An Article In Meteorite Times Magazine
by Jim Tobin

 

Spheres and Sluglets

In my continuing fascination with Meteor Crater metallic spheroids I took a small sample from one of the bottles in my collection. This extracted sample consisted of about a hundred particles which were all responsive to a magnet. I had gotten this supply a while ago and had done some weight measurements only but no real examination.

The small sampling immediately revealed under the microscope that it was in fact an unsorted sample of magnetics from the soil around the crater. It contained a high percentage of metallic spheroids but, also had what Nininger had named sluglets, the occasional volcanic cider fragment as well as very rarely some small objects that looked and acted like tiny impactites.

The process to separate the various items from the mix was a challenge that plagued Nininger for a long time. He finally devised a method that would make use of their individual magnetic responsiveness and allow him to concentrate the individual products and eliminate the ciders and iron shale particles. I found no bits of shale in my small sampling so some sorting for that may been done.

In the follow micrograph are examples of the different components of the mix. It is about 90% what I would call spheroids based on photographs of what Nininger called spheroids. Though many are not spherical most of his were not very elongated either. They are often shown in his photos as what could be called blobs they are often flattened. Many of this form can be seen in the following photograph.

The sluglets on the other hand are much more linear and often curved. In the photo two can be seen that have hook like projections on one end. They are not smooth on the surface. They are not bubbly looking like some of the metallic spheroids. They look like tiny bits of torn metal. In fact as Nininger observed they look like tiny Canyon Diablo meteorites. On this point Nininger spoke in terms of a continuum of size from these tiny sluglet through very small meteorites to the larger Canyon Diablo individuals. Tests on sorted and cleaned sluglets gave metal content consistence with Canyon Diablo meteorites. This would seem to confirm that they are not condensation product as the metallic spheroids are. Metallic spheroids tested for Nininger had percentages of cobalt and nickel very different than that of ordinary Canyon Diablo meteorites.

Nininger also described a particle in the soil that he called oxide spheroids some of these appear to be in this sampling as well. Oxide spheroids as pictured in Nininger’s books are very round and smooth surfaced. Several much more spherical and smooth surfaces particles can be seen in the photo offered here. Several appear to have holes in them. Whether these holes are bubbles exposed by cracking away a thin surface of melted material or whether the holes were always present is not something I can not determine.

Gilbert and Barringer along with Tilghman had all found the spheroids in the soil around Meteor Crater. All of them had attributed their presence there to melting during flight. This has always raised the question in my mind of what would be the nature of the particles that melted off the asteroid as it burned through our atmosphere. Much smaller meteoroids leave smoke and dust trains in the atmosphere that last for hours. From the painting of the Sikhote Alin passage they can be tremendous clouds of smoke and particulate residue. Only in recent years has any attempt been made to collect this material with high altitude aircraft. Not knowing the entry direction for sure and the great length of time since the impact makes searching now for products of the atmospheric passage of Meteor Crater's asteroid a futile quest in this writer’s opinion. To attempt it would seem to require that the investigator go much farther away from the crater to avoid finding normal condensation products. They would then take thousands of samples from a vast ring perhaps as large as one or two hundred miles in circumference. Hoping to recover particles that settled out 50,000 years ago and remain still today.

My guess on Meteor Crater is that the material of the smoke train would be very much like the oxide and metallic spheroids left from the explosion of impact. I would guess that oxide particles would predominate since there was likely less oxygen starvation than was present in the explosion cloud which allowed the metallic spheroids to form. Like a welder can control what is formed by his oxy-acetylene flame. So the amount of oxygen in the explosion cloud at Meteor Crater seems to have dictated the type of particles produced. The outer areas with oxygen being present produced oxide particles like the scaly and porous weld produced by an improper too oxygen rich torch flame. Whereas, the core of the explosion cloud in an oxygen starved state produced metallic particles. Continuing my analogy; the reducing flame of a properly balanced welding torch applies heat only without creating waste oxides.

Winds during and just after the impact distributed the various particle types around the crater. Though Nininger found evidence that the concentrations in the soil were greatest on the northeast side and extended out from there. Anyone visiting the crater often, has noted that the prevailing winds are still from west to east. Some redistribution of the particles has likely occurred by wind activity since the impact. The crater itself may have provided some wind break for the redistribution.

At about the one o’clock position in the photo can be seen the light colored egg shaped particle that resembles the impactites found at the crater though much smaller. It is not a lump of sand grains. It is a poorly fused object with a rust spot on it. It could be a metallic spheroid that has a not quite glass coating. It is interesting whatever it really is.

I never stop being amazed by these little melted bits of meteorite. Imagine a rain of hot metal falling as chunks amid blocks of rocks. Then tremendous choking dust and ash mixed with the fall of metallic rain. The pressure wave from the explosion would have scoured off the land surface for miles and the heat blast would have incinerated everything for many miles as well. With a mushroom cloud rising to miles in the air, boiling like that of any A-bomb detonation.

Of the thousands of tons of nickel iron that hit that high plateau 50,000 years ago the greatest amount that remains is not in the form of the Canyon Diablo meteorites we have in our collections or see in museums, it is the tiny particles in the soil. The survivors: spheroids and sluglets.

A Tale of Two Crusts

As a short second offering this month I submit a pair of pictures with descriptions. The meteorite is an unclassified NWA weighing 280 grams. It is a relatively fresh stone with black fusion crust covering about 90% of its surface. However, the fusion crust is distinctly different on one of the sides. It is a nearly perfect depiction of what Foote described seeing in some of the Holbrook specimens. Here is that quote:

“The primary crust, begun on the entrance of the meteor into our atmosphere with its high planetary velocity, and prior to the first explosion, is almost universally present. It coats broadly rounded surfaces and is generally dull black, being about 0.3 mm thick. A checking or crackling of this crust, due to unequal expansion, is often noticeable, as shown in fig. 4. The secondary crust, formed on the fractured surfaces produced by this first disruption, is somewhat shiny and thinner than the older crust. Moreover, the fractures it covers are hackly and irregular, and it even fails to hide occasional protruding chondrules, indicating that the superficial dissipation by combustion had not proceeded far enough to round off the sharper corners amid smaller prominences.”

As you can see in the accompanying photos the fusion crust on most of the specimen is smooth with only shallow regmaglyphs. But, on one face the fusion crust is clearly a well yet incompletely melted fractured surface. Though the fusion crust on the entire meteorite is very thin, this secondary fusion crusted area is very thin indeed. Many time we see meteors that break up and continue to display bright luminous tails from many of the resulting pieces. This secondary fusion crust is one of the features that results from break up early in flight where velocity is still high and ablation continues after fracturing.

 

A primary fusion crusted surface on an unclassified NWA of 280 gram weight.

Secondary fusion crust on the surface of the same meteorite as pictured above. The bumpy surface of the fracture is still visible though covered by fusion crust.