Characterization of micrometeorites and explanations of the database

Even if every micrometeorite is individual in its overall appearance, micrometeorites can still be classified or grouped on the basis of characteristic features.

In the database https://www.micrometeorites.org/database, such information from surface analyzes of several hundred micrometeorites (mainly cosmic spherules) as well as light photos, scanning electron images and EDX data are available. For some micrometeorites there is also information from sectioned and polished parts thus giving insights into the interior of the particle.

The database not only wants to give insights into the characteristics of the micrometeorites, but also to give an impression of their frequencies. The selection of the filter criteria does not follow any recognized guidelines but my personal selection.

The data can be filtered in the selection view according to selected characteristics. In the single view (detailed view), all available information is compiled for each micrometeorite.

Selection View

The selection view offers the following options (see image below):

  • Click on an image or on the ID of a micrometeorite in the displayed list to open it in the single view and get detailed information
  • Switch the page if there are more micrometeorites in the selection than show on the screen
  • Use the filter to define your selection (explanation on the filter options see below)
  • Reset filters
Selection view of a set of micrometeorites with explanations

Single View

When switching to the single view of a micrometeorite you have the following options (see image below):

  • Activate image series by clicking on the magnifying glass
  • Hover the cursor over the active image to magnify it
  • Click on any small image to make it the active image
  • Click on the arrows in the top right corner to go to the previous or subsequent dataset
Single view of a micrometeorite with explanations

Using database filter

By using the filter you can modify the selection of micrometeorites displayed in the selection view. The particles criteria are:

  • Classification
  • Metal differentiation
  • Magnetite appearance
  • Chromite crystals
  • Various other surface properties
  • Size class
  • Available documentation

Their sub-categories (e.g. „Fe / Ni bead“) are primarily used as selection criteria. The superordinate categories „Classification“, „Metal differentiation“ etc. are only used for grouping these selectable criteria. Clicking on the plus sign to the right of the category name opens its sub-categories, clicking on a minus sign closes them again.

Several selection criteria can be combined. An „and“ link applies here, i. H. the selection is limited to micrometeorites that meet all of the selected criteria.

The number of available data records (depending on the current selection) is displayed in brackets behind each selection criterion.

The selection criteria are explained in detail and examples are given below.

Classification

The following figure shows a typical representative of each of the classification sub-categories that occur. The classification is based on Genge et al. 2008 and regarding the subdivision of type CC according to Suttle and Folco 2020 (see also here). It should be noted, however, that the variation is quite high and there are particles that are difficult to assign in the classification based on surface characteristics only.

Light images of typical representatives of the micrometeorite classification
SEM images (BSE, backscatter electron images) of typical representatives of the micrometeorite classification

Metal differentiation

When the particle melts as it passes through the Earth’s atmosphere, the metal and silicate phases often separate. After solidification, these appear as metal spheres or metal droplets on the outer edge of the silicate body or as exit holes when the metal droplet has completely detached from the particle. Metal beads contain mainly iron (Fe) and often also nickel (Ni), in smaller amounts sometimes cobalt (Co) or other small admixtures of elements. The degree of oxidation varies. The following graphic with SEM-BSE images shows some representatives.

SEM images (BSE, backscatter electron images) of different metal differentiations

Appearance of magnetite and chromite

Magnetite almost always forms at the surface of cosmic spherules and other micrometeorites when iron gets in contact with atmospheric oxygen and it oxidites. Depending on the speed of this process and the chromium content of the particle, different shapes can result, which are shown in the following graphic. Chromite crystals form at particularly high Cr contents, but could also be relics of the unmelted material.

SEM images (BSE, backscatter electron images) of different magnetite appearances and chromite
SEM images (BSE, backscatter electron images) of micrometeorites with various magnetite (and chromite) appearances

Surface properties

Here a choice of various other frequently observed characteristics are summed up (see figure below).

SEM images (BSE, backscatter electron images) of micrometeorites with miscellaneous surface properties

Size class mean

These categories divide the micrometeorites into certain size classes, according to which they can be filtered.

Documentation

This defines which information about the particle is available in the database. Mostly these are:

  • light image: using a microscope, digital camera, ring lighting and photo stacking
  • SEM image: backscattered electron (BSE) imaging at scanning electron microscope (SEM)
  • EDX data: chemical analysis by energy dispersive X-ray spectrometry (EDX, EDS)
  • sectioned: SEM image: backscattered electron (BSE) imaging at scanning electron microscope (SEM) of sectioned and polished particles set in epoxy resin
  • sectioned: EDX data: chemical analysis by energy dispersive X-ray spectrometry of polished particle interiors (EDX, EDS)

Suggestions

Still no idea how to use the filter options? Here are some simple query suggestions:

  • Show me all micrometeorites that have sections and that belong to the Porphyritic Olivine (PO) type
  • Show me all G-type micrometeorites
  • Show me all micrometeorites that are larger than 250 µm and that have a metal bead

Would you like to use the database to answer certain questions? Here is an Example:

Question: Where do metalbeads occur most often, within type PO, BO or CC?

Action:

The total number of micrometeorites is currently (in this example) 315, of which 114 are assigned to type CC, 119 to type BO and 73 to type PO. In total, a metal bead was observed in 53 of the 315 micrometeorites (16.8%). If you restrict the selection to type CC, there are 22 with a metal bead (22 of 114 = 19.3%), for type BO there are 15 (15 of 119 = 12.6%) and for type PO there are 14 (14 of 73 = 19.2%). So metal beads are a little rarer for type BO than for types CC and PO. Whether this is statistically significant or a random observation remains open.

However, if one analyzes the escape holes from metal beads for these three types at the same time, one finds that these are significantly more common with type BO. This leads to the conclusion that metal beads might peel off more frequently in the BO type than in the CC and PO types.

The critical observer may have noticed that in rare cases a particle is classified into 2 types. This is e.g. the case, when the classification based on surface features deviates from that which was carried out after viewing the interior of the particle. This is then noted accordingly on the detail page of the particle.

Feel free to explore the data and make your own assumptions!