Rynersonite, a new species isostructural with synthetic CaTa2Oe, is found in the Himalaya pegmatite-aplite dike system, Mesa Grande district, San Diego County, California. It occurs as creamy-white to reddish-pink felted masses of fibrous to lath-like crystals. Individual laths average less than 0.5 mm in length and 0.05 mm in thickness. Electron diffraction patterns show that the laths are elongated along a and flattened on (010). Hardness is about 4.5; luster earthy; streak white; fracture uneven. Rynersonite is biaxial positive with moderate 2V; indices of refraction are greater than 2.05; birefringence is about 0.14; X = c, Y = b, Z = a; with X = Y = straw-yellow, Z = light straw-yellow.

Reddish-pink rynersonite is orthorhombic, space group Pmnb, with a = 7.505(1), b = 11.063(2), c = 5.370(1)A, Z = 4, Gcalc = 6.394; for the composition Ca(Ta1.21Nb0.77)O6, Gmeas = 6.402. The six strongest lines of the X-ray pattern (d, in A; intensity; hkl) are 3.038 100 031, 4.835 91 011, 2.964 91 211, 3.754 71 200, 2.683 50 002, and 2.359 50 231. Heating in air for 20 hours at 550°, 800°, and 1050°C produces no irreversible changes in cell dimensions or structure.

Electron microprobe analyses indicate a range of composition from Ca(Ta1.44Nb0.51)O6 to Ca(Ta1.21Nb0.77)O6. Rynersonite formed as an alteration product of stibiocolumbite—stibiotantalite crystals and is intimately associated with minor amounts of antimonian microlite and fersmite, Ca(Nb,Ta)2O6. Although rynersonite and fersmite are both orthohombic AB2O6 compounds, they differ structurally. The new mineral has space group Pmnb; fersmite, Pcan.

The mineral is named for Eugene B. and Buel F. Rynerson, and their late father, Fred J. Rynerson, in recognition of their lifelong interest in mining the gem-bearing pegmatites in Mesa Grande, California.

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