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Northwest Africa 1685
Basic information Name: Northwest Africa 1685
     This is an OFFICIAL meteorite name.
Abbreviation: NWA 1685
Observed fall: No
Year found: 2002
Country: (Northwest Africa)
Mass:help 11.56 kg
Classification
  history:
Meteoritical Bulletin:  MB 101  (2012)  LL4
Recommended:  LL4    [explanation]

This is 1 of 360 approved meteorites classified as LL4.   [show all]
Search for other: LL chondrites, LL chondrites (type 4-7), Ordinary chondrites, and Ordinary chondrites (type 4-7)
Comments: Approved 15 Sep 2012
Writeuphelp
Writeup from MB 101:

Northwest Africa 1685 (NWA 1685)

(Northwest Africa)

Purchased: 2002

Classification: Ordinary chondrite (LL4)

History: Purchased by D. Bessey in 2002 from Morocco in two lots. Fragments were sold under the name "BL" to a number of collectors.

Physical characteristics: Total known mass >11.56 kg in multiple pieces. Individuals have a characteristic glossy, desert-polished black fusion crust. Broken and cut surfaces often show a breccia with angular igneous clasts to 2 cm set in a chondritic matrix (igneous clasts ~20% of volume). Weathering is minor, limited to Fe-oxides developed in association with the fusion crust and with some Fe,Ni metal blebs. Bulk physical properties: Grain density 3.42 ± 0.03 g/cm3 (n=3 pieces); Bulk density 3.15 ± 0.03 g/cm3 (n=4); calculated porosity 7.9%. Magnetic susceptibility of four pieces averages log χ = 4.10.

Petrography: (P. McCausland, UWO) Two polished thin sections with an area of 4 cm2 show a large (2 cm) and several small (~mm) subangular metal-poor igneous clasts in a matrix of chondrules and chondrule fragments to 2 mm, as well as minor Fe,Ni and troilite grains <1 mm. Igneous clasts have sharp contacts with the chondritic matrix. The clasts consist of 40-50% fresh, subhedral ~0.1 mm olivine grains set in 55-45% symplectic to skeletal low-Ca pyroxene in a feldspathic glass with accessory chromite; also present within clasts are relict mm olivine grains, which retain inclusions of low-Ca pyroxene and troilite and that host ~0.2 mm inclusion-free olivine rims. Clasts have sinuous fractures and irregular patches filled with glass (dark in cross-polarized light). The fractures and associated melt patches sharply offset clast olivine grains and also contain partially resorbed clast olivine and pyroxene-feldspar symplectite; they do not extend beyond the igneous clasts.

Geochemistry: (R. Flemming, P. McCausland UWO; UCalgary) Chondritic matrix: olivine (Fa28.4±0.6 n=25; FeO/MnO = 57.5±5.0); low-Ca pyroxene (Fs23.0±0.6 Wo2.4±0.8, n=22; FeO/MnO = 32.7±3.4); high-Ca pyroxene (Fs9.1±1.2 Wo43.9±0.6, n=3). Igneous clast: olivine (Fa27.1±0.4 n=49; FeO/MnO = 53.5±4.7); relict olivine (Fa27.8±0.5 n=7); low-Ca pyroxene (Fs22.2±0.6 Wo2.4±0.5, n=42; FeO/MnO = 31.4±2.6); melt veins of ~pyroxene composition (Fs26.4±0.6 Wo2.7±0.6, n=8; with ~5% total Na, K, Al).

Classification: NWA 1685 is a polymict breccia consisting of an LL chondrite matrix and metal-poor igneous clasts, which have experienced variable to significant shock. The similar olivine and pyroxene mineral compositions between the igneous clasts and the host LL chondrite suggest that the clasts are a melt product derived from the LL chondrite host, with selective loss of Fe, Ni, S. A subtle but sharp drop in olivine Fe content visible in BSE images across contacts between matrix and clast olivine implies that the overall bulk meteorite cannot be equilibrated, despite the apparent uniformity of chondrule olivine compositions. The igneous clasts have a similar appearance to those of LL4 Yamato 74442, and to a lesser extent with the igneous clasts in LL chondrites Krähenberg and Bhola.

Specimens: A type specimen (28.64 g) and two polished thin sections are on deposit at UWO. Main masses: 1640 g,  Gregory; 624 g R. Jirasek.

Data from:
  MB101
  Table 0
  Line 0:
Place of purchase:Morocco
Date:P 2002
Mass (g):11560
Pieces:many
Class:LL4
Shock stage:S2-4
Weathering grade:W1
Fayalite (mol%):28.4±0.6; 27.1±0.4
Ferrosilite (mol%):23.0±0.6; 22.2±0.6
Wollastonite (mol%):43.9±0.6 (n=3) in chondrules
Magnetic suscept.:4.10
Classifier:P. McCausland
Type spec mass (g):28.64
Type spec location:UWO
Main mass:D. Gregory
Comments:NWA 1685 is often referred to as "BL" in reference to its unusually dark, glossy fusion crust; submitted by P. McCausland (UWO)
Institutions
   and collections
UWO: University of Western Ontario, Department of Earth Sciences, BGS 1026, 1151 Richmond St. N, London, Ontario, Canada N6A 5B7, Canada (institutional address; updated 18 Jul 2015)
Bessey: Dean Bessey ,P.O. Box 40810, Glenfield, Auckland 1310 , New Zealand; Website (private address)
Gregory: David Gregory, 230 First Avenue, Suite 108, St. Thomas, Ontario N5R 4P5, Canada (private address)
Catalogs:
References: Published in Meteoritical Bulletin, no. 101, MAPS 50, 1661, September 2015
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Photos:
CreditPhotos
Photos from the Encyclopedia of Meteorites:
Don Edwards   
Photos uploaded by members of the Encyclopedia of Meteorites.
    (Caution, these are of unknown reliability)
AR   
bphd         
Mirko Graul   
tett         
Geography: 
Coordinates:Unknown.

Statistics:
     This is 1 of 6877 approved meteorites from (Northwest Africa) (plus 2037 unapproved names)

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