Overgrowths

These chondrules show the effects of multiple heating events coupled with accretion. They are chondrules and mineral grains that have gathered nebular dust, been heated and ended up with porphyritic olivine and pyroxene jackets.

A pair of related barred olivine (BO) chondrules appear blue/yellow in this otherwise porphyritic olivine pyroxene (POP) chondrule. NWA 4870 LL3.7 thin section in cross-polarized light (XPL). Field of view (FOV) is 3mm wide.

With the polarizing filters rotated to a different position the BO chondrules go to optical extinction, clearly revealing their position in the larger chondrule. NWA 4870 LL3.7 thin section in XPL, FOV = 3mm wide.

A POP chondrule with a BO chondrule in the center. NWA 1930 LL3 thin section in XPL, FOV = 3mm wide.

A large, partially disrupted POP chondrule with a BO chondrule occupying much of its center. NWA 1930 LL3 thin section in XPL, FOV = 6.4mm wide. POP chondrule 4mm wide.

A POP chondrule with a BO chondrule in the center. NWA 4522 LL3.5 thin section in XPL, FOV = 3mm wide.

With the polarizing filters rotated to a different position the BO chondrule goes to optical extinction revealing how much of the overall chondrule it occupies. NWA 4522 LL3.5 thin section in XPL, FOV = 3mm wide.

A chondrule consisting of a single large mineral grain surrounded by a thin layer of material that had gathered around the grain and then been heated. The whole is surrounded by opaque dust. NWA 3358 H(L)3 thin section in XPL, FOV = 3mm wide.

The same chondrule with the central mineral grain in extinction. NWA 3358 H(L)3 thin section in XPL, FOV = 3mm wide.

The chondrule below and to the right of center contains a dusky rusty red relict grain. NWA 3358 H(L)3 thin section in XPL, FOV = 3mm wide.

The irregular chondrule in the center of this field contains several dusky relict grains. NWA 3358 H(L)3 thin section in XPL, FOV = 3mm wide.

The chondrule just below center contains a BO chondrule and POP material. NWA 4522 LL3.5 thin section in XPL, FOV = 3mm wide.

With the filters rotated we can easily see that the BO chondrule occupies about half of the cross section. The chondrule just above center consists largely of two joined BO chondrules of different textures. POP material rounds out the chondrule. NWA 4522 LL3.5 thin section in XPL, FOV = 3mm wide.

Finally, another BO / POP chondrule. NWA 1930 LL3 thin section in XPL, FOV = 3mm wide.

With the filters rotated we can see that the bulk of the chondrule is composed of three discrete BO chondrules or chondrule fragments. NWA 1930 LL3 thin section in XPL, FOV = 3mm wide.

John Kashuba

John is a natural history enthusiast living in Oregon.