The Achi-Ido L4 ordinary chondrite

Visual Sample Catalogue

Twelve photomicrographs of Achi-Ido provide an indication of the silicate and metal textures in this interesting chondrite. They are variously taken in plane-polarized or cross-polarized light (PPL / XP), transmitted or reflected-light illumination (TL / RL). In terms of classification, the petrologic class (L) was determined by electron microprobe analysis of mineral chemistry (olivine, orthopyroxene and kamacite compositions), a result consistent with overall metal content, and further confirmed by bulk chemical analysis. The `metamorphic grade' (4) and degree of shock (S3) are gauged from the textural and microstructural details. The freshly-recovered fall shows minimal weathering, but metal on sawn faces oxidizes rapidly.



Coarse metal [55 kb] FeS-rimmed chondrule [66 kb]

1. Left: Large metal mass, largely kamacite, with a prominent rounded bleb of troilite (FeS), in a groundmass dominated by equant, porphyritic olivine. Almost indistinguishable from the kamacite in this photograph is a small amount of Ni-rich taenite- tetrataenite, at the far left end of the mass and in the core of the mass, halfway from the troilite bleb to the lower right end. 40X magnification, long-axis field-of-view roughly 2.8 mm, PPL-RL.

2. Right: A good example of a porphyritic olivine chondrule, structures which comprise roughly half the volume of the meteorite. This one is rimmed by troilite grains, with a "cap" of kamacite on one side. 40X, FOV 2.8 mm, PPL-RL.



Halo around chondrule [67 kb] Coarse olivine [113 kb]
3. Left: A striking halo around a rounded volume centred on a coarse olivine grain. Abundant fine-grained chromite (grey), plus troilite and lesser kamacite. 80X, FOV 1.4 mm, PPL-RL.

4. Right: Rotated, transmitted-light view of the same subject, highlighting the coarse olivine in a fine-grained granular matrix. 80X, FOV 1.4 mm, XP-TL.



Metal-rimmed chondrule [66 kb] Porphyritic olivine chondrule [101 kb]
5. Left: Kamacite-dominated rim on porphyritic olivine chondrule, cf. troilite-rich rim in Fig. 2 and chromite-rich rim in Fig. 3. 40X, FOV 2.8 mm, PPL-RL.

6. Right: Exactly the same view as Fig. 5, in transmitted light. Note the typical porphyritic texture in this chondrule, the granular groundmass beyond the metal rim, and the barred olivine chondrule at upper right (see Fig. 8). 40X, FOV 2.8 mm, XP-TL.




Barred olivine chondrule [63 kb] Barred olivine chondrule [92 kb]
7. Left: An unusual chondrule with parallel orientation of olivine domains, a texture intermediate between a typical barred olivine chondrule and porphyritic chondrules. 80X, FOV 1.4 mm, PPL-RL.

8. Right: Another chondrule, this one closer to the typical barred olivine form, but with relatively few layers of olivine crystallites. 40X, FOV 2.8 mm, XP-TL.




Barred olivine chondrule [65 kb] Olivine and orthopyroxene [101 kb]
9. Left: Alternating lamellae of olivine and glass stand in sharp relief in this view of an asymmetric barred olivine chondrule in a neighbourhood rich in kamacite and troilite. 80X, FOV 1.4 mm, PPL-RL.

10. Right: Compact orthopyroxene (on the right) and coarser olivine displaying brighter birefringence. Both minerals appear to have thin embayments of devitrified glass, but it is not clear whether they shared a common history prior to entrapment in the chondrite groundmass. 40X, FOV 2.8 mm, XP-TL.




Troilite in OPX chondrule [79 kb] Pitted kamacite with troilite [70 kb]
11. Left: Coarse anhedral troilite in an orthopyroxene chondrule rimmed by metal and lesser troilite. 40X, FOV 2.8 mm, PPL-RL.

12. Right: Anhedral kamacite and lesser troilite in olivine-dominated groundmass with trace of chromite. The coarse metal is scored by tiny pits, perhaps incipient alteration to troilite. 80X, FOV 1.4 mm, PPL-RL.


This meteorite, provisonally named Achi-Ido, is a recent fall in Nigeria, west Africa. The mineralogy, bulk chemistry and content of short-lived radionuclides in this chondrite are the subject of an article in preparation by M.P. Gorton et al. at the Department of Geology and IsoTrace Laboratory of the University of Toronto, and a taxonomic summary will be sent to the Meteoritical Bulletin.


See the hand specimen and thin section
(the source of these photos)

Compare with the Yafa (Yemen) H5 (S2) ordinary chondrite

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Photographic profile of Achi-Ido by Graham Wilson.
Posted on IsoTrace web site, last revised 10 June 2002.