NWA 1685 LL4 Bessey’s BL

NWA 1685 LL4 is a handsome, interesting meteorite. Numerous stones were introduced in two releases by Dean Bessey and called BL. Individuals are glossy black on the outside and clearly brecciated on the inside. The brecciation is distinct because chondritic clasts differ in shock darkening, impact melt rock is featureless grey and many clasts are bounded by thin dark impact melt glass veins.

Thumbprinted, dark and glossy individuals of NWA 1685 LL4. One centimeter scale cube.

 

Cutting diagram for thin section maker. Note brecciated texture and fine grained clast at edge of portion to be sectioned. NWA 1685 LL4.

 

Thin section with a few features noted. Many more are apparent with magnification and attest to multiple impacts that darkened, crushed and melted this material. NWA 1685 LL4 in transmitted light.

 

Angular clasts at top of slide. This and all close-up photos below are with transmitted light and partially crossed polarizing filters, hereafter pxpl. This method seems to distinguish smaller textural features better than either plane polarized or xpl.

 

A light, angular, bell shaped clast bounded by a thin black vein (actually, a sheet) of melt glass. NWA 1685 LL4 pxpl.

 

Shock-darkened angular clast bounded by dark shock melt. NWA 1685 LL4 pxpl.

 

Somewhat rounded (or faceted) shock-darkened chondritic clast bounded by dark shock melt. NWA 1685 LL4 pxpl.

 

Rounded light-colored chondritic clast. NWA 1685 LL4 pxpl.

 

Rounded light-colored chondritic clast. NWA 1685 LL4 pxpl.

 

Fracture through a clast and chondrule with injected melt. NWA 1685 LL4 pxpl.

 

Grains of olivine in the fine-grained clast crystallized from impact melted rock, set in glass. The larger grains are probably the relict olivine grains mentioned by Phil McCausland. NWA 1685 LL4 pxpl.

 

Detail of olivine grains. NWA 1685 LL4 pxpl.

 

NWA 869 L3-6 is also known for its fine-grained impact melt rock inclusions. Analysis by Metzler and others found that some are simply melted parent material (minus volatiles) and some contain a foreign component, presumed to be an impactor.

 

Fine-grained olivine in impact melt clast. Note crack with terrestrial weathering products. NWA 869 L3-6 pxpl.

 

Detail of olivine grains and crack. NWA 869 L3-6 pxpl.

About the Author

John is a natural history enthusiast living in Oregon.
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