Length-slow calcite has recently been reported to occur only as an in situ replacement; the implication has been that length-slow calcite does not occur as a primary cement in carbonate rocks. However, the length-slow character is not necessarily unique to replacement calcite. Its apparent absence from cements may be due to either the erroneous belief that the optic axis and the long axis of cement crystals are coincident or the simple fact that it has not been recorded. The optic axis and greatest growth vectors of closed crystallographic forms are coincident if the forms are oriented with their optic axis vertical and with faces inclined more than 45° to the horizontal. Elongate crystals grown from such forms will be length-fast. Closed forms with faces inclined less than 45° to the horizontal have their greatest growth vectors at a high angle to the optic axis. Elongate crystals grown from these forms will be approximately length-slow.

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