Two IMR Inclusions in NWA 869

About 1% of the volume of NWA 869 L3-6 is impact melt rock (IMR) according to Metzler et al. Their study tells us a lot about these highly variable fine grained inclusions we find in cut stones. Some IMR clasts include mineral and rock clasts, some do not. The crystallized fallback material is depleted in nickel-iron metal and sulfides. Grains that crystallized from the melt are olivine. Any pyroxene crystals are included relict grains. Oxygen isotope analysis show that some IMR match bulk sample values while some do not. The latter perhaps reflect inclusion of impactor material.

These photos are of two nearly adjoining IMR inclusions in one thin section.

Two IMR clasts, the smaller, more coarsely crystallized at bottom left and the larger, more finely crystallized clast to the right. The larger clast contains clasts within the crystallized melt. Field of view (FOV) = 12.5mm wide, cross-polarized light (XPL), NWA 869 L3-6.

Same, with arrows.

Portion of the fine grained clast. FOV = 3mm wide, plane polarized light (PPL), NWA 869 L3-6.

Portion of the fine grained clast. FOV = 3mm wide, XPL, NWA 869 L3-6.


Close-ups of the fine grained clast showing skeletal euhedral grains. FOV = 0.3mm wide, XPL, NWA 869 L3-6.

Portion of the coarse grained clast. FOV = 3mm wide, PPL, NWA 869 L3-6.

Portion of the coarse grained clast. FOV = 3mm wide, XPL, NWA 869 L3-6.



Close-ups of coarse grained clast showing skeletal euhedral grains, all at the same scale as the close-ups of the other clast above. FOV = 0.3mm wide, XPL, NWA 869 L3-6.

About the Author

John Kashuba
John is a natural history enthusiast living in Oregon.
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