We present an empirical equation to link the thickness of a MgAl2O4 spinel layer growing at the interface between MgO and Al2O3 multianvil high-pressure assembly pieces to pressure (P), temperature (T), and time (t), extending the recent study of Watson et al. <2002> to the pressure range 6--16 GPa and temperature range 1673--2273 K. For the spinel thickness ΔX we obtain$Delta X left( mu {rm m} right) = sqrt { left( 5.3 pm 0.3 right) cdot 10^{8} cdot exp left( - displaystylefrac{ left( 33200 pm 2118 right) + left( 1424 pm 75 right) cdot P left( {rm GPa} right) } { T left(rm K right) } right) cdot t left( {rm s} right) }. $This equation can be used to assess the thermal gradient in any multianvil assembly where MgO and Al2O3 filler pieces are in contact at P-T conditions within the MgAl2O4 spinel stability field. As an illustration, we show that the central hot spot, in which temperatures can be considered constant, is close to 1 mm long in a common octahedral multianvil assembly with 8 mm edge length and rhenium furnace, while it extends to 3 mm long in an octahedron with 18 mm edge length using a straight graphite heating element. In addition, we present the results of a spinel growth experiment performed at 2273 K and 15 GPa with 18O enriched MgO, which shows that oxygen is mobile at a length scale exceeding that of the spinel layer. This finding raises the possibility that under some circumstances growth of spinel (and of reaction-product layers in other oxide systems) might be accomplished by concurrent fluxes of oxygen and cations. This mobile oxygen model differs from the more conventional Mg-Al exchange model proposed by Watson and Price <2002> for the lower-pressure experiments and might explain the observed differences in systematics between the high- and low-pressure data sets.