Hidden Hollow 2011
Sky Stones Meteorite Display
Are you curious about meteorites? A meteorite is a natural object that originates in outer space - a sky stone - that survives a fall to Earth. Meteorites that are recovered after being observed as they transited the atmosphere or impacted the Earth are called "falls". All other meteorites are known as "finds". As of mid-2006, there are approximately 1,050 witnessed falls having specimens in the world's collections. In contrast, there are over 31,000 well-documented meteorite finds. But what are they... And what do they look like?! Let's find out...
On February 12, 1947 all was quiet in the Sikhote-Alin mountains just north of Vladivostok and east of the China border. The Sun had just risen and the skies were clear and cold. Then it happened... At 10:38 a.m. local time a huge fireball appeared from 15 degrees east of north at a 41 degree angle, slashing through the skies at an estimated 31,000 miles per hour. For a few brief moments, it outshone the Sun... then about 3.5 miles above the Earth's surface, it exploded violently. What could have left a trail of smoke and dust 20 miles long? Evidence of its entry lingered in the air for hours afterwards and the light and sound of the impact were recorded from 200 miles away from the point of impact. The Sikhote-Alin meteorite fall is one of the most spectacular in recorded history. When it fragmented, the group fell to Earth together to cover an oval area that measured roughly half a square mile in size. One of the largest of the pieces formed a crater 85 feet in diameter and 20 feet deep! The explosion was so incredible that fragments were even lodged in trees... Just how big was this intruder from space? Experts judge the total mass of the Sikhote-Alin meteorite may have been near 1000 tons. The largest fragment of the Sikhote-Alin meteorite is a 1,745 kilogram specimen retained by the Russian government and now on display in Moscow. There are many specimens around that weigh as much as 1000 kg (displayed at the Russian Academy of Sciences), but most are smaller fragments. The Sikhote-Alin meteorite is one of the coarsest octahedrite known, with its Widmanstatten pattern nearly a centimeter wide. In smaller sliced specimens, this means you may not even be able to see crystal structure at the edges at all.
While Sikhote-Alin meteorites are all iron nickel alloy classified in Group IIB, they display two very distinct types. The first is known as a "Complete Individual". This group of Sikhote-Alin meteorites will display ablation and fusion crust. Many or most of these whole specimens will also contain regmaglypts, ablation cavities which look like small thumb prints in the surface of the meteorite. The second type of Sikhote-Alin meteorite fall specimen is called a fragment. These irregularly shaped chunks can only be likened to shrapnel - because this is what they are - the remnants of where the meteorite exploded above the Earth's surface. Some fragments may display shield shapes, show fine lines known as striations, or even give signs of their direction (orientation) from where they fell. The most well known and recent meteorite fall, the Sikhote-Alin, meteorites are classified into two categories. This meteorite qualifies as shrapnel, occurring during atmospheric explosions when the meteorite was descending, or during it's impact with the Earth. Come and take a look! Minerals present include: kamacite, taenite, plessite, troilite, chromite, and schreibersite. The shrapnel has a coarse and rigid surface, caused by the high temperatures and pressure on entering Earth's atmosphere. The specimens you see here will be approximately 93% iron, 5.9% nickel, 0.42% cobalt, 0.46% phosphorus, and 0.28% sulfur.
While you're looking, you might encounter a Canyon Diablo. Located in the heart of Arizona, Canyon Diablo Meteor Crater (the "Devil's Hole") was known to the Indians as a cursed place long before science began exploring its extra-terrestrial nature. The fall which caused what is now known as Barringer Crater occurred between 20,000 and 40,000 years ago and its mass may have been as much as 63,000 tons. When this 80 foot in diameter metal rock came flying down from the sky at 9 miles per second, it smashed into the desert floor unleashing 1.7 megatons of energy - the equivalent of 1.7 million tons of TNT. The result was a massive crater that spans 3400 feet across, runs 600 feet from rim to floor, and with a rim that raises above the desert surface an additional 200 feet. The first meteorites recovered and recorded from the Canyon Diablo meteorite fall gained research attention in the mid nineteenth century, but it was nearly the twentieth century before dealers began to make them available for collector trade. Because of their iron nickel content, geologist Daniel Barringer recognized the crater as a potential site for mining, staked a claim on the site and began searching for a huge mass - one which was never found. So what happened to the smoking gun? It vaporized on impact...
