Mesosiderites


Northwest Africa 1242

Gillio, Libya
Found 1985
Mesosiderite

NWA 1242. Image source: www.nyrockman.com/museum/ nwa-1242.htm

Two fully ablated pieces of a mesosiderite (total known weight ~7 kg) were found near the village of Gillio by unknown oil exploration workers and put into service as bookends for 13 years. Purchased in 1998 by an anonymous dealer. Reports of a strewn field are presently unconfirmed. Classification and mineralogy (T. Bunch and J. Wittke, NAU): type 2A; contains scarce, metal nodules up to 19 mm in diameter; Mg-rich pyroxene overgrowths on Ca-pyroxene clasts; moderate resorption of plagioclase. Orthopyroxene (Fs33Wo3) replaces pigeonite (Fs34Wo8) and exsolved augite (Fs43Wo42); plagioclase (An92); pyrrhotite, chromite (Al2O3 = 8.4 wt. %); kamacite, 5.8 wt. % Ni; rare taenite (42 wt. % Ni), silica and phosphate. Shock stage, S1; weathering grade, W0. Specimens: type specimen 20.2 g, thin section, NAU; main mass is with the anonymous purchaser.


Northwest Africa 1827

Morocco
Purchased 2003 June
Mesosiderite

NWA 1827. Image source: www.meteoriteguy.com/ nwa1827.htm

A complete stony-iron meteorite (877 g) was purchased in Rissani by a Moroccan dealer for N Oakes (Oakes) in 2003 June. Classification and mineralogy (T. Irving and S. Kuehner, UWS and T. Bunch, NAU): Igneous texture, mostly medium-grained but with some large mineral clasts of orthopyroxene and plagioclase (commonly surrounded by metal-rich areas). Predominantly orthopyroxene with about 10 vol. % metal (5 wt. % Ni), troilite, chromite, merrillite, and sparse, large anorthite clasts (An90.5Ab9.1Or0.4 to An95.1Ab4.6Or0.2). Large orthopyroxene clasts are more magnesian (Fs16.2Wo0.8, FeO/MnO = 34.8) than the predominant medium-sized orthopyroxene grains (Fs22.5Wo2.1 to Fs27.3Wo2.2, FeO/MnO = 29.7-32.7). Small elongate grains of a secondary Ca-Fe-rich phosphate mineral occur along fractures in merrillite grains, and are probably of terrestrial origin. Although this sample resembles a "metal-rich diogenite" rather than a mesosiderite, comparative studies by T. Bunch (NAU) of this sample, NWA 1879 and similar material provided by A. Jambon (UPVI) and M. Farmer (Farmer) have demonstrated that all (with an estimated total weight of over 22 kg) are part of a large, heterogeneous mesosiderite (tentatively assigned to class C) containing sparse eucritic and diogenitic clasts. Specimens: type specimen, 20 g, UWS, 1 g and polished thin section, NAU; main mass, Oakes.


Northwest Africa 1912

Morocco
Found 2002
Mesosiderite

NWA 1912. Image source: www.meteoriteguy.com/ nwa1912.htm

A tiny, single stone of 13.52 g was purchased in Erfoud in March 2003. Classification and mineralogy (T. Bunch and J. Wittke, NAU): type 2B, contains orthopyroxene fragments, Fs26.4Wo2; molar FeO/MnO = 33, with minor recrystallization at the margins; very calcic plagioclase (An96), chromite (52.8 wt. % Cr2O3), troilite, silica, and metal (6.4 wt. % Ni, 0.78 wt. % Co). Shock level, S1; sparse oxidation. Specimens: type specimen, 2.5 g, NAU; main mass, Farmer.


Northwest Africa 1979

Morocco
Found 2002
Mesosiderite

A 5.12 kg single mass was purchased in Erfoud in 2003. Classification and mineralogy (T. Bunch & J. Wittke, NAU): typical B2 mesosiderite, based on 8 vol. % plagioclase, with a partially recrystallized matrix. Modes (vol. %): silicate matrix, 44; metal, 33 ; eucritic clasts (gabbroic), 12; diogenite grains (up to 1.2 cm), 8; and sulfides, 3. Metal grains are typically >2 mm with a few cm-size, subrounded to irregular grains; eucrite clasts < 2 mm. Matrix pyroxene, Fs29.7Wo2 and Fe/Mn = 28.7; matrix augite, Fs13Wo43; plagioclase, An89.7. Gabbroic eucrite inverted pigeonite, Fs30.1Wo3 and Fe/Mn = 26.9; exsolved augite, Fs14.8Wo43; plagioclase, An91. Metal: Ni = 6.7 wt. %, Co = 0.82 wt. %. Not paired with NWA 1827. Weathered crust, but with very fresh interior. Specimens: 22 g, NAU; main mass, Boswell.


