Fractal Metal Shards all the way down

3 responses to “Fractal Metal Shards all the way down”

1. 

   jurvetson

   Aug 27, 2020 at 1:29 am

   From Yang’s detailed analysis (Geochemica PDF 2013) "Thermal and collisional history of Tishomingo iron meteorite: More evidence for early disruption
   of differentiated planetesimals":

   "The Tishomingo iron meteorite is a particularly interesting meteorite because it has a very unusual composition, a unique structure, and a low temperature thermal history that is constrained by its microstructure. Tishomingo is a chemically and structurally unique iron with 32.5 wt.% Ni that contains 20% residual taenite and 80% (hard-steel) martensite plates, which formed on cooling to between -75 and -200 °C, probably the lowest temperature recorded by any meteorite. The absence of cloudy taenite in these two irons shows that they cooled through 250 °C abnormally fast at >0.01 °C/yr. Thus this grouplet suffered an early impact that disrupted their parent body when it was still hot. Our noble gas data show that Tishomingo was excavated from its parent body about 100 to 200 Myr ago and exposed to cosmic rays as a meteoroid with a radius of 50–85 cm.

   Optical and SEM images of the microstructure:
   (a) Optical image of a typical region showing martensite plates (dark brown) and residual taenite (light brown) after etching with 2% nital. (b) At higher magnification, a SEM image of several martensite plates containing midribs. The residual taenite regions are structureless.

   (c) Optical image of unetched region containing a sulfide–metal intergrowth about 100 lm in size. Irregularly-shaped filaments of FeS are intermixed with high Ni FeNi and with low Ni high Co metal and daubre ́elite (light gray and indistinct). (d) Optical image after etching of the same sulfide–metal intergrowth as in (c). The low Ni high Co metal, which is concentrated in the rim of the intergrowth, is overetched and appears black. The martensite plates and retained taenite regions around the sulfide intergrowth appear unaffected. However, the sulfide intergrowth appears to extend slightly further into the martensite plates than the adjacent taenite.

   Helium Ion Microscope (HIM) images of decomposed martensite a2 and residual taenite at increasing magnification. In the decomposed martensite, the bcc kamacite phase (dark gray) was etched away and the unetched phase is fcc taenite (light gray). (a) A triangular smooth featureless region of residual taenite surrounded by the decomposed martensite:
   (b) A decomposed martensite regions containing crystallographically oriented particles of taenite.

   (c) At the highest magnification, the decomposed martensite plates are seen to contain both coarse and fine taenite particles 10–100 nm in size:
   And from all of the analysis, the inferred thermal history of Tishomingo:The meteorite cooled so rapidly at >0.01 °C/yr through 250 °C that cloudy taenite could not form (t1). It cooled to a temperature between -75 and -200°C so that 80% of the taenite was transformed to martensite. A second impact at t2 shocked the meteorite to <30 GPa and heated it to 320–400 °C for about a year causing martensite to decompose to kamacite and Ni-rich taenite. The third impact excavated Tishomingo as a meter-sized object around 100–200 Myr ago and exposed the object to cosmic rays.

   Reply
2. 

   jurvetson

   Aug 27, 2020 at 3:12 am

   and a close-up of mine

   Reply
3. 

   jurvetson

   Jan 24, 2021 at 2:15 pm

   I took some 5x Macro Zooms of this slice:

   Reply