This is kind of a step up, and down, from mullite. Cordierite has a relatively high melting point and remarkable expansion properties and is sold as all sorts of kiln (and other high temperature) hardware, but the melting point is lower than mullite's.
Named for the... uh, I can't find a reference! Cordierite is a crystalline magnesium aluminosilicate, 2MgO.2Al2O3.5SiO2. Natural cordierite is birefringent, meaning it shows different colors depending on the angle the light passes through the crystal. (Despite the tendancy of Fe(II) replacing Mg, cordierite is never green to black, always gray to dark blue, which must be a property of the lattice like the birefringence.) It is made by firing an appropriate mixture of the elements: a typical blend might involve magnesite (magnesium carbonate), clay and silica or alumina to balance, or preferably, use a more amenable form of magnesium like talc (magnesium silicate) with the clay, plus a dash of alumina as needed.
This is a better insulator than mullite, and has a far smaller expansion factor - in fact it can be quenched from near red heat with NO damage! Add some porosity and you can increase the insulating value and flexibility of the material (the modulus is already much lower than mullite's, resisting fracture better, though strength is lower). It can handle nearly cone 16, so you can't say it isn't refractory - true, it's much less than the heavy duty mullite which melts up at cone 34 or so, and I wouldn't trust it melting cast iron or nickel, but it'll handle cone 10 firings day in, day out, and will hold all copper alloys in liquid form. The only problem is, it has a very narrow firing range. One post on Clayart said it has a variation of +/- 9°F!? That has to be a typo, but nonetheless it does make light of the point that cordierite is, unfortunately for firing purposes, an eutectic composition. I would suppose you could add some alumina and drop the magnesia a little to encourage more mullite phase (also strengthening it a little), but I have no idea how well that would work. I would think a flux such as Li2O or B2O3 could also be used to extend firing range down (liquid phase sintering), but that also reduces melting point. I wouldn't suggest Na2O or K2O because those will increase expansion.
Haven't made any yet! ;-) Will as soon as the kiln is finished and tuned up though. I intend to mix ball clay (KT#1-4 is about 2Al2O3.7SiO2) with the appropriate amount of magnesite and alumina to get the formula right, then fire to the cone 14 area, or just 10-12 if the kiln itself feels like melting any higher! If I can't get right below the melting point, I'll hold temp as long as I can to get it to sinter well.