Into the Unknown
Every unclassified meteorite is kind of an adventure for me when I first begin to investigate it. I have had this particular broken stone for a number of years. It predates my numbering system so I am not fully sure where I got it. At some Tucson show before three years ago probably.
It had some things in its favor that kept me from passing it by as I normally would have such a seriously broken small stone. It had a reasonably fresh fusion crust. But, more important the crust had interesting cracks throughout the surface. I assumed as I still do since it was very fresh on the broken surfaces that they were contraction cracks. The stone itself has no cracks. The ground mass of the stone was eroding from around the metal giving the outside broken surface a spikey texture of tiny metal mountains with deep valleys between. The stone might have a soft friable nature I thought. There were little shiny spots all over the outside betraying the fact that there was a lot of metal inside. Or armored chondrules perhaps.
I’m sure back then that I did not pay more than a few dollars for the stone. And I have enjoyed it a great deal since. I have looked at it many times. One of the broken surfaces was nearly flat. I often thought that with almost no loss of material I could get a good look at the inside of the stone. Just a few seconds on the diamond lap and I would have a big window ground off.
So this month my article is the story of the start to finish home investigation of Meteorite X.
Its been a while since I wrote any ludicrously accurate weights in my articles. So I will use some this month. The stone before lapping was 72.66151 grams. After being lapped it now weighs 70.81526 grams. As I ground it I saw that there was a good amount of metal in the stone. I took off a little more than I originally thought to get a great surface of metal exposed. The stone was also very soft to grind. It cut very fast and easy. The powder stayed on the surface which I thought was odd. Soon I realized why. The meteorite has a grainy texture and quite porous ground mass. This maybe why the metal is so altered while the stone is still so fresh. The water gets in and rusts the metal but does not travel very far to stain the matrix. Just a guess. The metal grains all have rings of limonite with an area of stain beyond. While as the photos show the stone itself and the polished face are still mostly an off white color.
I have made no thin section of the stone since I don’t really desire to cut it. But, chondrules can be seen on the polished face and also on the broken exterior surfaces. Melted chondrules can be seen in the fusion crust as well. So I can begin to make some guesses about the petrological type of the stone. The chondrules are not perfect, their edges are a little rough, but they are still well defined. Many are radial and that structure can be easily seen in the microscope. A photo of one is included below for you guessing enjoyment. There are also many unround masses which appear to be oval chondrules. It seems that a type 4-5 is appropriate for this stone since chondrules are easily seen and good but still altered .
Now comes the trickier part. Is it an L or H or something else. The size of the grains of metal are consistent with many Ls in my collection. However, the density of the grains is greater than in most of my Ls. The grain size is larger than in the average H. Yet, there are lots of grains in a given space. Its magnetic response is fairly consistent with that of an H. I realize that how hard a magnet sticks to the surface of a stone is a pretty crude measurement. We are just having fun here. And we all do it. We all make guesses based on magnetic stickiness when we think about getting unclassified stones.
So are there other chondrite types with a bounty of metal grains and few chondrules with a fluffy internal sturcture? I began a short hunt to see if anything matched. There wasn’t anything that really made think other than H or L Ordinary Chondrite, but it couldn’t hunt to do a little research. I might learn something new along the way. I got out my picture books of meteorites beginning with Robert Haag’s collection book then Marvin Kilgore’s collection book. I did not see anything that exactly match my meteorite but there were some Ls that had similar size and density of metal grains. There were a few with very porous ground masses to be seen in the Encyclopedia of Meteorites. And I learned a little about the relationship of porosity to shock level.
I suppose that at some point I should make a disclaimer about never using one of my home spun guesses as if it was a real classification. I never would do that. But, one of the end goals for me in my study of meteorites is to be able to look at a stone and arrive at a good estimate of what it is. Today it is all microprobe and isotope ratio data that makes a classification. However, it was not always so. In the past it was weights and measurements, and percent of metal that determined what class a meteorite was. What has not changed so much is the use of microscopes and thin sections. That remains much as it was a hundred years ago. Since I do not want to crush a portion of this meteorite to extract the metal this guess will be based on only observations as a hand specimen. I have when the meteorite was large enough taken a small amount and done a percentage of metal analysis.
