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?
It is the peak of fall. Not the peak of color, but the peak of sound and the peak of smell.
A few weeks ago the ground was patterned, the leaves of each tree pooled around it in fallen skirts of color: yellow here, orange there, reddish brown over there. Now they mix promiscuously, stirred into an impossible puzzle of divergent shapes. The downed leaves, their colors muted but not gone, rustle as I shuffle through them, a few shattering with each step. The smell of tannins fills the air. There is nothing like it.
I did an early run at Minihaha Creek. Our summer, which after an exceptionally dry winter and spring, has been relentlessly moist, continues in its steamy mode. So morning is the time to run, when the temperature is still in the 70’s.
It was pleasant to run, feeling the air, heavy with water, slip by me. I feel immersed in it. And I notice smells a lot more. A lot of things are blooming, and though I don’t have a very sensitive noise, I notice that the scents shift as I run: here it is sweeter, now I get a bit of mildew, and later a the scent of wild roses, one of the few flower scents I recognize. A bit later I catch an acrid whiff of marijuana, no doubt drifting down from a car stopped along Minihaha parkway. It makes me appreciate the floral and herbal scents all the more.
Our not-much-of-a-winter has passed, notwithstanding a few flakes eddying about in the gusty air today.
I went for a run along Minihaha Creek yesterday. The ice is entirely gone, and the muddy spots from the last snowmelt are dry. We’ve not had much in the way of moisture, either snow or rain, and I’m told 80% of the state is in drought conditions. We shall hope for a moist spring, though not so moist as to bring disaster upon the farmers.
It is interesting to look through the columns of the forest, and see, here and there, a tree hanging onto last year’s leaves.
After a long unusually warm period, we’ve had a brief period of cold and snow. The creek is still flowing free in most parts, and ice is mostly opaque white granular frozen shelves that protrude from either bank. However, there are areas where recrystallization has occurred, often, though not exclusively, under bridges, and often, though not always, showing signs of melting/refreezing.
Here we can see melting refreezing, with the fine-structure of the crystals erased, and only the large acicular macro-crystals visible.
refrozen acicular crystalsrefrozen crystals
Something else that interests me is that many of the masses of crystal, both refrozen and ‘fresh,’ have lots of empty space in them. You can see that a little in the image on the upper left; it is more visible in the photo just below. In a book I’m reading, Above the Trees, about the alpine tundra environment, it describes how sheets of ice form across the tops of small depressions, which then act as miniature greenhouses, warming the air below them. I wonder if heat rising from the flowing water is likewise trapped by the panes of ice above them, forming these honeycomb patterns of ice and air.
Sheets of ice, and the acicular crystals, often seem to me at an angle to the surface of the water of about 10-15°. On possibility is that the water in the deeper part of the creek is warmer, and so that there is a temperature gradient that affects the level at which ice crystals grow over the water. I think, but am not certain, the the ice crystals on either bank of the creek angle away from the water as they project out over the stream.
‘honeycomb’ of acicular crystals and plates
Finally, although much of the ice is refrozen, there appear to be areas of fresh crystals, or at least crystals that still show fine structure on the surface. One of the images below (right) is from under a bridge, where I would expect it to be cooler and sheltered from the sun, but the other (left) is from a more normally exposed area of the creek.
mat of acicular crystals with fine structureacicular crystals and plates
The image shown on the left is particularly interesting because it retains a lot of fine structure — etchings and dendrites — on the crystals. More detail below.
We continue to have above freezing weather during the day, dipping below the freezing point at night. The leads to a lot of melting and re-freezing on the creek. There are areas where the creek is entirely frozen over, and others where it runs free, except for a fringe along its edges. The ice itself is quite varied. Some is clear, some an opaque white; some has a dull granular texture, some is a palimpsest of crystals upon cyrstals
We are in the midst of an unusual warm spell. It is not unusual to have a few days of above-freezing weather in January – enough to melt the top layer of snow and ensure that in a few days everything will be a sheet of ice through Februrary – but this is different. It reached the fifties yesterday, and came close to that today. Almost all the little snow that we’ve gotten is gone. The brown grass is showing hints of green, and the bare trees have visible buds visible. Any more of this warmth and bulbs will sprout and buds will burst, and then cold will return and kill them.
I returned to Minihaha Creek five days after my previous visit for a run, and a look at the ice. The snow, not very deep before, has receded, and the fallen leaves arch above its surface, leaving a myriad of little cave-lets speckling the surface. I wonder if there are any organisms — macro or micro — that are adapted to take advantage of these niches.
We had a nice outing today. The main event was the play ‘Art’ at the Guthrie.
