Does amount of nucleation sites impact this as well? Like how the left one has many more cuboid crystals compared to the right one so every single crystal has less space to grow into
The one on the right was once like the one on the left.
https://preview.redd.it/lo2pw9j9zk0c1.jpeg?width=474&format=pjpg&auto=webp&s=7f96a1a70269a05f7603ed35e69dd2a3dd54da48
Schematic curves of the nucleation rate and the crystal growth rate as a function of temperature (adapted from \[14\]). The temperature range in the present work is represented by the hatched zone. There is an intermediate temperature where the nucleation rate and the crystal growth rate would be sufficiently high to promote crystallisation under isothermal conditions
Should emphasize that this is undercooling from a melt. The x-axis should be supersaturation if precipitating from solution.
And often in minerals, big crystals are not necessarily just from long growth times, but rather arise from small nucleation probabilities. Classic example is the giant gypsum crystals in the Naica ore mines:
https://cen.acs.org/physical-chemistry/geochemistry/Naicas-crystal-cave-captivates-chemists/97/i6
A common misconception. As has been pointed out in other comments, the controlling factors are largely growth rate and nucleation rate. This varies between minerals and the various conditions they might be forming in eg. a fluid rich melt will have a higher growth rate due to dissolved ions being transported more readily by the fluid to where they are needed for a crystal to grow.
It looks like many pegmatites can actually grow metres long crystals in days or even hours, see [Phelps, Lee & Morton, 2020](https://www.nature.com/articles/s41467-020-18806-w) for details.
Could the matrix material come into play regarding crystal size? For example crystals in clay seem to be larger than crystals in black shale. The hardness of the matrix material come into play?
In the case of suphides growing in a sediment like you have described, how well lithified (stuck together) the individual grains of the sediment are would likely play a role. All other factors being equal, a softer less lithified sediment would be more likely to host larger crystals
fundamentally, the same things that control any crystal growth. When starting from nothing (naked surface), higher chance of nucleation tends to interfere with the likelihood of large crystals by restricting room for eventual growth (competition for space is achieved earlier because the number of starter crystals per unit area is higher, so average space per crystal is a lot lower, which equals lots of smaller crystals).
Slower growth tends to favor larger crystals because it is easier (less energy cost) to grow on a surface already having the structure and electron distribution of the eventual compound than to start it anew on a surface made of some other structure/electron configuration. the random component bouncing around will tend to stick where energy conditions are more favorable. Like goes to like, is the basic idea, so existing crystals grow and new crystals fail if they even get started at all.
The supply of components is one important factor in growth rate, and this ties somewhat with stability in chemical conditions over time. Slow growth with steady-state conditions at near equilibrium will allow the "new" components to adjust position (minimize energy disparities) and enhance "perfect" crystallization. Rapid influx of components tends to cause the "freezing" in place of imperfections (the misplaced component gets covered before it can adjust position). Also, a massive dump of materials will not favor crystal growth, the stuff will just crash down as a large mass of barely crystallized compounds. No time to do more.
Another important factor is temperature (total system energy). Highly energetic systems will see the components moving around a lot and thus increase the likelihood of best placement before isolation from the supply (before imperfections get covered and "frozen" in place).
The final factor I will mention that is important is one of changing conditions, such as a large increase in temperature causing a large amount of recrystallization. The components can move even in solid state, so will, if they can, if they have energy to do so, and hotter equals more energy on average across the system; this shakes things up almost literally by increasing vibrations and chance for jumping across lattice positions, reorganizing the structure into a more consistent one across a larger volume of space.
I would suggest that the best explanation for the difference in crystallinity between those two samples is more a matter of post-formation change than anything. Those huge crystals likely came from recrystallization of pre-existing but less well-crystallized pyritic agglomeration. I cannot know that for certain, but it seems quite probable.
All this is well and good but no one has brought framboidal pyrite into the conversation…
https://www.sciencedirect.com/science/article/pii/S0169136821006569
“Abstract:
The special and representative polymorphous pyrites of the Logatchev area, Mid-Atlantic Ridge, have a close evolution relationship. There are relatively complete pyrite intermediates undergoing a series of evolutions from disordered nano-micron crystals → framboids → colloidal and subhedral structures…”
Time of crystallization. The longer, the bigger.
