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Why is it spelled in katakana, as "ヒラキ"? Isn't this just simply "開き"?
John F. It's mainly a way of pointing out it's a technical word (like our italics). Sometimes it's to cover up the possibility of several different characters being applicable, and sometimes to cover up rare characters or characters used in unusual readings. It's a modern thing. Old books didn't dumb down/try to be helpful (take your pick) like this.
Thank you! I still don't understand it, though. "開" is not a rare character, and the reading "開き" doesn't seem unusual to me. So is it that there are two different characters for the "ヒラ" part? Could the other one be "拓", by any chance? (Wouldn't that be neat: The hand with a stone!) But why would you want to avoid two different applicable characters, anyway? Lots of other languages have words that can be spelled in different ways. I guess I just don't understand those modern Japanese!
On second thought, never mind. Your first option, pointing out it's a technical word, is probably it. I realize now that the view from west to east is the opposite of the view the other way. For me, kanji are technical words, while katakana are mere transliterations (often of everyday items such as "テイベル"), but I understand now that for Japanese, it's probably the other way.