Sunday, March 8, 2015

Geo 1095: March 4, Day 793: Translucent Needles

Geologists use the term "acicular" to mean "needle-like," and here we see a nice example of acicular crystals of calcite. It looks to me as if this represents two periods of growth of this speleothem, the first very slow, with the initial crystals acting as seed sites for further growth. Water flowing over this surface deposited more calcite in a crystallographically consistent manner. You can see vague concentric color bands; those bands represent small variations in impurities during different times. However, you can also see that individual crystals extend through most or all of the bands, indicating that they grew over long periods. The outermost few millimeters, with the brownish crust, is much finer-grained, and I'm inclined to say chaotic-looking. But I can't make out individual crystals or their relationships to each other, so describing them as "chaotic" is beyond what I can actually observe. On the other hand, there was definitely some kind of environmental change between the deposition of the inner acicular crystals and the outer, finer-grained crust.

Photo unmodified. May 9, 2013. FlashEarth Location. (Since we're underground, I have only a vague idea where this is with respect to the surface.)

No comments: