My five-year-old is losing his apatite

My five-year-old has cavities. Four cavities. Two on his lower left, where his back teeth rub together, and two little ones in the same spot on his lower right and upper left. I've never had any cavities myself, so I may be more worried about the next dental visit than he is.

"Does he drink a lot of juice?" the hygienist asked me.

"No..." I frowned. "But I've been letting him brush his teeth himself. He might not be doing a very good job."

"The chewing surfaces look fine," the hygienist said. "He's got to get the sugar between his teeth to get his cavities. Usually it's fruit juice that does it."

"He drinks mostly milk and water," I said. "What else could it be? We don't do very good job flossing his teeth, I know. And... we were on well water until this summer. I never got the water tested for fluoride, and the dentist didn't want to give him extra fluoride, because the groundwater around here varies alot." I didn't go into detail about the diverse bedrock geology that contributes to the variability in water chemistry. I was busy feeling guilty because I had treated the problem as something to solve by knowing more about the bedrock, when I should have just sent the water to the local health department for a fluoride test.

See, I don't know much about teeth, except that brushing and flossing every day is good, eating lots of sugar is bad, and the enamel coating consists mostly of the mineral hydroxyapatite. And that tooth decay happens because sugar-eating bacteria excrete weak acids, which slowly etch the hydroxyapatite and destroy the enamel.

Apatite is a mineral that all geologists learn at least in passing – it defines “5” on Moh’s hardness scale, it’s the most common of the phosphate minerals, and it’s got a funny name. And it’s present in a lot of different types of rocks, igneous, sedimentary, and metamorphic. I usually miss it when glancing at a thin section, but find it on a microprobe, when I’m actually looking for tiny grains of plagioclase feldspar. But most of what I know about apatite comes secondhand, from collaborating with thermochronologists, who use apatite to figure out when rocks cooled to near-surface temperatures. (Well, that’s near-surface to someone who likes to work on metamorphic rocks.)

When I was in grad school, apatite was mostly used for apatite fission-track dating*. Apatite is mostly a calcium phosphate mineral, but it can contain tiny amounts of uranium. When the uranium decays by nuclear fission, it damages the crystal lattice of its host apatite grain. If the temperatures are high enough, the lattice is able to heal, but at low temperatures, the apatite grain collects these damage zones called fission tracks. You can’t see the fission tracks in a normal thin section – you need to separate the apatite grains, mount them in epoxy, polish them, and etch the surface with an acid. Then the heroic (and very patient) thermochronologist counts and measures all the little etched tracks. But there are complications. And one of those complications has to do with the amount of fluorine that substitutes for the hydroxy (OH-) ions in the structure.

I don’t remember exactly what the complication was. But I’m willing to bet that fluorapatite isn’t just harder than hydroxyapatite. I’m not sure, but I think that fluoroapatite doesn’t dissolve as easily in acid.

My son's cavitities are in baby teeth, which is a good thing. They will fall out before enough fission tracks can accumulate in them to be countable. They will fall out and be replaced by adult teeth. Which are currently growing. Which is why the dentist prescribed supplementary fluoride tablets, for a little while, at least.

In the meantime, the hygienist and I came up with another hypothesis for the source of the sugar.

“Ummm,” I said. “When he started kindergarten, he was able to choose from white milk or chocolate milk. I think he drinks chocolate milk a lot.”

“Chocolate milk has sugar in it,” she nodded.

In fact, I think he’s been drinking chocolate milk nearly every day since starting kindergarten. I had encouraged him to try the white milk, and to save the chocolate milk for special occasions. But, well, he’s five. There are a lot of special occasions.

But now, maybe he’ll listen to me when I tell him that chocolate milk will destroy his apatite.

* Since I left grad school, a new method, (U-Th)/He dating, has been developed and become an incredibly powerful tool for thermochronology.