‘Arts,’ a play by Yasmina Reza, is not about art per se, but rather a satire on friendship, and the dynamics of the power structures that underlie, or perhaps constitute, friendships. The play begins with a purchase of a modern painting – all white – by one of the trio of friends, and the ensuing reactions of the other friends. While the art purchase is that catalyst, it quickly becomes clear that what is at stake is really the changing relations between the friends. Although there are a lot of funny moments, the deepening disagreements and the vehemence with which they were conducted brought ‘Who’s Afraid of Virginia Wolfe’ to mind. The acting was excellent, particularly that of Max Max Wojtanowicz who played Yvonne, the ‘buffoon’ of the group. The lighting and sound design was also excellent, in particular in signaling moments of solliquoy by the various characters.
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.
We’ve had a remarkably long fall. Although we’ve had some chilly periods, the temperatures are still tending to edge above freezing during the day, and in the last month we’ve had days that make it into the forties, and even the fifties. We’ve had two light snows: one but a dusting, and the second perhaps an inch, enough to almost bury the downed leaves. So the world is mostly gray and brown: the dully gray winter sky arches overhead, and the grass, trees, and carpet of leaves is brown brown brown. Very dull.
I have been waiting, with anticipation, for the first ice crystals to form on Minihaha creek, where I do most of my runs. I enjoy looking at ice on the creek as the winter progresses. First, most of the water is open, and a crystalline fringe forms along the edges of still portions of the creek. Next, the fringes grow, meeting in a delicate and parlous surface in the middle. As the cold deepens, the ice thickens. In the early part of the winter the ice tends to be crystal clear — although depending on the way in which the crystals form the may make portions of the surface matte white.
On Saturday, October 21st, I was practicing the piano. As is my custom I had headphones (the piano is electronic, and I make lots of mistakes that I prefer not to share with the world). The headphones provide a pretty significant degree of sound isolation, but nevertheless at some point I began hearing yelling or shouting. I wondered if maybe it was some high spirited high school kids passing by, even though that is not something that happened before.
I had a peculiar dream. It was a segment in the midst of a long, loosely connected series.
It was just after sun set, or thereabouts. Twilight. The western sky was a deep blue, almost gray. I was at a beach, or perhaps near a beach; I did not see the ocean, but somehow knew it was nearby. I walked inland, moving uphill, following a twisting path through boulders and shrubs, and came to a flattened rise. There were others there. In the twilight I could not make out their features. They were more than shadows, but less than shades hades. They came together into a crude circle, and I joined them. I don’t know why. I just did. Then they began joining hands. I was a little reluctant, but the person next to me reached out and so I took his hand. Everyone crouched a bit, and so I did too. And then we all jumped, straightening our legs, propelling ourselves into the air. And up we went, slowly, and at the apex we floated for a moment, and then, captured by the faint pull of gravity, we sank back towards the earth, touching down softely. Again we jumped in unison, and floated, and sank. And yet again.
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.
I am sitting outside, at a picnic table, while my windshield is being replaced at Safelite Autoglass. It turns out that replacing a windshield is no longer a simple thing. Windshields no longer just protect you from the elements, they also contain sensors, which must be recalibrated.
Discovering that this would take a while, I choose the company who would replace it — it is all entirely paid for by insurance — by its proximity to the Surly Brewing company. I used to stop by fairly frequently three or four years ago when I was taking classes at the University. It was convenient to the bikeway that led to the U, and so it was a nice place to stop for a beer and a nibble and to do a bit of homework on the way home from classes. Unfortunately, I did not check its hours. In the post-pandemic employee-constrained world, it now does not open until 3:00pm, well after the time when I expect my car to be finished.
But at least the day is pleasant: the clouds from a storm passing by to our south are dissipating, the sun is shining, and the temperatures are cool — in the seventies. That said, with another one and a half hours to go, I see that my shade is disappearing and that I will soon want to find another spot.
For the first quarter’s curriculum, I laid out a set of aims, both general and specific. I revisit them below, with comments on how they went (or if they went) and what the results were, and then think aloud about what to aim for in Q2
Aims for Q2: • Study: Studies going fine; consolidate essay work. • Write: Focus on workshop; look for venues for publication and sharing. • Do: Continue local activity; plan regional/national/international travel. • Maintain: Ramp up running; learn to make seaweed salad & miso soup.
The vernal equinox arrived yesterday. And, indeed, for the last week or so there have been days where it’s sunny, a bit of thaw is in the air, and the birds are positively boisterous.
I started running again, after a lapse of nearly a month, and a sparse few weeks before that. There is still a layer of ice on everything, but with the temperature getting above freezing most days, running with spikes works. Minihaha Creek is flowing freely, and I think the period where I will see either ice crystals, or the faintly green algae-infused ice, is past.
Happy March 1st! There are now some cracks in the shroud of winter. Oftentimes, it seems to me, there is a period of a few days, usually in February, where the birds suddenly become more active. It is perhaps the first real hint of spring — real, in contrast to the phantom spring thaw that often happens in late January.
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.
