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Phosphate in a Nutshell


bone phosphate

Animal bone is a major phosphorus reservoir

Photo (c) Andrew Alden, licensed to About.com (fair use policy)

Life on Earth tends to run out of phosphorus first. It is an underappreciated element, but as the years flip by more of us will grow aware of how precious and strategic phosphorus is.

Phosphorus dislikes silicon, which puts it in the geochemical category of the incompatible elements. In the Earth's mantle and in the igneous rocks derived from the mantle, phosphorus occurs in a limited set of minerals, mainly the apatite group. From these primary rocks apatite steadily erodes and feeds its phosphorus to the biosphere, where it is carefully recycled from life to life on land and in the ocean.

Phosphorus in the Biosphere

Phosphorus has a key role in DNA and ATP, two of life's most important molecules. DNA (deoxyribonucleic acid) carries the genetic code of all living things, and ATP (adenosine triphosphate) carries the chemical energy that powers the activity of every living cell. In addition, phosphorus is essential in the composition of bones and teeth, which are intricate frameworks of proteins and the mineral apatite.

Bones and corpses are avidly degraded and their phosphorus taken up by plants. The guano of birds, insects and bats circulates phosphorus more widely in the biosphere, but some always gets away into the ocean. Seabirds and salmon bring phosphorus back to the land. A small trickle of phosphorus is buried in the seafloor to roughly balance what enters the cycle from igneous rocks.

Phosphorus in the Geosphere

Phosphorus is concentrated on the rare occasions when the ocean runs out of oxygen and goes dead. In these anoxic episodes, the biological balance between oxidizers and reducers is lost, and phosphate-bearing sediment accumulates with the seafloor clay along with carbon, calcium and other elements. The rocks that result are black shales, and in them calcium and phosphorus combine into apatite crystals. Where ocean currents winnow the clay away from the apatite grains, minable deposits of phosphorite form.

In the United States, most phosphate rock is mined in Florida and North Carolina in young black shales of Neogene age. In Idaho and Utah the Phosphoria Formation is mined, a large body of older, Permian-age black shale. Elsewhere, China, Morocco and the Middle East have large supplies, likewise derived from ancient anoxic seas. But as you may know, most seafloor deposits are eventually swept under the continents by subduction and reenter the mantle. And that closes the phosphorus cycle.

Very old igneous phosphates, from before the time of today's phosphorus cycle, occur in the usual places: the Archean cratons of Canada, Brazil, Russia, Africa. The largest Canadian deposit is a deeply eroded carbonatite in which apatite was concentrated.

Phosphorus in Commerce

About 90 percent of the phosphorus that is mined in phosphate rock goes into fertilizer; the rest serves many purposes in the chemical industry. U.S. production hit its peak in 1980 (54 million tons), and growing amounts are imported here from Morocco, where deposits are abundant and the government is firmly controlled.

Like coal and petroleum, phosphate rock accumulates heavy metals that can become a hazard. Chief among these accessory metals are uranium, radium and cadmium. Unlike the case with coal and petroleum, these do not enter the air because phosphate rock is not burned, but they remain in the sulfate waste from processing, called phosphogypsum. This waste may be spread on farmland, processed like gypsum into wallboard or simply put in massive piles. Either way, these unwanted metals can enter the human organism, adding to our load of toxins and radiation.

It is also said that radioactive contaminants in phosphate fertilizers account for a significant fraction of the cancers caused by smoking tobacco.

Phosphorus will only become more important as world population grows along with per capita resource use. Within a few decades not petroleum, but phosphorus may be the world's keystone resource.

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