Northwest Africa 11761 (NWA 11761)

(Northwest Africa)

Purchased: June 2016

Classification: Mesosiderite

History: The meteorite was purchased by an anonymous collector at St. Marie-aux-Mines, France, mineral show in June 2016 from an anonymous Moroccan dealer.

Physical characteristics: The stone of 2258 g total weight has irregular shape; the surface is reddish-brown; the fusion crust is not preserved.

Petrography: (C. A. Lorenz, Vernad) The meteorite has a brecciated texture; it is composed of lithic and mineral fragments (50 vol%) embedded into a troilite-metal-silicate melt matrix (13, 22, 15 vol% respectively). The mineral fragments are pyroxene, feldspar and silica; the rock fragments are fine-, medium- and coarse-grained pyroxene-feldspar rocks of equigranular, subophitic or gabbroic textures, fine-grained clastic and melt-matrix breccias and minor dunitic and peridothitic rocks. Coarse-grained pyroxenes contain the fine exsolution lamellae. The rock contains 11 vol% of fragments of >2 mm in size. The matrix has heterogeneous texture varying in troilite, metal and silicates composition. Matrix silicates forming intergrowths with troilite are fine-grained subhedral pyroxene, anhedral feldspar and glass. Metal-and sulfide-poor matrix surrounding large lithic fragments are fine-grained melt rock. Metal forms irregular and globular grains in the troilite matrix and is composed of kamacite with inclusions of taenite. Accessory minerals are silica and chromite.

Geochemistry: Mineral compositions and geochemistry: (Kononkova N. N., Vernad) Matrix pyroxene is En_21.5-56.2Wo_0.5-8.4 (Fe/Mn=16 - 30, at.), feldspar is An_84.7-90.2Ab_9.5-14.6; fragments pyroxene is En_20.4-31.0Wo_2.1-15.2 (Fe/Mn= 23 - 30, at.); feldspar is An_94.5Ab_5.5.

Classification: Mesosiderite of Type 4, anomalous; weathering is minimal. This mesosiderite is unusually troilite-rich and metal-poor.

Specimens: A piece and slices of 103 g total weight and thick section is on deposit in Vernad. Anonymous collector holds the main mass.