Magma nature
Magma is a term first introduced into geologic literature in 1825 by Scope, who referred to it as a “compound liquid” consisting of solid particles suspended in a liquid, like mud. Measurements on extruded magma (lava), together with evaluations of mineral geothermometers in magmatic rocks and experimental determinations of their melting relations, indicate that temperatures of magmas near the surface of the Earth generally range from about 1200°C to 700°C; the higher values pertain to mafic compositions, the lower to silicic. Very rare alkali carbonatitic lavas that contain almost no silica have eruptive temperatures of about 600°C. Extruded magmas are rarely free of crystals, indicating that they rarely are superheated above temperatures of crystallization. Densities of magmas range from about 2.2 to 3.0 g/cm3 and are generally about 90% of that of the equivalent crystalline rock.
Magma in general consists of a mobile mixture of solid, liquid, and gaseous phases. The number and nature of the phases constituting a magma depend, under stable equilibrium conditions, on the three intensive variables P, T, and X (concentrations of chemical components in the magma). At sufficiently high T, any rock melts completely to form a homogeneous liquid solution, or melt. Except for carbonatite magmas, melts consist mostly of ions of O and Si hence the alternate appellation silicate liquid but always contain in addition significant amounts of Al, Ca, H, Na, and so on.
Examples of different types of magmas are shown schematically in below figure. Only in some unusually hot systems will a magma consist wholly of melt and no other phases. In most instances, a melt is only part of the whole magma, but is always present and gives it mobility. Hence, melt and magma are generally not the same. To a significant extent, the properties of the melt largely govern the overall dynamic behaviour of the whole magma. Rare magma systems consist at equilibrium of two physically distinct melts one essentially of carbonate and the other of silicate, or both are silicate but one is silicic and the other very rich in Fe. Each of these immiscible melts has distinctive properties, such as density. Oil and water are familiar immiscible liquids.
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| Schematic possible magmas |
Atomic Structure of Melts
The configuration of ions in a melt its atomic structure largely dictates many of its significant properties. In pictorial representations of crystalline, liquid, and gaseous states, individual atoms have to be drawn as fixed in position relative to one another, but these are only their average, or instantaneous, positions. Even in crystals above absolute zero (0K), individual ions have motion. In glasses that are supercooled very viscous melts, ions experience vibrational motion: small periodic displacements about an average position. But at temperatures above a glass-melt transition, approximately two-thirds to three-quarters the melting T in degrees Kelvin, ions in the melt have more mobility and can break their bonds with neighbouring ions and wander about, forming new configurations. In a flowing melt, bonds are broken and bond angles and distances are distorted, but after deformation ceases, the ionic array may have sufficient time to reform into a “relaxed” equilibrium structure.
Many studies of melts in the laboratory using nuclear magnetic resonance, vibrational spectroscopy, and X-ray analyses reveal a lack of long-range (on the scale of more than a few atomic bond lengths) structural order and symmetry that characterize crystals. However, melts possess a short-range structural order in which tetrahedrally coordinated Si and Al cations are surrounded by four O anions and octahedrally bonded cations such as Ca and Fe2+ surrounded by six O anions roughly resemble those in crystals. Because silica is the most abundant constituent in most natural melts, the fundamental structural unit is the (SiO4)4 - tetrahedron, as it is in silicate minerals. Conceptual models of the atomic structure of silicate liquids can be constructed on the basis of these observations. Figure below depicts these models for liquid silica (SiO2) and CaMgSi2O6; the latter in crystalline form is diopside pyroxene.
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Conceptual models of atomic structures of silicate melts compared with the symmetric lattice of a crystalline solid. (a) Crystalline silica (high tridymite). Layers of hexagonal rings of Si-O tetrahedra with alternating apices pointing up and down are stacked on top of one another, creating a three-dimensional structure in which each oxygen is shared by two silicons. Tetrahedra with apices pointing up have the upper apical oxygen left out of the drawing so as to reveal underlying silicon. Dashed line indicates outline of one unit cell in the lattice. (b) Model of liquid silica. Si-O tetrahedra are slightly distorted relative to the crystalline lattice. Long-range order is absent. Structure is highly polymerized because all tetrahedra are interconnected by bridging oxygen anions. (c) Model of liquid CaMgSi2O6 showing less polymerization than that of liquid silica. Note presence of network-modifying cations (Ca and Mg) and non-bridging oxygen, neither of which occurs in the silica melt.
