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NWA 820 L3-5

Ahmed Pani was selling this brecciated L chondrite in Tucson in February 2017. The mineral dealer from Vienna had one slice with a distinct dark clast that I should have bought. Luckily he had a similar slice available when we saw him at the Munich show in October. It turned out to be beautiful in thin section.

The dark clast is similar to the impact melt rock (crystallized fallback material) found in NWA 869, an L3-6. Barred olivine appears in several forms. There are distinctly brown cryptocrystalline and radial pyroxene chondrules. And numerous chondrules are jacketed with fine grained dust. One chondrule has a dark glassy mesostasis surrounding large olivine phenocrysts.

The variety of textures makes this thin section a pleasure to peruse.

The original part slice was about 2 inches long.

 

The other side with the dark gray inclusion to the center left.

 

Thin section in incident light.

 

Thin section in transmitted cross polarized light (XPL).

 

The dark gray inclusion is a clast of impact melt rock, shown here in the upper 2/3 of the photo. It is similar to those in NWA 869. Field of view is 8.5 mm wide. XPL.

 

Detail of above photo. FOV=3.1mm. XPL.

 

Detail of similar clast in NWA 869 L3-6. FOV=3mm. XPL.

 

Cross section of a barred olivine (BO) chondrule with multiple sets of bars. Chondrule is about 2mm in diameter. XPL.

 

Same chondrule in plane polarized light (PPL).

 

BO chondrule with fine bars. FOV=3.1mm. XPL.

 

Upper left chondrule has olivine laths in many orientations hence in many colors. FOV=3.1mm. XPL.

 

Same chondrule in PPL.

 

Same chondrule up close. The olivines are somewhat skeletal. FOV=0.3mm. PPL.

 

Left, fragment of a typical BO chondrule with a thick rim. Right, chondrule with thick coating of opaque dust. FOV=3.1mm. XPL.

 

Chondrule with a thick dust jacket. Diameter=1.3mm. Notice that the mineral grains are larger on the outside of the chondrule than on the inside. XPL.

 

Chondrules with opaque dust jackets. FOV=4.4mm. XPL.

 

Cryptocrystalline chondrule. Diameter=1.0mm. XPL.

 

Radial pyroxene chondrule. Diameter~1.2mm. XPL.

 

Radial pyroxene chondrule with fine laths. Diameter~1.2mm. PPL.

 

Radial pyroxene chondrule. Diameter~1.9mm. XPL.

 

Chondrule in center with brownish glass mesostasis.

 

Closer. FOV=3.1mm. PPL.

 

Same. XPL.

 

Contrasting size and chondrule types. The large central chondrule is 2.9mm in diameter. Off its edge at 7 o’clock is a colorful 0.7mm diameter chondrule with a neat shell of olivine grains. And at 5:30 o’clock is a 0.5mm diameter dust rimed chondrule. XPL.

 

Contrasting textures. FOV=3.1mm. XPL.
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