We have an answer now ... but do try your hand at ID for fun!
I"ll put the description
of collecting location and other information below
all the photos
Collecting Location : Mount Kilimanjaro, Tanzania, mostly above timberline, above 11,000'
ORIGIN : These are obviously of volcanic origin (!), but I am very interested to hear more about them. Not every rock type on Kili had these inclusions or crystals, but many of them did. I"ll call them "crystals" from now on for simplicity.
CRYSTAL SIZE: The crystals varied in size (that I noticed) from around 6" long and 3" across down to about 1" long and 1/2" across. 2" x 1" was a very common size among the weathered-out crystals. The smallest ones were mostly visible only in a matrix (not weathered-out) and were often really flat in cross-section.
CRYSTAL SHAPE : In cross-section, a flattened diamond shape. 2-3 times longer than wide. About twice as wide as thick. I.e. one crystal might be 2.5" long, 1" wide, and 1/3" thick. The well-formed crystals have four flat surfaces, two on the top and two on the bottom.
CRYSTAL MATERIAL : Glassy or glossy, and filled with air bubbles. Grey to Black. The crystals must be just slightly harder than the matrix in which they were found ... they commonly weathered out of the matrix and remained intact, but they were also often found broken through (i.e. cracked across cross-section) and still in the matrix material. See last two photos above. We also found many weathered boulders where the crystals were still intact in the matrix.
MATRIX MATERIAL : Clearly volcanic. Almost certainly a basalt. Sometimes with air bubbles, sometimes not (or tiny). Greyish to blackish to reddish brown.
Please tell me
what you can about these crystals ... I'm all ears!
Also, I would love suggestions for a book to help me identify various kinds of volcanic / igneous rocks. I'm looking for something in between the Layman-level and the Hard-Core-Geologist level. Ideally, it would have lots of color photo samples that one could compare to actual rocks. I've read lots of books on Volcanism and Plate Tectonics, and already own a bunch of field guides to Rocks, Minerals and Gems. I'm looking for something more specific to Volcanic rocks, with photos to help me learn the field-level differences between Welded tuff, Volcanic breccia, and such.
THE STORY BEHIND THE CRYSTALS : As anyone who has climbed Mount Kilimanjaro will know, the guides make you walk really, REALLY slow. Ridiculously slow! This is their concept of acclimatization. They constantly say "Pole, Pole" which means slow, slow. It is pronounced Poe-Lay. In an effort not to get annoyed at the Pole, Pole, I examined and collected rocks all the way up. I had plenty of time to do this and still keep up, and it surely helped my attitude! Thus, we will forever call these POLE POLE rocks :-).
NIGEL notes : "Your
mystery mineral looks very much like rutile and the rock is perhaps
a pegmatite rather than a basalt. The only basalts that carry euhedral
crystals of the size you mention are the olivine basalts and the
crystals tend to be greenish (peridot) rather than grey-black.
ROB agrees : "The large crystals in your mineral specimens look alot like anorthoclase, a type of feldspar. I have some specimens of anorthoclase from Mt. Erebus, Antarctica. Best, Rob"
MIKE adds : "Pumice bearing Ignimbrite."
Thus the question is answered. THANKS! I hope you had fun guessing!
Thanks everyone! More responses are welcome!
See the previous mystery rock.