Seems our Canadian Wilbert Smith is responsible this micron-sphere story, all my books just repeat the same thing essentially...
The only other case I read about such spheres is when Art Bell received that metal, back in 96. Read about the
Metal fragments received by Art Bell in 1996, fragments which had nothing special about them, but the "electron
dispersion spectroscopy revealed that some of the samples were almost pure aluminum. The other items, various
odd-shaped pieces of metal, consisted of silicon granules..."
To close this chapter, for now, here's a brief recap of that Wilbert Smith testimony...
above top secret, page 188
"During a recorded interview with C.W. Fitch and George Popovitch in November 1961, Wilbert Smith admitted that a number
of fragments had been recovered and analyzed by his research group, including one that had been shot from a UFO
near Washington DC, in July 1952....There was iron rust-the thing was in reality a matrix of magnesium orthosilicate.
The matrix had great numbers-thousands-of 15-micron spheres scattered through it."
UFOs and the National Security State
There is an uncorroborated claim that the air force shot down a UFO during the July  crisis.
The source is Canadian gov official and UFO researcher Wilbert Smith. In an interview conducted
almost ten years later, Smith claimed to have shown his friend, U.S. Navy Rear Admiral H. B. Knowles,
"a piece which had been shot from a small flying saucer near Washington":
I showed it to the admiral. It was a piece of metal about twice the size of your thumb which had been loaned
to me for a very short time by your air force...As a general thing they differ only in that they are much harder
than our materials. [It was] in reality a matrix of magnesium orthosilicate. The matrix had great numbers-thousands-
of 15 micron spheres scatterd through it.
Unconventional Flying Objects
Relays the same incident, but Paul Hill gives us a bit more data on the origins of the info:
"Frank Edwards has a most interesting Chapter 4 in his 'Flying Saucers: Serious Business', entitled "Pick Up the Pieces"...
C.W Fitch of Cleveland Ohio, and George Popovitch of Akron Ohio. They arranged an interview with Mr. Smith, and they had
the foresight to record what was said. Thanks to them I have a copy of the tape and it is from that source that the following
material is taken."
Interview done on [November 1961] (As in the UFOs and the 'National Security State' book said, almost ten years later)
The quotes are, identical to the ones above so I won't repeat them needlessly.