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Northwest Africa 16479
Basic information Name: Northwest Africa 16479
     This is an OFFICIAL meteorite name.
Abbreviation: NWA 16479
Observed fall: No
Year found: 2022
Country: (Northwest Africa)
Mass:help 18 g
Classification
  history:
Recommended:  Howardite    [explanation]

This is 1 of 453 approved meteorites classified as Howardite.   [show all]
Search for other: Achondrites, HED achondrites, and Howardites
Comments: Approved 26 Feb 2024
Writeuphelp
Writeup from MB 113:

Northwest Africa 16479 (NWA 16479)

(Northwest Africa)

Purchased: Aug 2022

Classification: HED achondrite (Howardite)

History: Mr. Jasper Spencer purchased two stones with a combined weight of 18 grams (16 g and 2 g) via Facebook Messenger in August 2022. Mr. Spencer was told that the stones were found in Mauritania in 2022.

Physical characteristics: Physical Characteristics: Cascadia received a thin end cut. The exterior is a weathered broken surface in which angular black clasts can be seen amidst a dark orange-brown weathering rind. The largest black clast is roughly rectangular in shape (5 × 6 mm). The cut face displays black and medium brown angular fragments and metal grains surrounded by orange-brown halos set in a fine-grained light brown matrix

Petrography: (M. Hutson, A. Ruzicka, Cascadia): The thin section is composed almost entirely of monomineralic angular fragments of pyroxene and plagioclase feldspar, grading from a few very large (mm-sized, largest about 2.5 × 4 mm in area) to numerous medium (roughly 0.5 mm across) and small (<0.2 mm) clasts set in a fine-grained clastic matrix. Only two lithic fragments (each containing three partial grains intersecting near the clast center) were observed in thin section. No basaltic-textured clasts and no olivine grains were observed. Large clasts representing partial single pyroxene grains could be subdivided into three groups based on BSE imaging: one group composed of uniformly gray (compositionally uniform) grains, one group of grains with exsolution lamellae, and one group with noticeable grayscale variations indicating compositional zoning. Zoning is patchy rather than symmetric; lighter gray (Fe-rich) areas are found closer to clast edges and along fractures, suggesting modification following fragmentation and incorporation into the current meteorite. These zoned grains are typically the most heavily fractured and contain trains of small opaque inclusions. Plagioclase feldspar is relatively undeformed, with many grains showing sharp twins under cross-polarized light. Opaque "phases" in transmitted light contain kamacite surrounded by "fluidized" troilite. One rounded clast (dark in transmitted light) is composed of abundant crystallites of low- and high-Ca pyroxene, troilite, chromite, and phosphate in a feldspathic, vesicle-containing glass. A single irregular grain of a silica phase was seen in thin section. Both the rounded, crystallite-filled clast and the silica phase are surrounded by shells composed of laths of low-Ca pyroxene oriented perpendicular to the shell edge, surrounded by augite. Other phases observed are fluorapatite, ilmenite, and chromite.

Geochemistry: Seventy-seven low-Ca pyroxene grains, twenty-one high-Ca pyroxene grains or lamellae within grains, and thirty-three plagioclase feldspar grains were analyzed. Some grains had only one analysis, while others had multiple analyses. Data for each individual grain were averaged so as not to bias the overall average mineral chemistry towards a grain with a large number of analyses. Low-Ca pyroxene forms two connected clumps on a pyroxene quadrilateral; one clump has lower Fs contents and consists of the zoned and opaque inclusion-filled grains; their compositions are consistent with diogenitic pyroxene grains that are partially equilibrated with the surrounding material: Fs31.5±2.0Wo2.3±0.8 (range Fs27.8-33.9Wo0.9-3.9), molar Fe/Mn=30.7±2.5, N=16 grains (darker cores only). The other clump contains the low-Ca pyroxene grains which appeared uniform in BSE or contained augite lamellae, and has compositions consistent with pyroxene grains found in cumulate eucrites: Fs35.0±0.4Wo2.6±0.5 (range Fs34.2-36.6Wo1.1-3.7), molar Fe/Mn=29.5±1.0, N=61 grains. Augite grains (including lamellae): Fs15.2±1.4Wo42.9±1.4 (range Fs12.8-19.3Wo38.0-44.1), molar Fe/Mn=23.3±1.6, N=21 grains. Plagioclase feldspar compositions are variable: Ab9.5±5.1Or0.7±0.5An89.9±5.6 (range Ab3.9-28.8Or0.1-3.0An68.2-96.0), N=33 grains. Chromite contains Al, Ti, and Mg: Al2O3=8.6 wt%, TiO2=4.9 wt%, MgO=3.0 wt%, N=2. Kamacite: Fe92.5±0.1Ni6.7±0.1, N=4.

Classification: HED Achondrite (Howardite). The chemistry of pyroxene grains suggests a mixture of diogenite and cumulate eucrite fragments, with a small admixture of chondritic metal and sulfide. The proportions of diogenitic and eucritic material are difficult to estimate accurately. Of the 77 low-Ca pyroxene grains analyzed (biased towards larger grains), 26% were diogenitic. Major phases were estimated from a BSE montage of the section using ImageJ as approximately 65.5% pyroxene, 25.8% feldspar, and 8.7% opaque phases, although this is likely to slightly overestimate feldspar as it does not exclude all fractures. Estimates made using grain estimation charts suggests that feldspar is around 20% of the section. Feldspar makes up 30-70% of a cumulate eucrite (Mittlefehldt 2015). Assuming only two components (diogenite and cumulate eucrite), a reasonable mid-range cumulate eucrite abundance of 40% plagioclase feldspar, the presence of 25% plagioclase feldspar in the thin section suggests that cumulate eucrite fragments comprise about 71% of the section, with diogenitic components around 29%. Regardless of the actual abundance, the diogenite and cumulate eucrite components certainly exceed 10% each.

Specimens: Cascadia holds 4.2 g in one piece, as well as a polished thin section and material in an epoxy butt; Mr. Jasper Spencer holds the main mass.

Data from:
  MB113
  Table 0
  Line 0:
Place of purchase:FB Messenger
Date:P Aug 2022
Mass (g):18
Pieces:2
Class:Howardite
Ferrosilite (mol%):35.0±0.4, N=61 (main)
Wollastonite (mol%):2.6±0.5, N=61 (main)
Classifier:M. Hutson and A. Ruzicka, Cascadia
Type spec mass (g):6.4
Type spec location:Cascadia
Main mass:Mr. Jasper Spencer
Comments:Lab number CML 1668; submitted by Melinda Hutson
Institutions
   and collections
Cascadia: Cascadia Meteorite Laboratory, Portland State University, Department of Geology, Room 17 Cramer Hall, 1721 SW Broadway, Portland, OR 97201, United States; Website (institutional address; updated 28 Oct 2011)
Catalogs:
References: Published in Meteoritical Bulletin, no. 113, in preparation (2024)
Find references in NASA ADS:
Find references in Google Scholar:
Photos:
CreditPhotos
Public domain photographs:
Melinda Hutson            
Geography: 
Coordinates:Unknown.

Statistics:
     This is 1 of 9829 approved meteorites from (Northwest Africa) (plus 1851 unapproved names)

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