L/LL4 (LL4)
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Fell March 12, 1899
60° 20.42' N., 25° 42.30' E., approx.

At 10:30 in the evening, a highly friable stone meteorite fell through thick sea-ice near the NW shore of Bjurbölemalmen Cape in southern Finland (Moilanen and Kettunen, 2016). The meteorite broke into many fragments lying beneath 8 m of mud and clay sediments, which delayed recovery of the first fragments for a month. Over 328 kg were ultimately recovered, and a laboratory analysis determined the classification to be L4 ordinary chondrite. Further studies of Bjurböle resulted in a revision of this classification, and it was recognized as a member of the L/LL4 transitional group. Recent analyses conducted by Maksimova et al. (2020) based on X-ray diffraction, magnetization measurements, and Mössbauer spectroscopy indicate a likely genetic relationship to the LL chondrite group (see diagram below).

Classification of Bjurböle Using Mössbauer Parameters
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Diagram credit: Maksimova et al., Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, vol. 248:119196, (2021)
'Bjurböle L/LL4 ordinary chondrite properties studied by Raman spectroscopy, X-ray diffraction, magnetization measurements and Mössbauer spectroscopy'

The extremely low crushing strength of Bjurböle implies a crystallization at low pressures and/or within a small parent body. The meteorite has a porosity of 19.8% with a moderate degree of metal grain foliation, the latter caused by a relatively gentle impact (Friedrich et al., 2019). Two types of chondrules have been identified in Bjurböle: 1) those composed primarily of quenched glass, and 2) those consisting of olivine and pyroxene crystals along with a minor abundance of glass. While both chondrule types are rimmed with FeNi-metal and FeS grains, the chondrules containing silicate crystals also show a ubiquitous distribution of these opaque phases as fine grains (Rushmer et al., 2013).

Previous parameters used to delimit the L/LL group from the L and LL groups include the fayalite molar percentage and the content of cobalt in kamacite. New studies utilizing cluster analysis statistics on the minor and trace element data conclude that the L/LL meteorites are chemically more closely related to each other than they are to either the L or LL group. Furthermore, the enriched REE concentration and negative europium anomaly that are found only in the L/LL meteorites easily distinguishes them from the other groups; the evidence is now stronger for the existence of a separate L/LL, low total-Fe, chondritic parent body.

It was demonstrated by Szurgot (2016) that the mean atomic weight (Amean) of meteorites can be used to resolve the OC groups, including the intermediate groups L/LL and H/L. Amean values can also be predicted through various equations based on other parameters such as atomic Fe/Si ratio and grain density, and these Amean values, as well as the magnetic susceptibility values derived from X-ray fluorescence (XRF) scanning, all consistently resolve these groups into the ordered sequence LL < L/LL < L < H/L < H. Bjurböle has Amean values of 23.52 (chemical composition), 23.33 (Fe/Si atomic ratio), and 23.47 (grain density). Furthermore, it was demonstrated that Amean values are lower for unequilibrated type 3 samples than for equilibrated samples within each OC group due to the presence of water; Amean values for petrologic types 4–6 are indistinguishable within each group.

standby for amean diagram
Diagram credit: M. Szurgot, 47th LPSC, #2180 (2016)
Amean based on chemical composition (Eq. 1), Fe/Si atomic ratio (Eq. 2), and grain density (Eq. 3)

Using paleointensity data for Bjurböle, an estimate for the minimum size of the L/LL parent body was derived by Bryson et al. (2019) based on accretion and thermal evolution models, and with an assumption that the measured paleomagnetic field is associated with the timing of thermally-driven dynamo activity. From the timing of dynamo activity 80–140 m.y. after CAIs that was previously established for Bjurböle, along with an assumption of its thermal diffusivity (9 × 10–7 m²/s), a diameter of >300–400 km was determined to be most consistent with their model criteria.

Other members of this intermediate chondrite group include L/LL3 Esperance, L/LL4 Seemore Downs 001 and Cynthiana, L/LL5/6 Sahara 97021, L/LL6 Acfer 041, and several others. A few meteorites are only partially resolved into this intermediate group, including L/LL3 Inman, L/LL5 Knyahinya and Qidong, and L/LL6 Holbrook and Sultanpur. The specimen of Bjurböle shown above is a 0.2 g micromount with a small detached chondrule.

Bjurböle Impact Through Ice
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Bjurböle Largest Fragments of 80 kg and 21 kg
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Photo credit: W. Ramsey and L. H. Borgström