Bleached Chondrules

Bleached Chondrules are radial pyroxene and cryptocrystalline chondrules that are the result of low-temperature alteration by aqueous fluids flowing through fine-grained chondrite matrix prior to or during thermal metamorphism.

The bleached zones usually occur in the outer 100μm of a chondrule and encircle the darker unbleached core. The obvious optical difference between the dark cores and the outer white bleached zone is a consequence of the aqueous fluid interacting with the outer part of the chondrule. This interaction resulted in much of the chondrules' outer mesostasis (interstitial glass between the larger mineral grains) dissolving and being partially replaced by other minerals. Calcium rich zones also formed in these areas.

This is much of the reason the bleached zone is rather porous and highlights the difference in colour. The bleached zone is usually a white to light grey colour but can also be brown/yellow/orange due to terrestrial staining. There is an example of this in the NWA 1289 slice below where there is an orange/brown rimmed bleached chondrule in the top left corner.

There is a very informative and comprehensive abstract available for download below which gives a complete overview of Bleached Chondrules. The information used here is a very brief and basic overview sourced from this thorough study. The images below may be clicked to enlarge in a pop-up window.

Bleached Chondrules: Evidence for widespread aqueous processes on the parent asteroids of ordinary chondrites. (Grossman, Alexander, Wang & Brearley) (4.60MB)

NWA 3119 (LL4)	NWA 2288 (L3)	NWA 1289 (H3.8)
NWA 3112 (LL3)	NWA 2490 (H/L3)	NWA 2892 (H/L3)
Parnallee (LL3.6)	NWA 2398 (LL3.5)	NWA 3128 (LL3.8)

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