"Rock of the Month # 271, posted for January 2024" ---

The Saratov L4/LL4 ordinary chondrite

is a "textbook" meteorite in that its textural and mineralogical features are beautifully defined. Saratov fell in Russia (at 52°33'N, 46°33'E) at 15:00 local time on 06 September 1918. Saratov lies west of the Volga river in the southeastern part of the European (western) portion of Russia, north of Volgograd and southeastwards of Moscow. We will look at some of what's known of Saratov, but first ...

The wonderful rabbit-hole of chondrite classification!

The reader will note the "L4/LL4" label above. At the risk of circular arguments, we will look briefly at the classification of the ordinary chondrites, which comprise roughly 5 out of every 6 meteorites recovered at the Earth's surface. Some weighty references are cited, for those who wish to spend longer in the rabbit hole.

The ordinary chondrites are labelled H, L or LL, a series of classes that pertain to the iron contents of the samples (high, low, lower...). Proceeding from H to LL, ideally with a tray of well-chosen examples in a museum, we note a decline in the abundance of metal (Ni-Fe alloy) and a less-visual rise in oxidation state. The latter is revealed, upon analysis of mineral grains, in increases in the fayalite (Fa, the iron component of olivine) and ferrosilite (Fs, the iron component of orthopyroxene / low-Ca pyroxene). This partitioning of iron between metal and the oxidized (silicate) state is represented in a chart of Fa versus Fs contents in textbook reviews, as in Dodd (1981, p.80).

There is a second component of classification besides composition, and that is the so-called petrologic grade. This is basically the extent to which a meteorite shows evidence of reheating and recrystallization (metamorphism) and is rated from 1 to 7. Ordinary chondrites lie between 3 and 6, or rarely 7. There is a big caveat: so-called unequilibrated ordinary chondrites (UOC) of type 3 show dramatic variation in composition within and between crystals of a given mineral. The preservation of chondrules is one key index of grade that can be judged by eye. We will not consider this further here, but the topic is well-covered in textbooks (e.g., Dodd, 1981; Hutchison, 2004, Grady et al., 2014). See also an illustrated example here, of the classification of an "UNWA" stone, eventually recognized as the L4 chondrite NWA 12807.

The L chondrites are the second-most abundant class of meteorite, of the early solar system, in our collections, with 26,159 known (35.75% of 73,173 officially recognized meteorites: Meteoritical Bulletin, 18 November 2023). This is a close second to the 27,545 H chondrites (37.64%) and far more abundant than LL chondrites (8,211, 11.22%).

L chondrites contain proportions of nickel-iron metal grains, and olivine compositions (most commonly Fa_22-26.5) intermediate between those of H (Fa_16-21: more metal) and LL chondrites (Fa_26-33: less metal). The olivine compositions offer a guide, but the margins between classes may be blurred, with a few overlaps in composition between H and L and, especially, L and LL.

Saratov has mineral chemistry (olivine) that puts it on the border of definitions for L and LL chondrites. It has low metal content but relatively abundant sulphides. The analyses of olivine quoted for Saratov show appreciable spread: if each analytical session was well-calibrated, perhaps there are olivine-bearing elements in Saratov that are not fully equilibrated with their matrix?

The "intermediate cases" of L/LL stones are much less common than typical L or LL chondrites, but not especially rare, given the large numbers of ordinary chondrites available to us. A recent addition is the Golden (British Columbia, Canada) L5/LL5 (S2, W0) fall of 04 October 2021. Matrix and chondrule olivines and low-Ca pyroxenes in Golden are strongly equilibrated at Fa_26.3±0.2 and Fs_22.5±0.2 (Wo_1.3±0.2), respectively (Brown et al., 2023).

