Northwest Africa 2211 (NWA 2211)

(Northwest Africa)

Found: Jan 2005

Classification: Ordinary chondrite (L)

History: A single stone, weighing 27.46 kg, was purchased by David DGregory in January, 2005.

Physical characteristics: (Katrina van Drongelen, ROM) The main mass weighs 27.44 kg (38x34x21 cm), pieces used as type specimen at UCLA total 85.9 g, two slices at UCalg total 165 g, for a total mass of 27.69 kg. The surface of the meteorite consists of a fairly uniform, ~1 mm thick, pitted fusion crust that exhibits minor weathering. This fusion crust is variable in color from black to red-brown. The surface has many irregular to circular depressions and appears to have lost fragments during atmospheric passage. The main mass has one dark gray cut face, which reveals a melted aphanitic section that contains vesicles and comprises approximately 2/3 of the face. The other component of the cut face is an unmelted chondritic fragment containing sub-millimeter to millimeter chondrules and little Ni-Fe metal. The sample exhibits variable magnetism, suggesting an uneven distribution of the metallic components within the sample.

Petrography: (Katrina van Drongelen, ROM) Three slides were used to study the petrography of this meteorite, each from a texturally distinct section. Each represents a significant degree of thermal metamorphism, ranging from petrographic grade 6 to achondritic. The least thermally altered of the samples contains poorly delineated chondrules, barely distinguishable from the matrix material. The chondrules range in size from 0.75 to 1.25 mm and are of a variety of types, including porphyritic pyroxene, porphyritic olivine, and radiating pyroxene chondrules. The corresponding section contains vesicles on a macroscopic scale, also visible in thin section as very irregular voids (0.5 to 1.5 mm) containing minor crystallization on the interior wall. Minor glass is present. The second thin section is of a section of the meteorite which has lost its original chondritic texture due to thermal alteration. The groundmass of this section is very fine grained and glassy with a fairly uniform distribution of opaques (troilite, taenite, and kamacite). Remnants of chondrules are present in minor amounts (including a porphyritic olivine chondrule with its original outline lost). The groundmass contains 40 μm to 0.25 mm subangular olivine and pyroxene crystals, as well as irregular vesicles (0.5 to 1.5 mm). Subtle flow (or ductile deformation) can be observed in the groundmass. A glassy vein cuts across this slide, abruptly separating the main achondritic section from a petrographic grade 6 section containing poorly defined (but relatively intact) chondrules. The third, most thermally metamorphosed thin section does not contain any definable chondrule remnants. The groundmass is fine grained and glassy, with pyroxene and olivine crystals within it. The crystals are 80 μm to 1.2 mm in size and are generally angular to subangular. The groundmass contains opaque grains with a moderately uniform distribution, but with a minor degree of parallel orientation, suggesting flow. Vesicles are also present in this section, but are generally smaller (0.4 to 2.0 mm; average 0.5 mm), associated with opaques, and infilled with glass and crystalline material.

Geochemistry: Mineral compositions and geochemistry: Oxygen Isotopes: (Karen Ziegler, UCLA) δ¹⁷O = 3.952‰, δ¹⁸O = 5.718‰, Δ¹⁷O = 0.933‰

Classification: (Alan Rubin, UCLA) Ordinary chondrite (L, impact melt breccia), S6, W3.

Specimens: Type specimens: A total sample mass of 27.44 kg (main mass) is on deposit at the ROM. A total sample mass of 85.9 g (three pieces plus fragments) and three thin sections are on deposit at UCLA (catalog # LC2256). A sample consisting of two slices totaling 165 g are on loan to Dr. A. Hildebrand at UCalg.