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Because the entropy of melting of crystalline silica (i.e., the change in entropy from the crystalline to the liquid state) is relatively small, there can be little change in the degree of order in the atomic structure of the melt relative to the crystalline state. Thus, a model for liquid silica is a three-dimensional network of somewhat distorted Si-O tetrahedra, not unlike the corresponding structure of crystalline silica. Short range order is roughly similar to that in the crystalline state, but long-range order, as would be evident in a symmetrical crystal lattice, is absent. The silica melt can be viewed as a three-dimensional network of interlinked chains, or polymers, of Si-O tetrahedra.
On the other hand, in the model of the CaMgSi2O6 melt, these string like polymers are shorter, less intricately linked, and interspersed among octahedrally coordinated cations of Ca and Mg. This melt is not as polymerized as liquid silica.
Four different types of ions can be recognized in these models (Figure above) on the basis of their relation to the polymers:
- Network-forming cations of Si4 within the interconnected tetrahedra of the polymers are strongly linked by
- bridging oxygens.
- Network-modifying cations of Ca and Mg are more weakly bonded to
- non-bridging oxygens in non-tetrahedral bonding arrangements.
The ratio of non-bridging oxygens to network-forming, tetrahedrally coordinated cations chiefly Si and Al is a measure of the degree of polymerization in a melt; small ratios correspond to high degrees of polymerization. In completely polymerized liquid silica, the ratio = 0. In partially polymerized liquid CaMgSi2O6 it is = 2/1 = 2.
The atomic structure of naturally occurring melts is more complex than these simple models. Despite considerable research, many details are not understood. Other ions of different size, charge, and electro-negativity, such as Al3+ , Ti4 +, Fe3+ , P5+ , H- , or F- make natural melts more complex. In this milieu, mobile cations compete for available anions, principally oxygen, in order to satisfy bonding requirements and to minimize the free energy of the melt. This is not quite the same situation as in crystals, where cations have more or less fixed sites of a particular coordination in the ordered lattice. In addition to the widespread (SiO4)4 - tetrahedra in melts, there are less abundant neighbouring tetrahedra of more negatively charged (Al3+ O4)5 - and (Fe3+ O4)5 -. The ionic charge and size of network-modifying cations, which generally form weaker bonds with non-bridging oxygens, can play an important role in melt structure. Network modifiers most commonly include monovalent K and Na; divalent Ca, Mg, Fe, and Mn, and more highly charged, but less abundant high-field-strength cations including P5+ , Ti4 +, and the still less abundant trace elements.
The most important dynamic property of a melt its viscosity depends strongly on its atomic structure. Because viscosity is a measure of the ease of flow of a melt and the mobility of ions, it should be intuitively obvious that more highly polymerized melts are more viscous. Alternatively, it can be said that, because non-bridging oxygen anions are less strongly bonded to neighbouring cations than bridging oxygens to Si and Al, viscosity correlates with the ratio of non-bridging to bridging oxygens. Increased concentrations of some components can depolymerize melts and reduce viscosity. Even small weight proportions of dissolved water or fluorine can depolymerize silicate melts, making them much less viscous. Also, high-field-strength, network-modifying cations whose charge is generally > 3+ have a strong affinity for oxygen anions and may successfully compete against network-forming Si4+ , Al3+ , and Fe3+ , thus depolymerizing the melt. The role of Fe in melt structures is especially significant because it occurs in two oxidation states. Fe2+ appears to be exclusively a network modifier, whereas Fe3+ can be either a network modifier or a network former. Changes in the oxidation state can therefore affect the degree of polymerization of a melt.
Increasing pressure appears to reduce the degree of polymerization somewhat. Because octahedral coordination of Si and Al is favoured in crystalline structures at high P over tetrahedral coordination, similar coordination changes might occur in melts at high P. Some experiments suggest that Al more readily shifts toward octahedral coordination with increasing P than does Si.
In conclusion, water-free (“dry”) rhyolite melts have virtually no non-bridging oxygens and are nearly completely polymerized and highly viscous. In andesite melts the ratio of non-bridging oxygens to network forming, tetrahedrally coordinated cations is about 0.2, and in basalt melts it is 0.4–1.2. Consequently, mafic, silica-poor melts are significantly less polymerized and less viscous than dry silicic melts.
