Radioactivity and Minerals

Radioactive isotopes are incorporated into mineral crystal structures in exactly the same way as their stable counterparts - as substitution defects, trace impurities, or as major structural constituents in minerals where the radioactive element is a primary ingredient of the formula. Over geological time, the nuclei of these isotopes decay to form new daughter elements. This decay has two consequences for mineralogy: it changes the chemistry of the mineral by adding new elements, and it damages the crystal structure through the energetic particles emitted during the decay process. ^[1]

Important Radioisotopes in Minerals

Six radioisotopes are of particular importance in mineralogy and geochronology. ^[1]

| Isotope | Half-Life (years) | Ultimate Daughter(s) | | ---------------- | ----------------------- | -------------------------------- | ----------------- | | ¹⁴C | 5,730 | ¹⁴N | | | ⁴⁰K | 1.248 × 10⁹ | ⁴⁰Ar, ⁴⁰Ca | | | ⁸⁷Rb | 48.8 × 10⁹ | ⁸⁷Sr | | | ²³²Th | 1.405 × 10¹⁰ | ²⁰⁸Pb, ⁴He | | | ²³⁵U | 7.038 × 10⁸ | ²⁰⁷Pb, ⁴He | | | ²³⁸U | 4.468 × 10⁹ | ²⁰⁶Pb, ⁴He | |

⁴⁰K is the most abundant of these and is found in all potassium-bearing minerals, including K-feldspar, the micas, and some amphiboles. Uranium and thorium are widely distributed but rarely abundant enough to form minerals in which they are major elements; most commonly they occur as minor constituents in minerals such as zircon (ZrSiO₄). ¹⁴C is produced in the atmosphere and enters biological material including the calcite, aragonite, and apatite of shells, teeth, and bone; its short half-life limits its usefulness to materials no older than late Pleistocene. ^[1]

Decay Mechanisms

Five decay mechanisms operate in minerals. ^[1]

Alpha decay ejects an alpha particle - a helium nucleus consisting of two protons and two neutrons - from the parent nucleus, along with gamma rays and heat release. The daughter element has an atomic number two less than the parent and an atomic mass four less. For example, ²³⁸U decays to ²³⁴Th through this mechanism, and the decay series ultimately terminates at ²⁰⁶Pb - the foundation of the U-Pb radiometric dating method. ^[1]

Beta (negatron) decay converts a neutron into a proton by ejecting an electron (the beta particle β^-) and an antineutrino from the nucleus. The daughter has an atomic number one greater than the parent but the same atomic mass. The decay of ⁸⁷Rb to ⁸⁷Sr by this mechanism forms the basis of the Rb-Sr radiometric dating method. ^[1]

Positron decay converts a proton into a neutron by emitting a positron (β⁺) and a neutrino. The daughter has an atomic number one less than the parent and the same atomic mass. About 0.001% of ⁴⁰K undergoes this decay, forming ⁴⁰Ar. ^[1]

Electron capture involves the nucleus capturing one of its own orbital electrons so that a proton is converted to a neutron. Atomic number decreases by one and atomic mass is unchanged. About 11% of ⁴⁰K decays by electron capture to ⁴⁰Ar, providing the basis of the K-Ar radiometric dating method. ^[1] The remaining approximately 89% of ⁴⁰K decays by beta decay to form ⁴⁰Ca. ^[1]

Fission occurs in the heavy nuclei of ²³⁸U and, to a lesser extent, ²³⁵U and ²³²Th. The nucleus spontaneously splits into two fragments of approximately equal mass. The two daughter nuclei fly apart at high velocity, tearing through the surrounding crystal lattice and leaving a trail of structural disruption in their wake. ^[1]

Radioactive Decay and Crystal Structure

When a radioactive atom decays, the daughter element that forms occupies the same structural site as the parent but is typically a different size and has a different charge. The decay of ⁴⁰K, for instance, produces both ⁴⁰Ar and ⁴⁰Ca. Neither daughter is the same size as K⁺, nor does either carry the same charge. The ⁴⁰Ar is an inert gas with an atomic radius of about 1.96 Å - it cannot form chemical bonds and is chemically foreign to the K site it occupies. The ⁴⁰Ca is divalent, considerably smaller than K⁺ in equivalent coordination, and its presence in a K site constitutes a substitution defect. ^[1]