Thanks to the work of Harvey Nininger, we learned that some of the iron condensed after it was vaporized and just where those tiny droplets might be found. The largest Canyon Diablo meteorite ever recovered weighed a modest 639 kilograms and it is currently on display in the Meteor Crater Museum. Very rare are any pieces which weigh 100 kg and only a very, very few have been recovered that tip the scales at 10 kg. Out of all of this huge amount of material, Nininger estimates that only 30 tons of documented Canyon Diablo meteorites have been recovered - and as much as 8,000 tons may yet exist as material as finely grained as sand. Much of it may still remain around the Barringer Crater, but it is now closed to meteorite hunting and only previously collected samples are available on the legitimate market. Canyon Diablo meteorites are Group I, coarse octahedrites, iron nickel with varying mineralogy depending on whether or not the sample was shocked at impact. About 90% are typical iron nickel alloy. Some specimens show may show evidence of partial melting, recrystallization, and deformations.
Look around! Maybe this time you'll spy a Campo! When the Campo del Cielo (Field of the Sky) Meteorite was first discovered on a desolate plain spanning approximately 320 square kilometers in Argentina, native Guaycuru Indians correctly believed it to be a gift from the heavens. As Europeans began their exploration of the new lands, they heard of the legend and, in 1576, a Spanish governor sent an expedition out to retrieve a piece of this huge iron mass known he referred to as "Maison de Fierro" (Table of Iron). From there, no further records of Campo Del Cielo meteorites exist until nearly 200 years later when the Spanish returned once again - believing the find to be silver. However, when they smelted the material - they found only iron. At this time, they dug out a specimen which may have weighed as much as 18 tons and could have been the legendary Meson de Fierro - but apparently it disappeared. Early in the nineteenth century, more iron pieces were found in the area which may have belonged to the original Campo del Cielo Meteorite fall. By the 1900's, even more pieces had been found, yet the mysterious Meson de Fierro still remained elusive.
In the 1960's Dr. William Cassidy directed a research group in conjunction with Argentine geologist Dr. Luisa Villar discovering more and new craters and in 1969 turned up "El Chaco", which weighed 37 tons and is considered the second largest meteorite in the world - now protected for the heritage of humanity by the Argentinian government who strive to keep such monumental finds in their original locations. There is a provincial park where this great meteorite is currently on display. Dr. Cassidy returned to Campo del Cielo in 2005 with the aid of the National Aeronautics and Space Administration, Chaco Association of Astronomy and support of the municipal government of Gancedo, Argentina. This time he unearthed "La Sorpresa", weighing 14,850 kilograms and "El Wichi" or Meteorito Santiagueno", weighing 7,850 kilograms. Many of these large meteorites from Campo del Cielo are on display at various Argentine museums.
Most Campo del Cielo meteorites available to collectors come from the area of 12 small craters that range between 78 and 56 meters wide and 5 meters deep which in the southwestern part of the original fall field. By carbon dating bits of charred wood found in the craters, scientist believe the fall date of Campo del Cielo to have been between 3950 to 5800 years ago. The majority of the Campo iron meteorites (with the exclusion of newly discovered materials at a higher elevation) are rusted and show signs of corrosion from exposure to terrestrial chemicals, however some still display areas of fusion crust. The structure of a Campo del Cielo meteorite is known as a polycrystalline coarse octahedrite, and it carries a Widmanstatten pattern just thin enough to keep the coarse octahedrite classification. Campo del Cielo meteorites available for collection are classified in Group I - they are a iron nickel alloy which accounts for approximately 90% of the recovered material available for specimen sales. They originate from reputable dealers in Gran Chaco Gualamba, Argentina and are Coarse Octahedrites with possible silicate inclusions and may contain varying levels of iron and graphite. Because not all of the material belonging to the Campo del Cielo Meteorite has fully recovered, no estimation is given to the amount that's available - but the original mass was believed to have exceeded 800 tons!
Wow! Look over there! Not only is that a Sky Stone - but it's a gemstone, too. These wonderful pieces are called pallasites. Pallasites are a rare type of meteorites. Only 61 are known to date, including 10 from Antarctica, and only 4 are observed falls.
