In recent years, important advances in our understanding of the pressu
re-temperature phase and transformation diagram for carbon have occurr
ed as a result of developments in both experimental and theoretical te
chniques. Graphite, diamond, liquid and vapor remain the major thermod
ynamically stable forms of carbon. However, due to the high activation
energies for solid-state transformations and the specific effects of
reaction paths, other metastable forms and a wide spectrum of complex
hybrid forms may be generated, and possibly quenched-in, to survive me
tastably. This paper focuses primarily on developments since the last
review of the carbon phase diagram published in 1989, but also include
s references to the reliable older work Some of the newer conclusions
include the following: the Clapeyron slope of the diamond melting line
, dT(m)/dP, is positive; the liquid is metallic and there appears to b
e no evidence for a transformation between electrically conducting and
non-conducting forms; melted droplets of carbon less than 0.2 mu m in
diameter quench to a giant fullerene structure even in the stability
held of diamond; graphite transforms to a transparent phase on compres
sion at room temperature; this phase reverts to graphite on decompress
ion at this temperature from pressures as high as 100 GPa.