Abstract

Mccrillisite, ideally NaCs(Be,Li)Zr 2 (PO 4 ) 4 .1-2H 2 O, a member of the gainesite group, occurs in the Mount Mica granitic pegmatite, South Paris, Oxford County, Maine. The mineral is a product of late-stage hydrothermal alteration and is associated with approximately twenty other silicate, oxide, carbonate, arsenide and phosphate minerals. Crystals occur in mm- to cm-sized cavities, and individuals are up to 1.2 mm in maximum dimension. The crystals are white to colorless, bipyramidal, with {111} dominant and {001} minor. Streak is white, and luster, vitreous; the mineral is translucent to transparent, and nonfluorescent. Hardness (Mohs) is 4 to 4 1/2. No cleavage was observed, and the fracture is conchoidal. D measured 3.125(5), D calculated 3.30 g/cm 3 . Mccrillisite is uniaxial positive, with omega 1.634(2) and epsilon 1.645(2), colorless and nonpleochroic. The mineral is tetragonal, I4 1 /amd, with a 6.573(2), c 17.28(2) Aa, V 746.6(9) Aa 3 , c/a 2.6289, Z = 2. The strongest six reflections in the X-ray powder-diffraction pattern [d(Aa)(I)(hkl)] are: 6.159(90)(101), 4.326(80)(004), 4.099(40)(112), 3.281(80)(200), 3.060(100)(105) and 2.896(30)(211). Chemical compositions (wt.%) for mccrillisite and cotype gainesite, respectively, are: Li 2 O 0.6, 0.8; Al 2 O 3 <0.1, 0.5; CaO <0.1, 0.7; MgO 0.1, 0.35; MnO<0.1, 0.7; FeO<0.1, 1.0; SrO 0.2, --; Na 2 O 4.2, 7.8; K 2 O 0.3, 2.4; Cs 2 O 15.3, 2.2; BeO 2.9, 2.3; ZnO 0.5, <0.1; ZrO 2 31.6, 33.0; HfO 2 2.5, 0.7; P 2 O 5 38.3, 45.0; Rb 2 O<0.1, 0.1; F 0.5, <0.1; H 2 O 3.0, 4.3; subtotal 100.10, 101.95; O = F 0.21, --; total 99.89, 101.95. An empirical formula for mccrillisite, based on 4 P atoms, is: Na1.00 (Cs (sub 0.80) Li (sub 0.18) K (sub 0.05) ) (sub Sigma 1.03) (Be (sub 0.86) Li (sub 0.12) Mg (sub 0.02) ) (sub Sigma 1.00) (Zr (sub 1.90) Hf (sub 0.09) Zn (sub 0.05) ) (sub Sigma 2.04) (P (sub 4.00) Si (sub 0.01) ) (sub Sigma 4.01) (O (sub 15.8) (sub F0.2) ) (sub Sigma 16.0) . 1.23H 2 O. The empirical formula for gainesite, based on 4 P atoms, is: Na (sub 1.00) (Na (sub 0.59) K (sub 0.32) Cs (sub 0.10) Rb (sub 0.01) ) (sub Sigma 1.02) (Be (sub 0.58) Li (sub 0.34) Mg (sub 0.06) Al (sub 0.06) ) (sub Sigma 10.4) (Zr (sub 1.69) Fe (sub 0.09) Ca (sub 0.08) Mn (sub 0.06) Hf (sub 0.02) ) (sub Sigma 1.94) (P (sub 4.00) Si (sub 0.01) ) (sub Sigma 4.01) O (sub 15.6) . 1.5H 2 O. Gainesite was originally considered to be anhydrous, but is shown to contain essential molecular water, as does mccrillisite. The mineral name honors the McCrillis family, particularly the late Dean McCrillis and Philip McCrillis, of Oxford County, Maine, and their more than 90 years spent working the granitic pegmatites of Maine.

First Page Preview

First page PDF preview
You do not currently have access to this article.