Shortly after I discovered the world’s first urban micrometeorite in 2015, I began engaging with fellow micrometeorite enthusiasts from around the world on social media. Every week I receive fascinating questions about these enigmatic particles. Some time ago, I decided to write a blog post series to summarize the various types of micrometeorites as they were classified in a seminal paper by Drs. Matthew Genge, Cecil Engrand, and Susan Taylor.

When cosmic dust particles end their journey, their path through our atmosphere most often transforms the rough, unmelted micrometeoroids into polished jewels. However, in the case of scoriaceous micrometeorites, which I discuss in this article, the particles remain in a mainly unmelted state due to variations in the angle of entry and initial mass and speed. As one might imagine, this leads to substantial differentiation in both chemical and morphological characteristics.

Jan Braly Kihle and I recently began a new research project with Dr. Roar Skartlien from the Institute for Energy Technology (IFE) in order to learn more about how micrometeorites are formed. Our hope is deepen the classification work that Genge et al. (2010) spearheaded.

Today I present a summary of scoriaceous (SC-type) micrometeorites; I hope you find it both fascinating and helpful. Enjoy!

Scoriaceous (SC-type) Micrometeorites

For reasons not fully understood, some micrometeorites survive atmospheric entry as unmelted, or low heated particles. Whatever the mechanism, the peak temperature of these enigmatic particles does not exceed 1,350°C (2,500°F). During this low heating, volatile elements expand and escape as gas bubbles resulting in a vesicular or scoriaceous micrometeorite.

Under a microscope, Sc-type extraterrestrial particles (ScMMs) are dominated by micron-sized equant olivine crystals erratically suspended in abundant glass. Occasionally, you will also see magnetite crystals scattered on the surface.

These micrometeorites are sometimes hard to identify, due to their lack of aerodynamic properties and characteristic textures. But, with experience and a strong understanding of Sc-type visual morphology, it is possible to find them in urban dust samples using a simple light microscope. However, the best way to confirm a ScMM is with an electron microscope, since it is easy to verify their extraterrestrial origin due to their classic chondritic chemistry.

The collage featured at the top of this blog post shows twelve of these fascinating scoriaceous objects, all in the size range of 0.2 to 0.7 mm, all of which were found on rooftops of industrial buildings. Among >4,000 micrometeorites, these are almost all I have found of this type, so they are quite rare. One important question that comes to my mind is whether this rarity is a true reflection of their distribution among other micrometeorite types or caused simply by the fact that they are difficult to recognize?

These barely rounded particles are mostly transitions between porphyritic and scoriaceous, sometimes closer to the one and sometimes closer to the other. From the Atlas of Micrometeorites, we know that exact classification of ScMMs is only possible on sectioned particles, because the inner foam-like structure is what defines a micrometeorite as scoriaceous. The presence of large openings visible from the outside is not a real criterion for scoriaceous micrometeorites and is often observed on porphyritic olivine (PO-type) micrometeorites too. On the outside, we usually see a magnetite rim composed of octahedral magnetite crystals. This rim may vary in quality, and sometimes it is barely visible, partially visible, or not present at all.

Here are some scanning electron microscope (SEM) images that illustrate some classic ScMM features.

This is a detail SEM image of the surface of scoriaceous micrometeorite NMM 2595. In the collage this particle is in the bottom row, second to the left. This is a strange mix of olivine and magnetite chunks. Truthfully, I do not even dare to call them crystals, even though they most likely are crystalline. This is a pre-stage to the order we observe in more heated particles, where elemental differentiation is accumulating all the dense elements (iron, nickel, platinum, etc) into a metal core and the olivine has developed into well formed crystals.

The SEM image above shows micrometeorite NMM 1459, which is in the top row of the collage on the left. You can see that the degassing of volatile elements was not completed and the matrix has not undergone elemental differentiation.

And, last, here is a detail SEM image of the surface of the NMM 1459. The white specks, which are composed of magnetite, are starting to develop into octahedral crystals but the nickel-iron and sulfide has not yet differentiated into a core.

I hope you enjoyed learning about scoriaceous micrometeorites! If you’re curious, the micrometeorites in the featured image of this blog post are outlined below:

• Top row: NMM 1459, NMM 3328, NMM 2073, NMM 2484

• Middle row: NMM 4016, NMM 2530, NMM 3451, NMM 1240

• Bottom row: NMM 2086, NMM 2595, NMM 828, NMM 601

Before I sign off, I’d like to extend my sincere gratitude to Thilo Hasse, Martin Suttle and Matthew Genge for their extraordinary contributions to this field and sharing information about these strange space rocks so generously. The exploration of the micrometeorites continues.

And, to you, I say a heartfelt THANK YOU for being here and, as always, if you have questions, please connect with me on Facebook, Instagram, or Twitter. For questions about specific micrometeorites in this post, please be sure to use the catalogue ID!

To finish, I will simply say: please remember that we are always surrounded by beauty, even if all we see is dust.

Yours truly,

Jon Larsen