Exterior and cross section BSE images from representative examples of various types of cosmic spherules. a, b) Porous ScMM with abundant relicts including forsterite with an Fe-Ni droplet (b, top left). c, d) A CG spherule dominated by a single large Fe-rich olivine relict and a smaller potassium-rich feldspar relict (d, bottom right); the exterior has an irregular morphology and consists of an Fe-rich melt with abundant magnetite with a snowflake-like habit. e, f) rb-PO with several forsterite relicts, of which one contains small refractory inclusions rich in Al, Ca, and Ti (f, cropped detail); the surface shows the remains of a highly diluted magnetite rim and a chain of beads around the particle's perimeter, indicating rapid spin during atmospheric entry. g, h) rb-PO displaying a large dark spinel relict with a corona reaction rim and a cumulate texture with small forsterite relicts clustered at the bottom by inertial forces from atmospheric deceleration. i, j) PO with large skeletal olivine crystals protruding at the surface from a glassy matrix. k, l) BO with parallel olivine lamellae and characteristic dendritic magnetite at the surface. m, n) Microcrystalline CC with both barred and randomly oriented textural domains. o, p) Normal CC with two beads on opposite sides. q, r) Turtleback CC with several distinct crystalline domains. s, t) Weathered vitreous spherule with small unaltered crystalline protrusions, indicating the initial perimeter of the particle.
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BSE, backscattered electron; ScMM, scoriaceous micrometeorites; CG, coarse-grained; PO, porphyritic olivine; BO, barred olivine; CC, cryptocrystalline. From Jonker et al. (2023).
Typical chemical spectrum of a micrometeorite with large peaks of oxygen, magnesium, and silicon, and low peaks of aluminium, calcium, and iron.
Classification of micrometeorites
As the micrometeoroids decelerate in the atmosphere, frictional heating causes them to melt and recrystallize forming so-called cosmic spherules. Depending on the peak temperature and primordial mineralogy, these spherules can crystallize in different forms, including porphyritic olivine (PO) with large skeletal, pyramidal, or euhedral crystals, barred olivine (BO) with elongated olivine crystals, and cryptocrystalline (CC) with submicron crystallites. When high cooling rates quench the particle before it crystallizes, it will become vitreous (V).
At lower temperatures, micrometeorites may retain their pre-atmospheric minerals, commonly referred to as relict minerals. These include in decreasing abundance olivine, pyroxene, Fe-Ni droplets, chromite, plagioclase, and spinel, and the associated particles are called either unmelted, scoriaceous, or coarse-grained. Consequently, the composition of cosmic spherules is commonly chondritic and may reflect their parent body. See also Genge et al. (2008) for an extensive review on the classification of micrometeorites.
Chemical composition
Originating from a diverse set of larger parent bodies, micrometeorites show some variation in terms of chemical composition. Nonetheless, most of them can be considered (close to) chondritic. This chondritic composition is generally characterized by high oxygen, magnesium, and silicon concentrations, followed by aluminium, iron, and calcium. In terms of bulk composition, these so-called S-type ("stony") micrometeorites generally contain trace amounts of nickel, chromium, sulfur, phosphorus, sodium, potassium, and titanium, but some of these elements can concentrate significantly within metal beads.
As with everything in nature, there are some exceptions, the most common being the I-type and G-type. I-type spherules consist of mostly Fe-Ni metal in the form of dominantly wüstite and magnetite. They may relate to iron meteorites or the metallic phases in ordinary and carbonaceous chondrites. I-type spherules are rare among rooftop collections since they are difficult to distinguish from anthropogenic metallic spherules. G-type spherules contain abundant magnetite crystals in a glassy silicate matrix and are intermediate between the S-type and I-type in terms of composition. Their origin still remains elusive.
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