normative composition - This refers to a calculated mineral abundance based on a bulk chemical composition. For example, the gehlenite-forsterite-anorthite ternary diagram used to classify CAIs (MacPherson et al., 1988) shows the normative compositions of type A, B, C, etc. CAIs.
metasomatic alteration - This is a type of open-system metamorphism in which the composition of the system changes.
volatility - This refers to the tendency for an element to prefer being in the gas phase rather than in a condensed phase. It is quantified by the Tc,50 values.
keplerian motion - This refers to the way that objects move in orbits, due to gravity alone. This motion is described by Kepler's three laws, which can be derived from Newton's laws of motion. Planetesimals and larger solids in the nebula would have keplerian motion. Gas would tend to revolve around the sun more slowly because it is strongly affected by thermal motion. Small particles entrained in the gas will therefore experience a drag force, which would tend to cause them to spiral in towards the protosun. The same result is achieved for the Poynting-Robertson effect, but in this case the drag force is caused by light pressure instead of gas.
Curie temperature - The temperature at which certain minerals become magnetic as they cool.
refractory element fractionation - This refers to the compositional variation observed in chondrites for Si and those elements more refractory than Si (e.g, Mg, Al, Ca). This is believed to have resulted from separation of refractory solids from nebular gas.
volatile element fractionation - This refers to the compositional variation observed in chondrites for those elements more volatile than Si (e.g., Na, S). This is believed to have resulted from separation of solids from gas at relatively low temperatures.
siderophile element fractionation - This refers to the compositional variation observed in chondrites for siderophile elements (e.g., Fe, Ni, Co, Ir, Ga, Ge). This is believed to have resulted from separation of metallic alloy from silicates in the solar nebula.
Table 1. 50% condensation
temperatures for selected elements
in a gas of solar composition,
with a total gas pressure of 10^-4 bars.
Condensation temperatures would
be lower for lower gas pressures.
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