Zoned Crystals

Most minerals that display solid solution do not crystallise with a uniform composition from core to rim. As the crystal grows and conditions change, the composition of newly added material changes too - producing a crystal whose inner portions record one set of conditions and whose outer portions record another. This phenomenon is chemical zoning, and it is one of the most informative records available of the history of a growing mineral. ^[1]

Zoning is detected in petrographic thin sections - slices of rock just 0.03 mm thick, mounted on glass microscope slides - where differences in chemical composition manifest as variations in colour, refractive index, or other optical properties visible under a polarising microscope. ^[1] The presence of zoning in a crystal is direct evidence that chemical and/or physical conditions changed during growth - the crystal is, in effect, a chemical diary written in concentric compositional layers. ^[1]

Plagioclase: The Defining Example

Plagioclase, the most abundant mineral in the Earth’s crust, forms a complete solid solution series between albite (NaAlSi₃O₈) and anorthite (CaAl₂Si₂O₈), with coupled substitution linking Ca²⁺ + Al³⁺ in anorthite to Na⁺ + Si⁴⁺ in albite. ^[1] It is also the mineral that most clearly illustrates the contrast between the two extremes of crystallisation behaviour: equilibrium crystallisation and fractional crystallisation.

Equilibrium Crystallisation

Under fully equilibrium conditions, crystals remain in continuous chemical communication with the melt throughout crystallisation, constantly exchanging atoms to maintain the correct composition for each temperature. For a plagioclase melt of bulk composition 55% anorthite (An) at 5 kbar water pressure, crystallisation begins at 1100°C with growth of An₇₇ plagioclase - strongly Ca-enriched relative to the melt, as the phase diagram requires. By 1050°C, crystals of An₆₈ composition are growing from a melt that has shifted to 43% An. Crystallisation is complete at 980°C, at which point all the plagioclase has the composition An₅₅ - the same as the initial bulk composition of the melt. The crystals have re-equilibrated their composition at every step, so the final solid is uniform throughout. ^[1]

Fractional Crystallisation

With fractional crystallisation, crystals are isolated from the melt the moment they form - no chemical exchange between old crystals and new melt is permitted. This changes the outcome entirely. Crystallisation again begins at 1100°C with the growth of An₇₇ cores, but now those cores are locked away from the melt. The melt behaves as if it has lost the Ca that went into the An₇₇ cores and must now crystallise from a slightly more Na-rich starting composition. At 1050°C, an outer layer of An₆₈ is applied to the exterior of the An₇₇ cores. Because no exchange with older crystal occurs, each increment of cooling is equivalent to starting afresh with a more Na-rich bulk and no inherited crystal. This allows the melt composition to shift progressively toward pure albite. The last thin layer of plagioclase to crystallise, grown from a melt that has become nearly pure albite, has an albite composition - and the overall crystal is zoned: Ca-rich at the core, Na-rich at the rim, with every intermediate composition recorded in the concentric layers between. ^[1] The bulk composition of all the crystals combined still averages to An₅₅ - the original starting composition - but the internal distribution of that composition is entirely different from the equilibrium case.

Diffusion Controls Whether Zoning Is Preserved

Real crystallisation lies somewhere between these two ideals. Whether a crystal ends up measurably zoned depends on how quickly the relevant elements can diffuse through the crystal lattice relative to the cooling rate. Diffusion through plagioclase is slow, so zoning is preserved even in deep-seated intrusive rocks that cooled slowly over millions of years - the Ca and Na simply cannot migrate fast enough to re-homogenise the crystal. Diffusion of Fe and Mg through the olivine lattice is relatively fast, so olivine crystals can re-equilibrate their composition during slow cooling and are only found zoned in volcanic rocks and shallow intrusions where cooling was rapid enough to outpace diffusion. ^[1]

Zoning Beyond Igneous Rocks

Chemical zoning is not confined to minerals growing from magmas. Zoned minerals are also common in metamorphic rocks and in minerals precipitated from aqueous fluids in hydrothermal ore deposits. In all these settings, the zoning documents the progressive evolution of temperature, pressure, and fluid chemistry during mineral growth. Deciphering the record encoded in zoned crystals is technically demanding but often highly informative - a single crystal can preserve the entire metamorphic or hydrothermal history of its host rock in concentric chemical layers. ^[1]

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