The Venus Fly Trap of the Mineral Kingdom
Certain crystals have tiny levers inside them that when tripped, will capture and entomb atoms.
The mineral kingdom is full of interesting and useful minerals that can mimic biological processes. Just as finding new disease cures from plants takes a lot of trial-and-error, walking down a trail and finding plants to eat, or using plants and minerals in art, minerals have amazing properties for advancing technology, solving socioeconomic issues, and even in pharmaceuticals. You don’t have to go to the far reaches of the Earth to find these unique minerals, however you do need to know a little bit about them, before we can get into the interesting parts.
Minerals are classified into classes, and of all the classes, the microporous minerals are the most diverse (and most interesting IMHO). Basically, these porous minerals have micro-tunnels running through their atomic framework, resembling something like Swiss cheese. But these tunnels are not random, are rarely empty, and they are usually stuffed with water and common elements you’ll find on a periodic table.
One group of microporous minerals is called zeolite, where the name comes from Greek: zeo (‘to boil’) and lithos (‘stone’). If you heat up a zeolite, the water will boil out of the rock. Plenty of YouTube videos show the reverse process where the zeolite rapidly absorbs water and releases heat. In addition to the water, the tunnels housemetals,which are required to balance out chemical formulae. These metals (like sodium, potassium, calcium, and magnesium) are exchangeable, and many times are extremely easy to swap, to the point where they do so spontaneously.
Ion exchange, and how it works
The process of pushing new atoms into the tunnels, and kicking out the old ones, is called ion exchange. Ions are atoms that have a charge, like sodium or calcium, and are needed to charge-balance the chemical makeup of the crystal. For example, if sodium resides in a crystal (having a charge of 1+), it can be replaced by another 1+ charged ion pretty easily. If a single 2+ ion wants to move into the crystal, then two sodiums will need to vacate the crystal to accommodate, which may or may not happen depending on the zeolite — but in the end, everything has to be balanced. If the charge differences are too disparate, then the crystal can ultimately weaken and break apart. There are sequences of ion preference that each microporous mineral has, and these have been experimentally determined. In general, we know that given two atoms of the same charge, the larger one is likely to be preferred, if it can still fit into the tunnel.
Another way to think about ion conduction is something like electrical conduction. For you to use electricity, electrons have to move through metal, to get from one place to the other in order to power stuff. It is the same thing with ion conduction — the calcium ions have to physically move through the crystal,while pushing the others out. There is a kind of friction during this process, and sometimes there are physical strains on the crystal. Nature has found some clever ways to do ion conduction in a variety of systems, and it is way more important than you think.
Ion conduction in biological systems is critical for survival. For example, Prof. Roderick MacKinnon found the mechanisms by which potassium ion conduction in human cells power much of our muscular control, including the heart. It’s so important that this work earned him a Nobel Prize in chemistry. The cells in your body regulate potassium for specific functions, through a molecular gate in the cell membrane. The gate is selective for just potassium, and all other atoms are rejected. Once the gate opens, it allows potassium to enter the cell and perform a function (e.g. move muscles). Have you ever been told to eat a banana to keep muscle cramps at bay? That potassium-rich food can supply lots of the needed nutrients for strong muscle function when you need it the most. The animation below shows just how this works.
How do these ion channels (tunnels) work?
In short, it’s a charge repulsion force that opens up the channel. The carbonyl groups have the positively charged hydrogen atom as the terminal at the end, pointing into the channel. When the positively charge potassium gets close to that carbonyl, it pushes the hydrogen to one side (because like charges repel each other), and this pushing force rotates subgroups of the protein and opens a channel. Only potassium can do that. Sodium is too small and has wrong charge density to force the channel open. There are different types of channels that move other ions in-and-out of the cell.
I imagine that this process happens at near the speed-of-light, because how else am I able to move my muscles and type out these words on the computer, to get this story written. It involves tiny levers inside the cell; little molecular levers that Mother Nature designed (i.e. evolution selected for). These molecular levers are not limited to biological cellular systems, as nature can do this in other systems, as well
The Venus Fly Trap
Here is example of how selectivity works on a larger scale. Take the Venus Fly Trap (Dionaea muscipula). The plant has a clam-like shape to it, and the clam-like parts are leaf-lobes. Inside the leaf-lobes are little levers. When a bug walks between the leaf-lobes and touches a lever, the plant rapidly clamps down to catch its prey. For this to work, the bug has to be sufficiently large to actually push the lever, but not so large that the bug cannot even touch the lever. Check out YouTube videos of this, you’ll notice that the bug has to be the right size to fit. So nature has built many types of selectivity gates, for bugs and for ions, to perform critical life functions.
