Whitlockite is a tricalcium phosphate, Ca3(PO4)2. Rhombohedral, probably scaleno-hedral—3 2/m, with a:c= 1:3.547. Habit rhombohedral {0112}, with subordinate {0001}, {1014}, {1120}. Structure cell (in hexagonal coordinates): a0= 10.25 ± 0.03, c0 = 36.9 ± 0.1; a0:c0= 1:3.600; arh= 13.65, α=44°612. Cell contents in the rhombohedral unit Ca21P14O56. Space group probably R3c. No cleavage. Hardness 5. Specific gravity 3.12 (obsvd.), 3.19 (calc.). Colorless and transparent. Uniaxial negative, with ω= 1.629 ± 0.002 and ϵ=1.626 ± 0.002 (for Na).

Found as a late hydrothermal mineral in granite pegmatite at the Palermo quarry, near North Groton, New Hampshire, in the following association and sequence: whitlockite and quartz—rhodochrosite—apatite—zeolite. Whitlockite is structurally distinct from graftonite, caryinite, fillowite and apatite. Named in honor of Herbert P. Whitlock, Curator of Minerals and Gems in the American Museum of Natural History.

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