Why is 'spatial' written with a t instead of a c?

The word 'spatial' is obviously derived from the word 'space'. So why is it usually written with a 't' instead of a 'c'?
Is there a historical reason or is it because of some grammar rule I do not know about?

Solution 1:

etymonline.com:

1840 (spacial is from 1838), "occupying space," from Latin spatium + adjectival suffix -al (1); formed in English as an adjective to space (n.), to go with temporal. Meaning "of or relating to space" is from 1857. Related: Spatially.

The historical reason why spatial is usually written that way is simply its origins in the Latin word spatium.

Solution 2:

Most spelling alternations between -c- and -ti- like this (there is another question that mentions some more examples: Keep with T or change to C in verb to derived word conversion) are based on sound changes that affected the pronunciation of Latin words in European languages like French. When Latin -ti- came before a vowel, the t was palatalized to a [ts] sound. (Unless there was an s immediately before it, as in question). In French and English, the same pronunciation was used for the "soft c" sound that developed from Latin c before a front vowel like e i y. So many Latin words spelled with -ti- + a vowel developed spelling variants with -ci-.

(In both French and English, the [ts] sound was later simplified to [s]. In English, [s] was furthermore altered to [ʃ] in many words because of the influence of the following palatal glide [j].)

Furthermore, words from Latin in French often reduced the Latin endings, so Latin spatium was turned into French espace. We see something similar with nouns ending in -ance or -ence which are often related to adjectives ending in -ant or ent: the noun endings are derived from Latin -antia/-entia, while the adjective endings are derived from Latin -ant-/-ent-.

In more recent times, there was a general movement in French and English away from the spelling -ci- in favor of the spelling -ti-, the "original" Latin spelling (for example, it was not uncommon in Middle English for words to end in -acion or -acyon, but in present-day English only a handful of words are spelled with -cion). In fact, this trend even led to some non-etymological spellings with -ti- where there was no Latin etymon with t: for example, the modern spelling practitioner and an obsolete variant spelling physitian in place of physician. But words like space and presence, where the i present in Latin has been lost, do not have spellings with t.

The use of the word spatial seems to be fairly recent—the Oxford English Dictionary's first citation is from 1847—and the preference for -ti- in this word seems to be explained by the trend discussed in the previous paragraph. Nevertheless, as ghoppe's answer mentions, it seems that spacial has existed as a minority spelling throughout the time period when this word has been used.