Closing Temperature and K-Ar Dating

Whether the ⁴⁰Ar produced by electron capture and positron decay stays in the mineral or escapes depends on temperature. Below a critical temperature known as the closing temperature (which varies for different minerals), diffusion of ⁴⁰Ar through the crystal lattice is extremely slow and the gas is effectively trapped. Above the closing temperature, ⁴⁰Ar diffuses out of the mineral and is lost. Measuring the ratio of ⁴⁰K remaining to ⁴⁰Ar accumulated in the mineral therefore gives the time elapsed since the mineral was last held at temperatures above its closing temperature - in effect, the date at which the mineral cooled through the closing temperature isotherm. Because different minerals have different closing temperatures, K-Ar systematics in a suite of minerals from the same rock can reconstruct the cooling history of that rock through different temperature intervals. ^[1]

Alpha Damage: Frenkel Defects and Metamict Minerals

Alpha decay has structural consequences beyond simply producing a daughter element. The ejected alpha particle carries substantial kinetic energy and travels up to about 10,000 nm (0.1 mm) through the surrounding crystal before stopping. As it decelerates it ionises the material it passes through, and near the end of its trajectory it typically dislodges several hundred atoms, producing Frenkel defects - ions knocked out of their normal structural positions into interstitial sites. The decayed atom also recoils for up to about 10 nm and may displace its immediate neighbours, creating a small zone of disrupted structure a few atoms across at the original decay site. ^[1]

Minerals that contain significant concentrations of uranium and thorium - such as zircon (ZrSiO₄) - can experience more than 10¹⁵ alpha decay events per cubic centimetre over a period of several hundred million years. Accumulated over time, this bombardment can completely destroy the crystal structure of the zircon. Minerals whose structures have been so thoroughly disrupted are called metamict. Metamict minerals lack long-range crystallographic order and behave for all practical purposes as glasses. ^[1]

Metamict minerals are recognisable by a characteristic suite of properties: lower density than their unaltered equivalents, darker colour, and reduced hardness. Most tellingly, metamict minerals do not produce characteristic peaks in X-ray diffraction because they lack the long-range order that produces diffraction. Heating a metamict mineral to sufficiently high temperatures allows it to anneal - the atoms rearrange back into the ordered crystal structure and the original properties are restored. ^[1]

Fission Tracks

The damage trails left by the two daughter fragments during spontaneous fission of ²³⁸U (and to a lesser extent ²³⁵U and ²³²Th) are called fission tracks. At temperatures below the closing temperature of the host mineral, the crystal lattice does not repair itself and the disrupted trails are preserved. After polishing the mineral surface and etching it with a caustic solution, fission tracks become visible under the microscope as elongated etch pits. The density of tracks depends on the amount of fissionable isotopes present and on how long the mineral has been below its closing temperature since the tracks started accumulating. The fission track dating method is based on careful counting and analysis of these tracks in minerals such as titanite, epidote, and apatite. Because different minerals have different closing temperatures, fission track analysis across mineral suites is particularly powerful for reconstructing the thermal history of uplifts, sedimentary basins, and orogenic belts. ^[1]

Pleochroic Halos

Because alpha particles can travel up to about 0.1 mm through crystalline solids, they can pass from their host mineral into the surrounding enclosing mineral. When a small radioactive-element-bearing mineral - such as a zircon grain included in biotite - emits alpha particles, those particles damage the biotite lattice in a spherical zone around the inclusion. The resulting radiation-damaged zone, visible in thin section as a dark halo around the inclusion, is called a pleochroic halo. The name reflects the fact that the damaged zone may have different optical absorption properties from the unaltered host, producing a colour change that varies with the orientation of the thin section relative to the crystal axes of the host mineral. ^[1]

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