Through the years I have studied a large number of small spherical particles, called spherules, of both terrestrial and extraterrestrial origins. Out of the many thousands of spherules that were easily classifiable, there have been a handful of cases where the origin remains uncertain. These enigmatic spherules are my favorite because they challenge existing knowledge and bring about a chance to learn something new.
A new mystery
Occasionally, in a very exciting twist of events, new findings throw light upon an old mystery from years ago. One such case is a spherule found on the roof of an industrial building a couple of months ago.
At first glance, the 0.45 mm (450 µm) object looks too round and too metallic to be a micrometeorite. It reminds me of an industrial two-phase iron oxide spherule of the type I’ve seen so many times before. The particle looks like a golden metal sphere, partly encapsulated in a mantle of silvery iron oxide. It seems that every clue points toward a terrestrial origin, but a deep instinct activates my curiosity.
I bring the mystery spherule in to the studio to be photographed by Jan Braly Kihle. My hope is that the increased magnification will enable me to study it in greater detail. However, even after the photo session, which reveals a huge golden sphere partly covered with steel grey iron oxide, the origin is still uncertain.
So next I bring the strange spherule for a scanning electron microscope (SEM) analysis. The analysis reveals that the gigantic golden core is composed of nickel sulfide, and the grey iron oxide was just what it appeared to be. I think to myself, sadly, that among 99 analyses this is the going to be the only one to not reveal anything conclusive regarding the origin.
Then, following another instinct, I zoom in on the structure of the nickel sulfide at 30,000 times magnification. Instantly, I see a phenomenon I recognize! The structure is exactly like something I had seen on a micrometeorite two years earlier.
Two new connections
NMM 2025 is a porphyritic olivine (PO) type micrometeorite, measuring approximately 0.2 mm (200µm), with an unusual crystalline nickel sulfide area measuring approximately 0.08 mm (80 µm). The texture has a distinctive “leaf-like” characteristic, where the orientation is changed in a polysomatic way. Could the new discovery, five times larger, be a micrometeorite after all?
The answer became obvious when I studied the back side of the new finding. A coating of unmistakable
porphyritic olivine crystals covers the gigantic metal bead like a thin silicate mantle. The result is a micrometeorite like no other I have seen before. Basically, NMM 3324 is a nickel sulfide ball!
This, in turn, reminded me of another mystery particle that had been put aside due to an unknown origin
years ago. It is a spherule measuring approximately 0.3 mm (300µm) of greenish nickel sulfide with the same leaf-like texture, just like the other two. Because of these new findings, I assert that this third enigmatic spherule, named NMM 1929 is also a micrometeorite.
Two new research questions
Given these findings, two important questions arise: Could this be an unknown type of micrometeorite? And, what is the minimum ratio between nickel-sulfide and silicate in a micrometeorite?
I dearly hope that you have enjoyed learning about this mystery and seeing photos and SEM images of these enigmatic spherules!