SION and TION .
Primitive words which end in d, de, ge, mit, rt, se, or ss, take sion in their derivatives; but all other words have tion.
Examples.
abscind, abscission
condescend, condescension
evade, evasion
intrude, intrusion
absterge, abstersion
emerge, emersion
admit, admission
remit, remission
revert, reversion
convert, conversion
confuse, confusion
revise, revision
impress, impression
confess, confession
admix, admixtion
promote, promotion
Irregulars.
adhesion
cohesion
compulsion
declension
decursion
depulsion
dissension
divulsion
evulsion
exesion
expulsion
impulsion
incursion
propulsion
recension
recursion
revulsion
scansion
tension
transcursion
version
attention
causation
distention
distortion
coercion
suspicion
crucifixion