Aragonite – A Mineral Ramble

I’ve been on a little internet ramble. The topic is aragonite, a form of calcium carbonate. Here are some pictures of inorganic crystaline aragonite, though mostly this post is about biogenic aragonite in sea shells.

I began with the following bits of knowledge:

Aragonite is one of two*^/** polymorphs. In terms of minerals, calcium carbonate has two polymorphs: calcite and aragonite. That is, both are calcium carbonate, but they have different crystal structures, and are thus considered different minerals.
It is metastable. I also knew that aragonite is metastable (that is, stable under very limited conditions, and easily nudged into instability).
It occurs in sea shells. I recently learned that aragonite occurs in sea shells and corals, and was curious why an ‘unstable’ mineral would be useful in a biological setting. Is it common in shells, or is it rare? Does it have advantages over calcite? Or is it just easier to synthesize?

A Geology Ramble: Tuff to Granitic Alteration

6 January 2024

This morning I was reading through the geology subs on reddit and came across something that I thought might be rhyolite or volcanic tuff, and then started wondering what the distinction between them was. I started searching, and soon my quest turned into more of a ramble. Here are some of the things I learned.

Rhyolite vs. Tuff. As it turns out the difference between rhyolite and tuff is that rhyolite is has a very fine-grained aphanitic texture, where as tuffs generally have a coarser and more varied texture, and may show some sorting. Rhyolite may have mineral grains (e.g. small quartz crystals) within it, but they are embedded in a ground mass of fine-grained material.

Welded tuff occurs when the ash is more than 600° C (1100° F). Essentially glass and pumice fragments adhere, “necking at point contacts,” and deform and compact together.
Unwelded tuff is relatively unconsolidated, but if it contains a lot of volcanic glass (a thermodynamically unstable mineral) it will lithify rapidly in the presence of water, which leaches alkali metals and calcium and forms new minerals (zeolites, clay, calcite) that cement the tuff.
Tuff Rheology. Tuffs may range from well-sorted, when produced by ash fall, to poorly sorted, produced by pyroclastic flows and surges — the latter my sometimes exhibit sedimentary structures such as dunes and anti-dunes produced by high velocity flows. In flows of tuff, the bottom will often be unwelded and poorly consolidated, due to compact with the cold surface.
Anti-dunes are flow structures in which material accumulates on the ‘upstream’ side of the structure (whereas with dunes material accumulates on the ‘downstream’ or lee side). Anti-dunes migrate ‘upstream,’ and grow rapidly as they move counter-current until they collapse.
Fiamme are lens-shapes, mm to cm, usually seen in volcanoclastic rocks like tuffs.

Somehow, I also got into ways in which granite can alter, but I’m not sure how

Miarolitic cavity — a crystal-line cavity that can be found in granitic pegmatites. Often formed by volatile portions of magma excluded from the crystal phases — eventually the volatiles will form cavities. These cavities, in turn, often contain unusual or rare minerals that are incompatible with silicate granite mineralogy.
Griesen is a highly altered granitic rock or pegmatite. Griesen is formed by self-generated alteration of granitic magma taking place at moderate to high temperatures; it involves hydrothermal-magmatic alteration and is often related to the release of volatiles in the later stages of solidification. Griesens exhibit variable alteration and range from coarse crystalline granite with miarolitic cavities to rocks rich quartz and muscovite.
Stockwork. A structural system of structurally controlled or randomly oriented veins.

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Mid-September: Clear Fall Light, Yosemite, etc.

13 September 2023

In my last entry I was sitting outside in full summer, waiting for my car windshield to be replaced, with my hopes for a beer at the nearby Surly brewery dashed by their continuance of ‘Covid-hours.’ Now, abruptly, after a string of hot dry days, fall is here. Oh, leaves have by and large not started to color, but the light has changed. The air is clear of humid haze, and perhaps, as well, the lowering slant of the sun does something to the color of the light.

May Day Miscellaney & ILSG 2023 Fieldtrip Notes

Monday, 1 May 2023

It’s been six weeks since I last wrote a daily entry. …What’s new?

Ice Crystals on Minihaha Creek

Saturday, 18 February 2023

The winter here has been weird – a couple of sub-zero weeks, a couple of warmish (32ish) weeks, repeat. I run several days a week, mostly along Minihaha Creek, which winds through south Minneapolis. With the variations in temperature there is a lot of thawing and refreezing, and that, combined with changing water levels in the creek, results in marvelous ice crystals. Mostly they are two dimension intergrowths of needle-like crystals, almost fabric-like in their structure.

Meta thoughts on meandering on the net: Taleb, Perthitic Textures and Chocolate

Thursday, 5 January 2023

Some days I spent a lot of time doing associative reading, where one text leads me to hop laterally to another text, and so on. I find this pleasurable, but often, after a few hours, have little sense of what I have learned.

Today I am going to try to track, at least partially, the path of my attention.

I did not begin here, but a good starting point is Taleb.

Life, the North Shore, and Geology

Wednesday, 25 May 2022

Two weeks and change have passed, and here I am again.

The notable events for the last couple of weeks include ‘graduating’ from physical therapy, a trip to the north shore, and… well, I think not much else.

Huff huff