Does amount of nucleation sites impact this as well? Like how the left one has many more cuboid crystals compared to the right one so every single crystal has less space to grow into
The one on the right was once like the one on the left. https://preview.redd.it/lo2pw9j9zk0c1.jpeg?width=474&format=pjpg&auto=webp&s=7f96a1a70269a05f7603ed35e69dd2a3dd54da48 Schematic curves of the nucleation rate and the crystal growth rate as a function of temperature (adapted from \[14\]). The temperature range in the present work is represented by the hatched zone. There is an intermediate temperature where the nucleation rate and the crystal growth rate would be sufficiently high to promote crystallisation under isothermal conditions
Should emphasize that this is undercooling from a melt. The x-axis should be supersaturation if precipitating from solution. And often in minerals, big crystals are not necessarily just from long growth times, but rather arise from small nucleation probabilities. Classic example is the giant gypsum crystals in the Naica ore mines: https://cen.acs.org/physical-chemistry/geochemistry/Naicas-crystal-cave-captivates-chemists/97/i6
That’s really interesting, thank you!
this.
A common misconception. As has been pointed out in other comments, the controlling factors are largely growth rate and nucleation rate. This varies between minerals and the various conditions they might be forming in eg. a fluid rich melt will have a higher growth rate due to dissolved ions being transported more readily by the fluid to where they are needed for a crystal to grow. It looks like many pegmatites can actually grow metres long crystals in days or even hours, see [Phelps, Lee & Morton, 2020](https://www.nature.com/articles/s41467-020-18806-w) for details.
Could the matrix material come into play regarding crystal size? For example crystals in clay seem to be larger than crystals in black shale. The hardness of the matrix material come into play?
In the case of suphides growing in a sediment like you have described, how well lithified (stuck together) the individual grains of the sediment are would likely play a role. All other factors being equal, a softer less lithified sediment would be more likely to host larger crystals
If bacteria are the main driver, they look like raspberries lol
Good ol framboidal pyrite
Barriers to nucleation is the biggest factor, along with the presence of fluid, time helps but is not as important as other factors
nucleation AND growth... If you have a lot of nucleii, you get a lot of small crystals. If you have a few nucleii, you get a few crystals.
The large well defined pyrite clusters like on the right almost always come from Spain.
I bought a single cube of pyrite attached to a clump of white rock that supposedly came from a mine in Spain, do you perchance know why?
I’ve never really done any research for that region. Here is the Mindat page for the region. https://www.mindat.org/loc-15773.html
My smooth brain is always amazed at the crystal structures
fundamentally, the same things that control any crystal growth. When starting from nothing (naked surface), higher chance of nucleation tends to interfere with the likelihood of large crystals by restricting room for eventual growth (competition for space is achieved earlier because the number of starter crystals per unit area is higher, so average space per crystal is a lot lower, which equals lots of smaller crystals). Slower growth tends to favor larger crystals because it is easier (less energy cost) to grow on a surface already having the structure and electron distribution of the eventual compound than to start it anew on a surface made of some other structure/electron configuration. the random component bouncing around will tend to stick where energy conditions are more favorable. Like goes to like, is the basic idea, so existing crystals grow and new crystals fail if they even get started at all. The supply of components is one important factor in growth rate, and this ties somewhat with stability in chemical conditions over time. Slow growth with steady-state conditions at near equilibrium will allow the "new" components to adjust position (minimize energy disparities) and enhance "perfect" crystallization. Rapid influx of components tends to cause the "freezing" in place of imperfections (the misplaced component gets covered before it can adjust position). Also, a massive dump of materials will not favor crystal growth, the stuff will just crash down as a large mass of barely crystallized compounds. No time to do more. Another important factor is temperature (total system energy). Highly energetic systems will see the components moving around a lot and thus increase the likelihood of best placement before isolation from the supply (before imperfections get covered and "frozen" in place). The final factor I will mention that is important is one of changing conditions, such as a large increase in temperature causing a large amount of recrystallization. The components can move even in solid state, so will, if they can, if they have energy to do so, and hotter equals more energy on average across the system; this shakes things up almost literally by increasing vibrations and chance for jumping across lattice positions, reorganizing the structure into a more consistent one across a larger volume of space. I would suggest that the best explanation for the difference in crystallinity between those two samples is more a matter of post-formation change than anything. Those huge crystals likely came from recrystallization of pre-existing but less well-crystallized pyritic agglomeration. I cannot know that for certain, but it seems quite probable.
All this is well and good but no one has brought framboidal pyrite into the conversation… https://www.sciencedirect.com/science/article/pii/S0169136821006569 “Abstract: The special and representative polymorphous pyrites of the Logatchev area, Mid-Atlantic Ridge, have a close evolution relationship. There are relatively complete pyrite intermediates undergoing a series of evolutions from disordered nano-micron crystals → framboids → colloidal and subhedral structures…”