Measurement of mineral compositions has become more routine, and much less onerous, following the development and increasing availability of the electron microprobe, from the 1950s to the 1980s. As noted above, the most common mineral-chemical parameters for chondrite classification include the Mg numbers (proportions of Fe and Mg) of olivine and low-Ca pyroxene. The basics appear in the textbooks and were reviewed, using a far larger database of analyses, by Grossman and Rubin (2006). An elegant third option (Rubin, 1990; Krot et al., 2005) is the Co content of kamacite, generally the most abundant Ni-Fe alloy in meteorites. Even here, there is a little overlap between L and LL. In practical terms, however, it is unlikely that an ordinary chondrite, recognized as such by old-fashioned petrographic examination of mineralogy and textures under the microscope, will receive microchemical analysis of all 3 minerals. Even given an an academic discount rate, laboratory time must be paid for, and time itself is valuable, so a minimalist approach may be taken in most cases, and unless the meteorite has a specific interest to justify a detailed study. In-situ determination of compositions may also be possible using modern developments in x-ray crystallography.

While the L and LL stones are collectively known as the ordinary chondrites, Saratov is in some ways extraordinary.

The Saratov meteorite

Saratov was a substantial fall, and in total some 328 kg was recovered. The chondrules are very nicely preserved, even for a petrologic type 4, where textural features are often intact. Saratov also evidently underwent no more than modest exposure to impact-related shock in space (shock stage S2), which further serves to preserve its original mineralogy. Saratov has been used for more research than we have space for here (only half the MINLIB bibliographic records are cited herein). Topics include physical properties (Kohout et al., 2008); a test medium for experimentally induced shock; crystal structures of orthopyroxene; occurrence of porous aggregates of silicate fragments (e.g., Semenenko et al., 1992; Girich and Semenenko, 2003); the fusion crust; bulk chemical studies; trace metals such as Tl, Pb and Cd; rare gases in bulk and in chondrules (e.g., Matsuda et al., 2010); B isotopes (Zhai et al., 1996); Li isotopes (Seitz et al., 2010); magnetic properties of metal phases; and more.

The well-preserved chondrules are a textural treasure trove (e.g., work associated with the Geological Survey of Canada in Ottawa: Herd et al., 2003, 2004, 2009). At least six classes of chondrules can be identified. Laura Dixon made a study of some 370 chondrules >100 µm in diameter, and focused on 19 chondrules, in one 20x30 mm polished thin section, taking 573 BSE images and 212 EDS spectra (Herd et al., 2009). Four relative sizes of crystals are defined, megacryst, macrocryst (equant or elongate), mesocryst (in mesostasis) and microcryst. By analogy, some crystals cooled slowly as in terrestrial gabbro, while others were quenched, as in komatiite. Saratov is illustrated in several reviews, e.g., Norton (2002); Lauretta and Killgore (2004); and McCall (2006). The metal phases have also been studied (Semenenko and Tertichnaya, 1994; Reisener and Goldstein, 2003).

Herd et al. (2003) made a detailed microprobe and SEM study of Saratov and found that the olivine was not appreciably zoned, in contrast to unequilibrated chondrites of petrologic grade 3. This finding held upon further analyses, with olivine compositions of Fa_22-26 (Herd et al., 2004). Glassy mesostasis in chondrules is largely recrystallized, forming calcic plagioclase feldspar. Knight et al. (2004) compared a Saratov chondrule to a particle of smelter dust from the Horne smelter, a major metal refinery in Rouyn-Noranda (Quebec, Canada). There were significant textural similarities between the rapidly quenched smelter dust and the chondrule (containing olivine, Fa₂₃). Girich and Semenenko (2003) described three highly porous aggregates, dimensions varying from 0.4 to 4.3 mm. Porosity is as high as 65 vol.%. The mineralogy of the aggregates mirrors the bulk meteorite, dominated by olivine (which they reported as Fa_17.1-18.2: more like H than L/LL) and pyroxenes, and with very little metal and troilite. The survival of these delicate bodies within the meteorite is surely a clue to the mechanism of accretion of the material that formed the parent body of the L chondrites.

The meteorite was widely distributed after the fall: Saratov figures in catalogues from Arizona State, the Southwest Meteorite Collection, the Leonard Collection at UCLA and other compilations.

Fig. 3: This smaller rough slice is 30 x 25 x 3 mm (Blaine Reed), mass 7.80 grams. Some mm-scale metal grains reflect the light. Chondrules and other silicate textural features can be seen, even in the rough surface. I should make a thin section! (in prep., January 2024). For some photomicrographs of Saratov, see Lauretta and Killgore (2004, pp.32-35).