Radium and Geology

Uranium-238 is a primordial radionuclide with a half-life of 4.47 billion years. It heads a decay chain (the "uranium series" or "4n+2 series") that ends at stable lead-206. The steps leading to radium and radon are:

²³⁸U → ²³⁴Th (alpha, 4.47 billion years) → ²³⁴Pa (beta, 24.1 days) → ²³⁴U (beta, 6.7 hours) → ²³⁰Th (alpha, 245,500 years) → ²²⁶Ra (alpha, 75,400 years) → ²²²Rn (alpha, 1600 years) → continuing to stable ²⁰⁶Pb

Radium-226 is the "fifth daughter" of ²³⁸U, produced directly from the decay of ²³⁰Th. This position is critical: several intermediate nuclides (²³⁴U, ²³⁰Th, ²²⁶Ra) have long half-lives, allowing them to be physically separated from parent uranium by geological processes and migrate independently. (Learn more about radon's fundamental properties.)

The 1600-year half-life of ²²⁶Ra sustains a steady supply of ²²²Rn over human and geological timescales. Where ²²⁶Ra sits in soil and rock, radon is continuously generated.

Radium (Ra, atomic number 88) is the heaviest member of Group 2 — the alkaline earth metals (Be, Mg, Ca, Sr, Ba, Ra). Its electron configuration is [Rn] 7s², and it readily loses its two valence electrons to form a Ra²⁺ cation. The +2 oxidation state is essentially the only state radium adopts in nature.

Radium's chemistry closely mirrors barium, which is why Marie and Pierre Curie used barium as a carrier to concentrate radium from pitchblende. Because radium is chemically similar to calcium, radium taken into the body is deposited mainly in bone, where it delivers long-term internal radiation dose.

Radium is a silvery-white metal that blackens on exposure to air, reacts with water, and imparts a carmine-red color to flames. It has a density of about 5.5 g/cm³ and a standard atomic weight near 226.

Radium has 34 known isotopes, all radioactive. Four occur naturally as members of the three primordial decay series:

Only ²²⁶Ra matters for indoor radon. Its 1600-year half-life ensures persistent radon generation, and its daughter ²²²Rn lives long enough (3.82 days) to migrate through soil into buildings. By contrast, ²²⁴Ra produces thoron (²²⁰Rn, 55.6 s half-life), which decays too quickly to travel far.

Radium is far too scarce to form its own minerals; instead it substitutes into the crystal lattices of barium, strontium, and calcium minerals. Substitution is governed by ionic charge (all are 2+) and ionic radius. The size trend is Ra²⁺ > Ba²⁺ > Sr²⁺ > Ca²⁺.

Because Ra²⁺ is closest in size to Ba²⁺, barium minerals — especially barite (BaSO₄) — are the best hosts. Barite and radium sulfate (RaSO₄) are isostructural, making barite an almost ideal radium carrier.

Different minerals have varying affinities for radium based on the size match between Ra²⁺ and the host cation. Barite (BaSO₄) is the near-perfect host, while calcium carbonate minerals show low affinity.

Radium's mobility is controlled by local geochemistry. In oxidizing, sulfate-rich water, Ra²⁺ co-precipitates with barite as (Ra,Ba)SO₄ solid solution, immobilizing it. This is the most important radium sink.

Under reducing conditions or in sulfate-depleted water, barite dissolves and radium is released into solution. Radium is more mobile in reducing aquifers, low-pH water, and high-salinity environments where competing cations (Ca²⁺, Ba²⁺, Na⁺) displace Ra²⁺ from adsorption sites.

Alpha recoil — the physical ejection of a newly formed ²²⁶Ra atom when its parent ²³⁰Th decays — can liberate radium (and radon) from mineral surfaces into pore water, supplying "fresh" dissolved radium independent of bulk dissolution.

At low salinity, radium adsorbs onto clays, iron and manganese oxides, and organic matter, reducing its mobility.

Global average radium-226 in soil is about 35 Bq/kg (UNSCEAR 2000), with a reported range of 1–900 Bq/kg. Igneous rocks like granite tend to be higher; carbonates and many sandstones lower; black shales and phosphate rocks much higher.

Secular equilibrium occurs when a very long-lived parent (like ²³⁸U) feeds a shorter-lived daughter: given enough time in a closed system, the daughter's activity rises until it equals the parent's, and every member of the chain decays at the same rate.

Disequilibrium arises when chemical or physical processes separate chain members. In the ²³⁸U series this is common because several intermediates are long-lived and chemically mobile:

Uranium is soluble under oxidizing conditions and leached by groundwater, leaving ²³⁰Th and ²²⁶Ra behind. Radium can be preferentially dissolved, adsorbed, or co-precipitated, decoupling it from its parent and daughter. Radon is a gas and escapes.

Because ²³⁴U, ²³⁰Th, and ²²⁶Ra have half-lives of tens to hundreds of thousands of years, disequilibrium can persist for millennia. The ²³⁸U chain can take a million years to re-establish equilibrium after major disturbance.

Practical implication: measured uranium in rock does not reliably predict radium or radon — the two can be badly out of step. This is a root cause of unpredictable radon levels.

Radon potential is notoriously local. Approximately 33% of the US population lives in geologic provinces where average indoor radon has the potential to exceed 4 pCi/L (USGS). Radon concentrations in dwellings vary by more than two orders of magnitude.

Several independent factors stack up, and each can change over short distances:

1. Bedrock uranium/radium content — Different rock units carry vastly different uranium. A uranium-rich granite may sit next to a uranium-poor sandstone. Geologic contacts, plutons, and sedimentary beds change over meters to hundreds of meters.

2. Fractures and structure — Faults, joints, and shear zones channel radon from depth to the surface. A house over a fracture can have much higher radon than a neighbor on unfractured ground nearby.

3. Mineralogy and radium disequilibrium — Radium can be leached or concentrated in specific minerals (barite, phosphates, clays, Fe/Mn oxides), so radium available at the surface may not track bulk uranium.

4. Soil permeability and moisture — Radon moves less than an inch in water-saturated soil but can move several feet through dry, permeable material. Permeability can change abruptly.

5. Groundwater movement — Local flow paths, water-table depth, and recharge zones redistribute radon and radium.

6. Glacial and surficial cover — Thick glacial till can mask or amplify the bedrock radon signal.

Because these factors combine multiplicatively and each varies on meter-to-kilometer scales, two nearby sites can have very different radon potential. This is why site-specific testing, not just a regional map, is essential.