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
to give you a chance to guess at what the rock is without any clues.

Top view : Weathered-out specimen. Approx : 2" x 1"

Bottom view : Weathered-out specimen. Approx : 2" x 1"

End view : Weathered-out specimen. Approx : 1" x 1/2" (cross-section view)

Some of the "crystals" still in matrix.

Another sample of the "crystals" still in matrix.

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!
(I'll include responses at the bottom of this page as they come in)

Brenda@WoodenSki.com

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 :-).

The moment of sunrise at the top of Mount Kilimanjaro : Glacial remnant, Mount Meru (small), and the large shadow of Kili.

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.
Cheers, Nigel"

2Neat adds : Nigel is entirely correct that the crystals are NOT olivine / peridot ... that is common around here and we would recognize it. We also have a lot of granite pegmatites here in the Colorado Rockies and those mystery rocks didn't come from anything similar to our pegmatites. However, we do know that other kinds of non-granite pegmatites do exisit. The shape of the crystal is certainly similar to Rutile if not the same.

Our reading on rutile led us to the following two statements :
1) "anorthoclase crystals in trachyte from the Kibo cone of Kilimanjaro." And
2) "The most famous Anorthoclase localities are Mount Kilimanjaro, Tanzania; Pantelleria Island, Italy; Tjolling, Norway; and Obsidian Cliff in Yellowstone National Park, Wyoming."
What do you think? - Anorthoclase?

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"

2Neat adds : Further searching using Rob's input led to this information and a photo of a sample much like ours : "A strange, elongated twinned crystal of anorthoclase (a member of the feldspar group and the midmember of the microcline and orthoclase series) without matrix from Mt. Erebus, Ross Island, Antarctica! The crystals occur inside volcanically-ejected "bombs" which then weather away to expose the anorthoclase. "

Thus the question is answered. THANKS! I hope you had fun guessing!

Thanks everyone! More responses are welcome!

See the previous mystery rock.

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