The air you breathe is about 21% oxygen and 78% nitrogen by volume. People suffering from serious Covid-19 need much higher oxygen concentrations to hopefully buy some time. Hospitals need to keep local stocks of liquid oxygen or high pressure oxygen tanks filled to provide treatment. Patients can be treated at home with a portable oxygen supply and lower the burden on the hospital. Often this means oxygen concentrators.

Around 1800 a mineralogist noticed heating stilbite produced steam. The class of materials was called *zeolites* from the Greek for "to boil" is *zeo* and lite is a mineralogy shorthand for *lithos* or stone. There are a few hundred such materials each with a similar internal structure or framework. The framework creates tiny pores - pores that can absorb substances and even be used as molecular sieves. Wikipedia shows a structure for a common zeolite called mordenite. It's based on a silicate (SiO_{4}) building block - I show it as this framework is common to zeolites and there's a nice public domain image.

Now back to oxygen concentrators. Rather than getting bogged down in their operation just note regular air is passed through a molecular sieve made out of a different zeolite that traps nitrogen and allows oxygen to pass. Relatively inexpensive machines for home use can produce about 10 liters per minute of enriched air that is 80 to 90 percent pure oxygen. These are regularly used by people who need oxygen therapy and have kept a lot of people out of the hospital during the pandemic.

A wonderful invention, but there's something extremely beautiful about zeolite.

Chemists and physicists have their own way of looking at its repeating framework - the one called the secondary building unit. Mathematicians have their own interesting way. To get there a bit of a digression is warranted.

Take a deck of cards. Assuming it's well shuffled (say a dozen shuffles or so) I guarantee the order of the cards has appeared only once and will never appear again. The reason comes from the fact there are an enormous number of possible combinations of cards: 52! (52*51*50 ... *3*2**1)* or something over 8 *10^{67} unique orderings! One way of thinking about it a mathematician might consider each possibility a point in a 52 dimension space. It's a bit abstract as we are only used to three physical dimensions, so here's a way to think about these configuration spaces.

A mathematician looks at a cube and says "that's a three dimension cube..." A square is a 2 dimension cube, a line is a 1 dimension cube and a point is a cube of 0 dimensions. She can go in the other direction too.. 4 dimension cubes and up. OK - now back to the cards. Take three cards labeled 1, 2 and 3. There are six (3! = 6) possible orderings: {(1,2,3), (1,3,2), (2,1,3), (2,3,1), (3,1,2), (3,2,1)} Make yourself a 3d grid with an x, y and z axis and plot the six points.^{2} Now connect them up - pretend you have a rubber band to snap around them.^{3}

Sweet! You get a hexagon. Shine a light on it at the right angle and it will cast a hexagon shadow. The combinations of the three cards are linked to a shape!

Now take four cards labeled 1,2,3 and 4. There are 4! = 24 possible ways to order them. Plotting them on a 4 dimension grid doesn't work for our spatially limited brains, but if you do the same trick of connecting them up you get a 3 dimensioned gem-like shape .. the exact shape of zeolite's framework (the secondary building unit). If we lived in a place with four physical dimensions the shape of that object would cast a three dimensioned shadow..

Thinking about the structure of an important molecular sieve with four playing cards.. math can be a curious playground.

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^{1} The repeating unit is fairly complex: (Ca, Na_{2}, K_{2})Al_{2}Si_{10}O_{24}ยท7H_{2}O

^{2} you can also do it with two 2d grids.. say x,y and z,y and see the pattern

^{3} the convex hull

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