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My recommendation:
You need 1/4 inch hardie board on the subfloor to keep the mortar under the tile moist while it sets. You want it to cure, not dry out and wood sucks the moisture out of it. Use thinset between the hardie board and the subfloor and screw the hardie down with 1 1/4" screws every 8 -12 inches (this may be a climate issue, around here we have a wide temp/humidity variance and really need to keep things tight). Marble is touchy because you have to be very careful setting it into the mortar to get it flush and level. It cracks more easily than porcelain so you can't pound on it as much. Have 4 foot and 2 foot levels and keep checking for level and flush. Take your time, mortar is cheap.
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