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Iron Meteorites

 

Prior to the age of professional meteorite hunting in hot deserts and their robotic recovery in the ice fields of Antarctica, most meteorite finds were irons. Due to their metallic composition and their extraordinary weight, even a layman can tell them from ordinary rocks, and they are easily recognized as foreign intruders. Moreover, most iron meteorites are quite resistant to terrestrial weathering, permitting them to be preserved much longer than any other type of meteorite. Finally, irons are usually much larger than stony or stony-iron meteorites. Irons rarely are fragmented upon entering the atmosphere and suffer much less from the effects of ablation during their passage through the atmosphere. In fact, the largest meteorites are irons. All iron meteorites taken together comprise a total known weight of more than 500 tons, and they represent approximately 89.3% of the entire mass of all meteorites known. Despite these facts, iron meteorites are rare since they represent just 5.7% of all witnessed falls.

 

 

Iron meteorites are composed largely of nickel-iron metal, and most contain only minor accessory minerals. These accessory minerals often occur in rounded nodules that consist of the iron-sulfide troilite or graphite, often surrounded by the iron-phosphide schreibersite and the iron-carbide cohenite. Despite the fact that some iron meteorites contain silicate inclusions, most have fundamentally the same superficial appearance.

 

 

 

Presently, iron meteorites are classified under two established systems. Just a few decades ago, iron meteorites were exclusively classified according to the macroscopic structures revealed when their polished surface was etched with nitric acid. Depending on these structures, they were separated into three classes: octahedrites, hexahedrites, and ataxites. Beyond that, modern research employs very sophisticated tools such as electron microprobes and X-ray spectroscopes, devices that enable us to detect minute amounts of trace elements such as germanium, gallium, or iridium. Based on the specific concentrations of these trace elements and their correlation with the overall nickel content, iron meteorites are classified into several chemical groups, and each group is thought to represent a unique parent body. We will elaborate on the different classification schemes and groups of iron meteorites below.

 

 

 

Structural Classification

 

 

Nickel-iron metal in iron meteorites occurs in the form of two distinct alloys. The most common alloy is kamacite, named for the Greek word for "beam". Kamacite contains 4 to 7.5% nickel, and it forms large crystals that appear like broad bands or beam-like structures on the etched surface of an iron meteorite. The other alloy is called taenite for the Greek word for "ribbon". Taenite contains 27 to 65% nickel, and it usually forms smaller crystals that appear as highly reflecting thin ribbons on the surface of an etched iron. Depending on the occurrence and the distribution of these nickel-iron alloys, etched iron meteorites display characteristic structures that are used to classify iron meteorites into octahedrites, hexahedrites, and ataxites.

 

Structural class

 Symbol

 Kamacite mm

 Ni %

 Related chemical classes

 

Hexahedrites

 H

 > 50

 4.5 - 6.5

 IIAB, IIG

 

Coarsest octahedrites

 Ogg

 3.3 - 50

 6.5 - 7.2

 IIAB, IIG

 

Coarse octahedrites

 Og

 1.3 - 3.3

 6.5 - 8.5

 IAB, IC, IIE, IIIAB, IIIE

 

Medium octahedrites

 Om

 0.5 - 1.3

 7.4 - 10

 IAB, IID, IIE, IIIAB, IIIF

 

Fine octahedrites

 Of

 0.2 - 0.5

 7.8 - 13

 IID, IIICD, IIIF, IVA

 

Finest octahedrites

 Off

 < 0.2

 7.8 - 13

 IIC, IIICD

 

Plessitic octahedrites

 Opl

 < 0.2, spindles

 9.2 - 18

 IIC, IIF

 

Ataxites

 D

 -

 > 16

 IIF, IVB

 

Chemical Classification

 

Modern meteoritics classifies iron meteorites according to a chemical classification system using nickel and the trace elements gallium, germanium, and iridium, to define distinct chemical groups. Other trace elements used to resolve groups are antimony, arsenic, cobalt, copper, gold, thallium, and tungsten. The concentrations of the trace elements are plotted against the overall nickel content on logarithmic scales to resolve well-defined chemical clusters, each representing a distinct chemical group. Fourteen groups, designated by Roman numbers and letters, such as  "IAB", have been recognized so far, with each group comprising five or more members. It is believed that the iron meteorites of each chemical group share the same origin and formed on a common parent body.

 

In the following, we will briefly discuss each chemical group, its primary properties, its relationship to certain structural classes, and its most famous members. However, we have to take into account that over 15% of all iron meteorites don't fit easily into the existing classification scheme. These irons are designated as ungrouped, probably representing more than 50 different parent bodies. We also have to consider that we won't be able to identify these parent bodies because most of them must have been destroyed in order to become a source for iron meteorites. Most iron meteorites were formed in the cores of small differentiated asteroids that were disrupted by devastating impacts shortly after their formation. They are true remnants of other worlds that once existed in the early solar system.

 

 

When cut, polished and etched with acid, most of the iron meteorite reveals a unique pattern called Widmanstatten figures. This particular crystal structure is formed by nickel-rich and nickel-poor metallic bands as a result of an extremely low rate of cooling over thousands/millions of years. This Widmanstatten structure is absolute proof of a meteoric origin for many iron meteorites, as it is not something that can be copied or forged in a lifetime.....in the laboratory, in has only been replicated on the microscopic scale.

> Meteorite Classification Index


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