Northwest Africa 2639

Northwest Africa
Purchased 2004
Mesosiderite

Three complete stones (TKW, 539 g) with black to brown surfaces, were purchased in Erfoud, Morocco. Description and classification (T. Bunch and J. Wittke,NAU): mesosiderite with nearly equal portions of metal and silicates; metal composed of mostly continuous aggregates of metal balls (0.2-0.4 mm diameter) that resemble grape clusters and enclose clumps of silicates. Orthopyroxene, Fs23.1-25.5Wo1.7-3.3 (FeO/MnO = 23.5-27.9); plagioclase, An88.9; chromite, Cr/(Cr+Al) = 82; metal, Ni = 6.1-7.0 wt. %, merrillite and troilite. Shock level, S2 and a weathering grade of W1. Specimens: type specimen, 21.1 g, NAU; main mass, A. Hupe.


Northwest Africa 2711

Morocco
Found 2004
Mesosiderite

History: A complete stone of 433 g was purchased in Erfoud, Morocco.
Petrography: (J. Wittke and T. Bunch, NAU) The specimen contains a relatively even distribution of metal and silicates with coarse orthopyroxene grains with margins that are recrystallized and contain numerous merrillite inclusions. The matrix is recrystallized. Modal content (vol. %): Orthopyroxene = 66, metal = 16, plagioclase = 10, Ca pyroxene = 4, merrillite = 2, chromite and sulfide = 2.
Composition: Orthopyroxene (Fs26.8Wo3.1); FeO/MnO = 31); plagioclase (An92.9).
Classification: Achondrite (mesosiderite); low shock, minimal weathering.
Specimens: A 24 g type specimen is on deposit at NAU. G. Hupé holds the main mass.

NWA-2711. Image width = 13 mm. Image © T.E. Bunch, 2007


Northwest Africa 1924

Algeria
Found 2005
Mesosiderite

History: Over 180 kg (several thousand pieces) have been recovered from a small strewn field in Algeria. The largest known piece weighs about 5000 g; most are small pieces that weigh less than 200 g.
Petrography and Geochemistry: (T. Bunch and J. Wittke, NAU) Fine-grained (<0.5 mm) matrix consists of brecciated and partially recrystallized subophitic to granular basalts that enclose larger (up to 2.5 mm) subrounded to subangular olivine (Fa28), pigeonite (Fs31Wo5.4; FeO/MnO = 25.7; Cr2O3 = 0.87, Al2O3 = 1.28 [both wt. %]) that rarely shows twinning. These cataclastic basalts contain unusually calcic plagioclase (An99.3) with little FeO (<0.04 wt%); highly twinned pigeonite is compositionally similar to the much larger clast pigeonite (Fs32Wo5.7), but with very little Cr2O3 and Al2O3. Merrillite is also present. Metal blebs (Ni = 6.6 wt. %) are irregular in shape and typically <2 mm in size with the exception of metal aggregates and rare, centimeter-size metal nuggets with included pigeonite.
Classification: Mesosiderite; low shock, variable degree of weathering dependent on the specimen size.
Specimens: A 85 g type specimen is on deposit at NAU. Farmer holds 16 kg of sample.


Miscellaneous Medosiderites

IDLocationDateClass.Mass (g)StateFaFsComments
NWA 1927Safsaf, Morocco3/2003Mes14.3--24.6metal: 6.4 wt. % Ni; shock darkened
NWA 2782Erfoud, Morocco2005Mes330S2, W2-26.5FeO/MnO = 27.5-30.0(photograph below)
NWA 2786Erfoud, Morocco2005Mes30S2, W1-28.1FeO/MnO = 27; An93 (photograph below)

NWA 2782. Metal is black in this PPL image. Image © T.E. Bunch, 2005.

NWA 2786. (left) Metal is black in this PPL image. (right) XPL. Images © T.E. Bunch, 2005.