NWA 15915 achondrite-ung

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Northwest Africa 15915, achondrite-ung, is the remains of a long gone inner solar system body. That planet was large enough to differentiate then cool slowly. The 2.84 kg meteorite is nearly all pyroxene – mostly iron-free augite and 5% to 20% iron-free enstatite, depending upon the sample. Small opaque spherical grains lie within and between the millimeter size pyroxene grains.

 

 

We bought our sample from Mark Lyon. Dave Mann thin sectioned it. It is 32 mm long.
Much of the edge is heat affected but no fusion crust remains.

 

 

This portion is magnified below.

 

 

Heat affected mineral grains along the sample edge. Field of view is 3 mm wide. Plane polarized light, PPL.

 

 

Same area in cross-polarized light, XPL.

 

 

This 8 mm wide area is magnified below and shown in PPL.

 

 

The opaque spherules are mostly sulphides with iron metal. The sulfides are Daubréelite – Fe2+Cr3+2S4, Troilite – FeS and Alabandite – MnS.

 

 

The same area in XPL.

 

 

This 2.6 mm wide area is magnified below.

 

 

Most, but not all spherules have stained their surroundings by weathering. FOV=2.6 mm. PPL.

 

 

The same view in reflected light.

 

 

The same view in XPL. Enstatite exsolution lamellae tend not to extend to the edge of augite grains.

 

 

The leftmost of the four sulfide / metal spherules. Apparent diameter is 0.35 mm. Reflected light

 

 

The second spherule. Apparent diameter is 0.22 mm. Reflected light.

 

 

The third spherule. Apparent diameter is 0.2 mm. Reflected light.

 

 

The fourth spherule. Apparent diameter is 0.13mm. Reflected light.

 

 

Again, the fourth spherule. Apparent diameter is 0.13mm. Reflected light.

 

 

One last spherule.

 

 

There is layering even within the metal in this 0.3 mm diameter cross section.

 

 

Top left quadrant of the above cross section. FOV=0.2 mm wide. Reflected light.
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