So how does it work in minerals?
Surprisingly, it is similar to biological cellular systems. Here is an example. The mineral sitinakite naturally forms with sodium and potassium in the tunnels. A group from Texas, led by Prof. Abe Clearfield, found that this mineral is extremely ion-selective, but only after the mineral absorbs hydrogen (when treated with a weak acid). After it absorbs hydrogen, it will only readily exchange with cesium. It’s crazy, because hydrogen is the smallest atom and cesium is the largest! That has a super-interesting application. Since cesium is a toxic element, and very radioactive when found in nuclear waste, sitinakite could be used for remediating nuclear waste and disasters (see Fukushima cleanup). The Texas group and I collaborated, and I found that only certain atoms can then re-enter the crystal, once sitinakite exchanged the hydrogen.
What was causing this? It took me several years of work to decipher the mechanism for this molecular processes, but I finally cracked it in 2008. All that experimental design, data collection, analysis, and work is summed up in the graphic below.
Just like the carbonyl in potassium channels in biological systems, sitinakite forms little hydrogen levers that cover the walls of the tunnel. Only a certain-sized ion, and one of a particular charge, is allowed to pass through. In the case of sitinakite, that ion is cesium. When we tried to reverse the process — remove cesium with hydrogen and other ion — we found that cesium wasn’t able to exchange back out; it is entombed in the crystal, until the crystal itself dissolves away (and that is no easy task).
When I made that discovery, I was literally shaking because I found an inorganic equivalent to the organic world. I immediately starting thinking about other inorgnaic-organic systems, and how they interact with each other. Later, Bob Hazen was clever enough to think of a larger picture of the mineral-biological world inteaction into what he called Mineral Evolution.
Maybe there are more of these types of connections between inorganic and organic materials.
As I continue to work on these microporous minerals, I find that sitinakite isn’t an isolated case. Just about all porous minerals behave like this, and depending on the size/shape/composition of the tunnels, they can be tuned to absorb different elements. So in effect, the minerals can be tailor-made to scavenge the most toxic of elements and lock them away inside the crystal. Once the ions are locked up, then they are no longer available for biological interactions, because they can’t exchange back out. At this point, we cannot predict which microporous minerals will be good at absorbing which elements. We still have to do all the experiments to figure out what each mineral is good for, and that takes a lot of time, and I’m working on it.
Or maybe the reverse process is more useful; where you want to release something to a particular environment. Cancer therapy should be taking note of this work. Instead of having a single ion in the tunnels, you could have a small drug molecule there instead. This drug would be exchanged out of the crystal when it encounters atoms that it prefers to absorb, and kick the drug out. Cancer is the body creates an acidic environment outside the cells, meaning that there is excess hydrogen, while the inside of the cells are alkaline. When a mineral loaded with the drug, encounters the cancer, the mineral begins to exchange the drug for the hydrogen. This makes for a targeted chemotherapy solution that delivers the drug directly to the cancer cells. Sound familiar? It was already figured out how this could work way back in 2008. If you work for a pharmaceutical company and are reading this, I’d be happy to collaborate :)
It is important to note that much of the microporous mineral properties in the human body have not been studied, and this includes the common zeolite supplements you see in nutritional stores and on websites. Zeolites can absorb calcium, potassium, zinc, and a whole host of important elements that are essential for life function. If you purposefully ingest a‘zeolite’, you are running the risk of potentially depleting yourself of important elements, necessary for your body to work properly. The zeolite might absorb heavy metals, but it is also just as likely to absorb other good things. Zeolite is not a mineral, but a group of minerals; and the companies that sell zeolite supplements don’t list what type of zeolite you are ingesting or precisely how it’s working. They just call it zeolite and that it cleanses the body. On top of that, you don’t know what elements the zeolite is good at absorbing in the body, and honestly, I don’t either. Lots of work has to be done first before it can be used for supplement purposes. Talk to your doctor before taking any kind of ion-absorbing material. Check out this story to read more about supplements in general.
Aaron Celestian is the Mineralogy Curator at the Natural History Museum of Los Angeles. He researches how minerals interact with their environments and with living things, and how those minerals can be used to solve problems like climate change, pollution, and disease.
Image credits that are not referenced in the text: mesolite photo by Stan Celestian, zorite structure by me, sitinakite panel by Bailee DesRocher.