QUANTITATIVE ANALYSIS OF CRYSTALLINE TEXTURES AND STRUCTURES IN METEORITES CONTAINING FREE IRON

The purpose of this study is to complete a quantitative analysis of textural features of free nickel-iron found in meteorites of various groups. Free iron in meteorites occurs in the intergrowths of Fe-Ni alloys taenite and kamacite, called Widmanstatten patterns. The widths of the resulting lamellae are useful in that they correspond to the Ni-Fe content as well as the cooling history of the meteorite. Ataxites, with a high Ni content, have microscopic a Widmanstätten pattern. Hexahedrites, with low Ni content, have such large lamellae that a sample is often composed of just one, while octahedrites show ideal patterns with 0.2-50 mm widths. Pallasites, a group of stony-iron meteorites, contain olivine grains supported in an iron matrix that may display the same textures. Stony meteorites containing 25-31% iron are classified as H chondrites. This iron can be stored as either free iron or in other minerals. Four cut and polished samples were examined in this study, representing various types of iron bearing meteorites; an octahedrite (Odessa), a pallasite (Milton) and two distinct chondrites (Richardton – H5 and Abee enstatite). Samples were imaged with a digital camera and reflected light scope together with a high-resolution scanner. Images were analyzed for separate classes based on visual characteristics, such as color and light reflectance. The classes were identified as kamacite-taenite, troilite or silicates through examination of the images and hand samples. Individual image classes were loaded into a GIS suite and analyzed on the basis of shape and distribution. In the Richardton ordinary chondrite, for example, free-iron minerals make up ~13% of the analyzed area with a total of 379 free-iron mineral grains identified. These mineral grains have irregular boundaries resulting in an average roundness of 0.772 with 1.00 corresponding to a perfect circle and 0.778 to an equilateral triangle. Free-iron minerals on the surface of the Abee E chondrite occupied ~28.8%, with grains averaging a roundness of 0.641. The Milton pallasite has a continuous metal matrix supporting olivine mineral grain. This metal matrix represents ~21% of the analyzed meteorite surface and also has irregular grain boundaries. The observed lamellae on the Odessa iron averaged at 1.445mm, but varied from 0.11 to 2.9mm, showing a thermal gradient.