THe High Possil Meteorite under the Microscope

The High Possil Meteorite under the microscope

The images here are thin-section photomicrographs. Thin slices of rock are made with a diamond saw, then stuck onto a glass slide, and ground so thin that they become transparent (30 microns, or thirty thousandths of a millimetre is usual). This makes it east to see the constituent minerals, and textures of the rock. Some of these photos were taken from a slice which was polished on the upper surface as well. Such slices allow opaque minerals to be examined in refllcted light, as well as the normal transmitted light.

Chondritic meteorites like this are fragmental rocks: they are aggregates of older grains and other rocks stuck together. Chondrites are distinguished by many of these grains being chondrules. These are roundish blobs of high-temperature minerals such as olivine or pyroxene, and are thought to have formed by shock melting of the primordial dust during the very earliest part of Solar System formation. Chondrites also contain other material: in the case of High Possil, larger crystals of the same minerals, and blobs of iron-nickel alloys, again, formed by early heating events in the dust cloud forming the Solar System.

Mineral compositions are consistent in the High Possil meteorite. The olivine is all close to Fo75Fa25, orthopyroxene around En78Fs21Wo1, and clinopyroxene around En48Fs7Wo45. (Bevan et al, 1985)

click image to enlarge	Barred olivine chondrule in thin section. Crossed polars. Note the "crater" or "bite" at the left side of the round chondrule. These are common in chodrules, and may be caused by impact, or where round blobs of another mineral have broken away. The field of view is about 1mm wide.
click image to enlarge	The same, in plane polarised light. The field of view is about 1mm wide.
click image to enlarge	An irregular-shaped chondrule showing radiating fibrous crystals, probably of olivine and orthopyroxene. The bright grain just NE of the centre is a rounded olivine crystal. Crossed polars. The field of view is about 1mm wide.
click image to enlarge	The same, in plane-polarised light. Note the opaque minerals: these are blobs of iron-nickel alloys, and iron sulphides. The field of view is about 1mm wide.
click image to enlarge	A barred olivine chondrule. Crossed polars. The field of view is about 1mm wide.
click image to enlarge	The same in plane polarised light. The field of view is about 1mm wide.
click image to enlarge	An unusual barred olivine-plagioclase chondrule. The texture here may be the result of textural equilibration of a fibrous chondrule which has been kept at high temperature for a while. This can result in olivine fibres "pulling in" to form coarser rounder blobs. Crossed polars. The field of view is about 1mm wide.
click image to enlarge	A low-magnifiction view, under crossed polars, showing a round chondrule set in a crystalline interchondrule matrix, mainly made of pyroxene. The field of view is about 2mm wide.
click image to enlarge	The same, in plane-polarised light. The field of view is about 2mm wide.
click image to enlarge	A radiating fibrous orthopyroxene chondrule in polished-thin section. Crossed polars. The field of view is about 1 mm wide.
click image to enlarge	The same, in plane-polarised light. The field of view is about 1 mm wide.
click image to enlarge	The same, in reflected light. In this mode, transparent minerals appear dark, because they are not very reflective, but shiny metals, which are opaque, appear bright. The paler grains are iron-nickel alloy minerals (mainly kamacite), while the browner one are troilite, an iron sulphide. The field of view is about 1 mm wide.
click image to enlarge	Iron-nickel alloy (grey) and troilite (brown) in reflected light. The field of view is about 1 mm wide.
click image to enlarge	Coarse barred olivine-rich (or orthopyroxene?) chondrules, with granular/porphyritic chondrule (top) Crossed polars. The pale birefringence coolours in this section may be due to it being ground too thin. The field of view is about 1 mm wide.
click image to enlarge	The same, in plane polarised light. The field of view is about 1 mm wide.
click image to enlarge	The same, in reflected light. The field of view is about 1 mm wide. Grey blobs are iron-nickel, brownish are troilite.
click image to enlarge	Barred olivine-plagioclase chondrule, with nearby euhedral olivine crystal (pale, just ENE of centre). The field of view is about 1 mm wide.
click image to enlarge	The same, in plane-polarised light. The field of view is about 1 mm wide.
click image to enlarge	olivine (or orthopyroxene?) rich chondrule (cross-polarised light). The field of view is about 0.5mm wide. The low birefringence colours may be due to the section having been ground too thin.
click image to enlarge	coarsely crystalline interchondrule orthopyroxene (clear), with metal/sulphide (black) in plane polarised light. The field of view is about 1 mm wide.
click image to enlarge	The same, under crossed polars. The field of view is about 1 mm wide.
click image to enlarge	The same, in reflected light, showing pale iron-nickel (kamacite?), and brown troilite. The field of view is about 1 mm wide.

This text and photos by John Faithfull (2.6.2004).

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