How did やわらかい gain its い?


As we know, there is a class of adjectives that end in `〜らか`: `滑【なめ】らか`,
`明【あき】らか`, `清【きよ】らか`, `安【やす】らか`, etc.

`やわらか` (`柔らか` or `軟らか`) is also in this class of adjectives. However, it can
also be written as the イ-adjective `やわらかい`. So when and how did this adjective
pick up an い to become a separate イ-adjective?

Can this be done arbitrarily with any of the `〜らか` class adjectives? Like
`滑【なめ】らかい`, `明【あき】らかい`, `清【きよ】らかい`, `安【やす】らかい`. My gut feeling is that it
cannot, but only because those forms sound strange to me. If it cannot
arbitrarily be done, are there any other `〜らか` class adjectives that **can**
take on the extra い?




Firstly, as was noted in the comments (by Tsuyoshi Ito), that the same thing
cannot be done with other adjectives ending in `～らか`.

On the other hand, there exists a number of adjectives, which can function
both as イ-adjective and as ナ-adjective, e.g.

[code]

    大きい   大きな
    小さい   小さな
    真っ白い 真っ白な (etc.)
    細かい   細かな
    暖かい   暖かな
    四角い   四角な (etc.)
    柔らかい 柔らかな
    
[/code]

(but, of course, やわらかい is the only one ending in `～らかい`). An investigation of
the differences here might deserve a separate question.

As for the difference in usage, I have talked to quite a few native speakers,
who maintain that there is no substantial difference in meaning between
`やわらかい` and `やわらかな`, which is only a handful of anecdotal evidence. On the
other hand, [here](http://www.yano.bc.ca/vansin/vansinpo095.htm) is an article
(blog post?) about the difference between `やわらかいお餅` and `やわらかなお餅`, where the
author says something along the lines, that a `やわらかなお餅` has a more gentle feel
to it (he says `「やわらかな」のほうが落ち着く感じがする`). Which, too, is at best anecdotal
evidence.

One can only guess that maybe he means that `やわらかい` is a more objective
statement about the physical properties of the `お餅` (softness), whereas
`やわらかな` suggests a subjective experience of softness, or rather gentleness.

But even if this is the case, not every Japanese person seems to perceive
these subtleties in the same way...

P.S. I noticed that I haven't answered your question, regarding the history of
the difference. I will delete my answer, if it isn't contributing anything to
the problem of answering your question.