A complete classification of a meteorite today will include the weathering and shock levels as numbers on their respective scales. So on to the weathering of Meteorite X. As can be seen in the pictures there is staining around the metal grains. There are no cracks so we will not see staining along any of those. The matrix remains very light in color further out from the metal grains. The metal grains are altered. The limonite is a halo around the grains. So using the guidelines of the weathering scale we may be able to make a guess at the weathering stage. There appears to be no involvement in the silicates yet. They are unaltered. This eliminates stages 5 and 6 which have chemical alteration of the silicates as part of their description. There is clearly visible oxidation of the metal grains with staining; so W0 is not applicable either. It is reserved for only the very freshest meteorites. Usually those remaining dry and found within a few days will be W0.
So we are left with W1,W2, and W3 as possibilities. At W1 there is only minor oxidation of the metal grains and other iron rich compounds like troilite. Nothing is said at this stage about percentage of metal which has been altered. I would make a guess from this that there would be very minor staining with a thin rim of altered metal since that seems to be what one could expect with “minor oxidation”. Since W2 begins with 20% altered metal less than that would seem to fall in W1. Ten percent alteration would be a thin rim around the grains and hard to estimate accurately by visual means. Some staining would probably also be noted close to the metal grains. We do not have W1 with Meteorite X.
W2 brings us to moderate oxidation of metal grains, and a percentage of alteration to limonite of 20-60%. The accompanying photos of a typical grain in Meteorite X will help to make an estimate of the amount of alteration that has occurred. The true color photo shows the metal and the alteration ring. The staining is going off frame as a much lighter orange color. The areas that are green in the false color picture are quite accurate for the limonite altered portion of the grain. So, what’s a good guess? A wide all encompassing guess would be 25% to 40% alteration. I think 30% is close. However, the exact number does not matter. It is clearly not 60%+ the W3 range. So I think we have our answer. W2 is the weathering grade of Meteorite X.
Now for the stickiest part of home guessing. The shock stage. It is almost impossible to determine shock accurately without a thin section. Since I do not want to cut this stone I will not be preparing a thin section of it. But, that does not make shock completely undeterminable. It just makes some of the levels untestable. All the low levels of shock S1, S2, and often S3 show nothing visible to the naked eye. All the shock features of the stone are alterations to mineral grains. These alterations can only be seen under a microscope mostly in crossed polarized light. S1 of course is an unshocked stage. Beginning with S3 however, opaque melt veins start to be visible in some stones. Their absence is however not a reliable measurement for less shock. But, if they are present, spider webbing through the stone then you certainly have some visual evidence of shock. Likewise melt pocket begin being seen at Stage 3.
Melt pocket and metal veins will increase through S4-S6. A predominance of metal pockets in a stone makes a good indicator for S5-S6. Finally, shock beyond S6 destroys the chondrite structure and the stone becomes an igneous rock.
We have no visible shock features and a stone that is very porous. With easily visible crystals in the matrix that have perfect shape. Voids and pores in the rock, well defined chondrules, and no veins or melt pockets does seem to make for a low shock number of S1-S3. Closer than that is hard to guess without that thin section. Just to be conservative I am going to say S2 since most meteorites are probably not shockless at stage S1.
My guess after all this for Meteorite X is that it is a L4 W2 S2. There were some surprises and there is still the matter of the amount of metal. But it feels at this time like an L. This kind of exercise can be a lot of fun and a great learning opportunity. Many of us are going to have a big group of unclassified meteorites in our collections forever. There will probably never be a compelling reason for most of them to be sent out to a lab. We might as well get to know them. Even if it is only your best guess.