Inspired by https://japanese.stackexchange.com/a/38733/3295

Some examples from Wiktionary:

真っ暗 (makkura, "total darkness")
真っ向 (makkō, "directly opposite", "right in front")
真っ黒 (makkuro, "pitch black)
真っ最中 (massaichū, "infull swing, in the midst")
真っ青 (massao, "deep blue") 真っ赤 (makka, "bright red")
真っ先
真っ直ぐ
真っ二つ
真っ白

Also possibly related: 真ん丸, 真ん中.

The usual rules for germination don't seem to apply: 真{ま} doesn't end in つ or く, so where does っ come from?

@user4092 puts forward this theory in a comment:

As for 真っ白, some claim that it derives from prefix "ma" with reduplicated adjectives, which is shared with Filipino or Melanesian languages. This feature is well reserved in "ma aka aka" → まっかっか.

To be honest I'm somewhat skeptical of this.

For now my pet theory is that it's related to the reading まこと, but is there a commonly accepted explanation?

Inspired by https://japanese.stackexchange.com/a/38733/3295

Some examples from Wiktionary:

真っ暗 (makkura, "total darkness")
真っ向 (makkō, "directly opposite", "right in front")
真っ黒 (makkuro, "pitch black)
真っ最中 (massaichū, "infull swing, in the midst")
真っ青 (massao, "deep blue") 
真っ赤 (makka, "bright red")
真っ先
真っ直ぐ
真っ二つ
真っ白

Also possibly related: 真ん丸, 真ん中.

The usual rules for gemination don't seem to apply: 真{ま} doesn't end in つ or く, so where does っ come from?

@user4092 puts forward this theory in a comment:

As for 真っ白, some claim that it derives from prefix "ma" with reduplicated adjectives, which is shared with Filipino or Melanesian languages. This feature is well reserved in "ma aka aka" → まっかっか.

To be honest I'm somewhat skeptical of this.

For now my pet theory is that it's related to the reading まこと, but is there a commonly accepted explanation?