Kosnarite, ideally KZr2(PO4)3, has been identified as part of a late-stage, secondary phosphate mineral assemblage from the Mount Mica pegmatite at Paris, and from the Black Mountain pegmatite, Rumford, Oxford County, Maine.

Kosnarite from Mount Mica occurs as pseudocubic rhombohedral crystals, as much as 0.9 mm in maximum dimension, that display the dominant {102} form. Color ranges from pale blue to blue-green to nearly colorless. The mineral has a white streak, is transparent, has a vitreous luster, and is nonfluorescent in ultraviolet light. It has a hardness of 4.5, is brittle with a conchoidal fracture, and has perfect {102} cleavage. Measured and calculated densities are Dm 3.194(2) and Dc 3.206. Optically, this mineral is characterized as uniaxial (+), with refractive indices of Nω = 1.656(2) and N = 1.682(2) and is nonpleochroic. Twinning was not observed.

The mineral is hexagonal (rhombohedral), space group R3c with a = 8.687(2), c = 23.877(7) Å, V = 1560.4(8) Å3, Z = 6. The six strongest diffraction lines [d(Å), hkl, I/Io] from the Mount Mica occurrence are 6.41, 012, 50; 4.679, 104, 50; 4.329, 110, 100; 3.806, 113, 90; 2.928, 116, 90; 2.502, 300, 50. Mean analytical results are Na2O 1.4, K2O 8.7, Rb2O 0.25, FeO 0.2, MnO 1.0, ZrO2 44.5, HfO2 0.5, P2O5 43.3, F 0.20, sum 100.05, less O for F 0.08, total 99.97 wt%. The empirical formula (based on O + F = 12) is (K0.93Na0.08Rb0.01)Σ1.02(Zr1.81Na0.15Mn0.07Fe0.01Hf0.01)Σ2.05P3.06(O11.95F0.05)Σ12.00. Kosnarite from Black Mountain is almost pure KZr2(PO4)3 with only trace amounts of Hf, Mn, Na, and Rb. The mineral is one of three known alkali zirconium phosphates; the others are gainesite and the Cs analogue of gainesite.

The name is for Richard A. Kosnar of Black Hawk, Colorado.

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