Heterogeneous nucleation and epitaxial crystal growth of magmatic minerals

Electron backscatter diffraction analysis of dendritic clinopyroxene (cpx) forming in rapidly cooled basalt reveals two features that are unexpected for phases growing from a liquid: (1) helical growth about {010}_cpx, the crystallographic b-axis, with incremental rotation (up to 0.4° μm-1) within branch segments and large rotational jumps (10°-46°) between closely spaced branch segments, and (2) strong crystallographic preferred orientation (CPO) between cpx and titanomagnetite (timt) decorating branch tips, such that {010}_cpx aligns with one of the six symmetrically equivalent {110}_timt face poles. More than 80% of timt crystals occur within 5° of the CPO with cpx substrate, as do 22% of timt crystals in contact with euhedral cpx in natural Etna basalt. The probability distribution of distances between an arbitrary unit vector and the nearest {110}_timt normal unit vector was found by numerical simulation and indicates that the observed alignment to within 5° occurs with frequency 0.024; thus the CPO occurs 9-33 times more commonly than expected from randomly distributed crystals. The CPO matches previous observations of epitaxial relationship between spinel and host pyroxene during subsolidus exsolution, but has not previously been reported among magmatic cpx and timt. Conspicuous contiguity among phenocryst phases is inferred to result from heterogeneous nucleation facilitated by epitaxy associated with CPO, an inference supported by high-resolution electron microscopy observations of 150 nm timt crystals adhering preferentially to silicate substrates. Epitaxial relationships among phases nucleating from a melt may contribute to fabrics in magnetic properties, dictate the textures of igneous rocks by promoting development of crystal clusters, and even influence magma transport and eruption styles through a control over magma rheology.