the last several years like many other collectors I have accumulated a number of unclassified meteorites. Call them Sahara, NWA, or something else the result is the same. I donít yet know what type most of them are for sure. Many of them are complete or mostly crusted stones that I have not wanted to disturb by cutting open. On a few occasions they had a broken flat on one surface that I lapped to make a window. Those meteorites I have made guesses about what they are. I have many of these unclassified meteorites sitting around my office. I have been selective in what I got. I sought stones with good fresh fusion crust that did not look like NWA 869 or one of the other plentiful finds. I have many NWA 869 stones and love then too of course.
I needed a way to store many of these stones that were never going to be on display and to remember where they were gotten. I discussed my painting numbers on them a few months ago. So the remembering part is handled. As for their storage I have put them in plastic containers designed for an entirely different purpose. In sporting goods and bait stores you can find clear plastic containers that have a lid with snap clasps. They have little plastic dividers that slide into channels. You can create compartments of many different sizes. Originally intended to keep hooks, sinkers and snap swivels separate it was a great way to store all the smaller stones after they were numbered. The containers will not handle the bigger stones, but up to about 200 grams they will often still be thin enough for the lid to close. The NWAs have never presented a problem requiring a dry box like much of the rest of my collection. They are all very stable.
Not every meteorite I have is a black crusted beauty, however. I have on a few occasions bought a meteorite that was very weathered or really ugly. I have been interested as a meteorite hunter in how they become terrestrialized. What do they look like as their characteristic appearance is slowly destroyed by our moist environment? I have noticed several things that are all seen in the following old stone.
Cracks which have often been developing from early on continue and become greater in number. They widen till finally portions of the stone fall away and continue weathering on their own. If it was not the desert of North Africa I might suggest that freeze and thaw was involved in the enlarging of the cracks. But the apparent lack of much moisture to freeze and expand makes me desire a different process for the expanding of the cracks and the break up of the masses into smaller fragments. Of course over the thousands of years involved there must have been occasionally rain followed by freezing temperatures even in North Africa's deserts. And the climate has changed too over time.
The fusion crust on meteorites is a remarkably durable surface. It does however disappear through a variety of chemical and mechanical processes. We have seen during the last decade many desert meteorites with surfaces blasted clean of their fusion crusts. Their exterior now a smooth polish with desert varnish. Initially the color, sheen, texture and other characteristics of fusion crust are the best visual means of recognizing meteorites while searching. Later in the weathering process we are left with dark-brown, rusty cracking-apart rocks to test by magnet and diamond file. Often only these test can confirm that we have a meteorite and not just a brown rock. After a long time little of the fusion crustís assistance is left to help the visual hunter. But even at the late stage of cracking that the pictured meteorite is at the fusion crust in this case is still present. It has changed from black to a tan brown. In this stone even some of its texture of fine lines is still present. I lapped an area flat on this stone. You can see in the following photo the dark thin line of heat alteration of the fusion crust.
I have found that even when the meteorites are badly weathered and have been reduced to angular and layered ugly chunks, the internal state of the rock is often still of great interest and value. Often the metal is still there. Many times the ground mass of the meteorite is still moderately light in color. I have seen the opposite as well; meteorites with fresher external characteristics that were totally darkened inside. It would seem to be a personal journey to oblivion that each meteorite must take on Earth.
I understand the use of ugly in describing the outward appearance of some stones, and I have some really ugly meteorites. But, they are still meteorites, and they still offer us the opportunity to learn. Even by the way that they become unrecognizable they provide us with knowledge.