A pallasite is a type of stony-iron meteorite. It consists of cm-sized olivine crystals of peridot quality in an iron-nickel matrix. Coarser metal areas develop Widmanstätten patterns upon etching. Minor constituents are schreibersite, troilite, chromite, pyroxenes, and phosphates (whitlockite, stanfieldite, farringtonite, and merrillite). Pallasites were once thought to originate at the core-mantle boundary of differentiated asteroids that were subsequently shattered through impacts. An alternative recent hypothesis is that they are impact-generated mixtures of core and mantle materials. They are named for the German naturalist Peter Pallas (1741-1811), who located in 1772 a specimen near Krasnojarsk in the mountains of Siberia that had a mass of 680 kg. The Krasnojarsk mass described by Pallas in 1776 was one of the examples used by E.F.F. Chladni in the 1790s to demonstrate the reality of meteorite falls on the Earth, which were at his time considered by most scientists as fairytales. They certainly are as beautiful as a fairytail!
What's that big rock over there? That's not a rock at all... That could be a chondrite! Chondrites are stony meteorites that have not been modified due to melting or differentiation of the parent body. They formed when various types of dust and small grains that were present in the early solar system accreted to form primitive asteroids. Prominent among the components present in chondrites are the enigmatic chondrules, millimeter-sized objects that originated as freely floating, molten or partially molten droplets in space; most chondrules are rich in the silicate minerals olivine and pyroxene. Chondrites also contain refractory inclusions (including Ca-Al Inclusions), which are among the oldest objects to form in the solar system, particles rich in metallic Fe-Ni and sulfides, and isolated grains of silicate minerals. The remainder of chondrites consists of fine-grained (micrometer-sized or smaller) dust, which may either be present as the matrix of the rock or may form rims or mantles around individual chondrules and refractory inclusions. Embedded in this dust are presolar grains, which predate the formation of our solar system and originated elsewhere in the galaxy.
Most meteorites that are recovered on Earth are chondrites: 86.2% of witnessed falls are chondrites, as are the overwhelming majority of meteorites that are found. There are currently over 27,000 chondrites in the world's collections. The largest individual stone ever recovered, weighing 1770 kg, was part of the Jilin meteorite shower of 1976. Chondrite falls range from single stones to extraordinary showers consisting of thousands of individual stones, as occurred in the Holbrook fall of 1912, where an estimated 14,000 stones rained down on northern Arizona.
Don't stop there... I see something shiny... Is that metal? You bet. Iron meteorites consist overwhelmingly of nickel-iron alloys. The metal taken from these meteorites is known as meteoric iron and was one of the earliest sources of usable iron available to man. While they are fairly rare compared to the stony meteorites, they only comprise about 5.7% of witnessed falls.
Iron meteorites have been linked to M-type asteroids since both types of objects have similar spectral characteristics in the visible and near-infrared wavelength regions. Iron meteorites are thought to be the fragments of the cores of larger ancient asteroids that have been shattered by impacts. The IIE chemical class may be a notable exception, in that they probably originate from the crust of S-type asteroid 6 Hebe. Chemical and isotope analysis indicates that at least about 50 distinct parent bodies were involved. This implies that there were once at least this many large, differentiated, asteroids in the asteroid belt - many more than today.
Is that all? No way. There will be hundreds of different types of meteorites for you to examine! The world of "Sky Stones" is as big as... well... as big as the sky! Meteorites are always named for the place where they were found, usually a nearby town or geographic feature. In cases where many meteorites were found in one place, the name may be followed by a number or letter (e.g., Allan Hills 84001 or Dimmitt (b)). Some meteorites have informal nicknames: the Sylacauga meteorite is sometimes called the "Hodges meteorite" after Ann Hodges, the woman who was struck by it; the Canyon Diablo meteorite, which formed Meteor Crater has dozens of these aliases. However, the single, official name designated by the Meteoritical Society is used by scientists, catalogers, and most collectors. Most meteoroids disintegrate when entering Earth's atmosphere. However, an estimated 500 meteorites ranging in size from marbles to basketballs or larger do reach the surface each year; only 5 or 6 of these are typically recovered and made known to scientists. So what you see here is not only rare, but very documented and very unique! Make time from your busy day to enjoy!