The left image is in plane-polarized transmitted light, the right-hand in cross-polarized light.
![eucrite in PPL [90 kb]](mbilppl.jpg)
![eucrite in XP [100 kb]](mbilxpl.jpg)
1. Left: Plagioclase feldspar crystals, slightly turbid, in a matrix of brown, granular pyroxene. Magnification 40x, long-axis field-of-view 2.8 mm.
2. Right: The feldspar laths are twinned, while the pyroxenes display concentric zonation. Scale as above.
These photomicrographs feature diabase samples from the
Lake Nipigon area, Thunder Bay district, northwest Ontario,
part of the
Keweenawan Midcontinent Rift.
At first sight they are fresh (unaltered) igneous rocks,
as is the meteorite.
![diabase [92 kb]](opapdia1.jpg)
![diabase [97 kb]](opapdia2.jpg)
3. Left: Olivine diabase, with olivine in a matrix of fine feldspar, pyroxenes and oxides, sample 29. Cross-polarized transmitted light, magnification 40x, long-axis field-of-view 2.8 mm.
4. Right: Intergrowth of pyroxene and feldspar in coarse diabase, sample 34. Details as above.
"Rocks of the Month # 17-18, posted for November-December 2002"
--- samples 1795 (the meteorite, obtained from David New), plus
N.W. Ontario samples 29 and 34 (diabase, collected 29 August 1999,
inclusive catalogue numbers 3365 and 3370).
These samples are juxtaposed
to exemplify the similarities of some Earth rocks and igneous
(achondrite) stony meteorites. Intricate details of chemistry
and mineralogy are avoided (but see references), the intent being
to compare and contrast the rocks in basic terms --- with luck, the
presence of a fusion crust would indicate the meteorite in most cases!
Millbillillie (a.k.a. Nabberu) is an example of the class of stony meteorites known as eucrites (calcic achondrites). This example is believed to have fallen in October 1960 in the Wiluna district of Western Australia. Ten years later, at least 25.4 kg was recovered. The thin section contains an estimated 54 volume percent clinopyroxene, 45% plagioclase feldspar and 1% opaque phases. The pale brownish clinopyroxene displays simple twinning and in some cases distinct chevron-like inverted-pigeonite exsolution textures. Some grains appear zoned with darker, relatively birefringent cores and paler rims. Maximum pyroxene grain size is 2.2x1.0 mm. The feldspar occurs as clear prisms with assorted simple, pericline and albite twin laws, up to 1.6x0.6 mm in size. The section is dominated by relatively coarse (1-2 mm) areas with ophitic texture, i.e., with plagioclase prisms partially enclosed by coarse pyroxene. Some areas are finer-grained (<0.1 mm), with the same major phases in an intergranular texture. A noncumulate eucrite, possibly a subvolcanic hypabyssal intrusive or the base of a lava flow on the parent body. This rock is massive, lacking obvious directed flow or settling textures, and appears generally similar to terrestrial basalt and diabase. This is a well-studied meteorite (51 MINLIB records, 1972-2013, as of June 2013). The Keweenawan diabase is a part of the relatively short-lived but voluminous continental flood basalt magmatism which occurred across much of what is now the American Midwest and the Lake Superior basin some 1,100 million years ago.
The question of identification of stony meteorites of igneous origin can be addressed in several ways. A fresh fall, or a rock of this kind on an Antarctic ice sheet or limestone plateau may be self-evident, particularly if the fusion crust is well-developed. The chemistry of eucrites is well-documented (e.g., Kitts and Lodders, 1998) but bulk chemical analysis of a suspected meteorite has several disadvantages: conventional approaches are destructive of the sample and can be relatively expensive and time-consuming. Detailed petrographic work (e.g., Yamaguchi et al., 1994) brings out details of zonation in feldspar, ilmenite and chromite, and the inverted pigeonite. Mineralogical study is usually advanced by microscopic analysis. The sample preparation is also destructive, with losses in cutting and polishing, but a polished thin section is inexpensive and, should a sample merit further attention, forms the basis for more detailed studies by microprobe or other techniques.
Colour index (proportion by volume of dark and opaque minerals) for each sample is roughly 45-60 percent. The igneous textures are quite similar, and the grain size ranges overlap (maxima of 2-4 mm in each case). Both terrestrial rocks show minor alteration of the olivine, and the coarse diabase also contains secondary amphibole and minor felsic mesostasis ("graphic granite"). Most notable, however, in terms of distinguishing the meteorite from a terrestrial rock, are signs of brecciation with areas of granulitic matrix and relict impact melts, indicative of shock events in the meteorite's past.
References
KITTS,K and LODDERS,K (1998) Survey and evaluation of eucrite bulk compositions. Meteoritics & Planetary Science 33, supplement, A197-213.
LABERGE,GL (1994) Geology of the Lake Superior Region. Geoscience Press, Boulder, CO, 309pp.
NICHOLSON,SW, SHIREY,SB, SCHULZ,KJ and GREEN,JC (1997) Rift-wide correlation of 1.1 Ga Midcontinent rift system basalts: implications for multiple mantle sources during rift development. CJES 34, 504-520.
PYE,EG (1997) Roadside geology of Ontario: North Shore of Lake Superior. OGS Rock On Series 2, 164pp.
SUTCLIFFE,RH (1991) Proterozoic geology of the Lake Superior area. In `Geology of Ontario' (Thurston,PC, Williams,HR, Sutcliffe,RH and Stott,GM editors), OGS Spec.Vol. 4, part 1, 709pp., 626-658.
YAMAGUCHI,A, TAKEDA,H, BOGARD,DD and GARRISON,D (1994) Textural variations and impact history of the Millbillillie eucrite. Meteoritics 29, 237-245.
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