Microprobe analyses (mean of 10 grains) of chayesite gave SiO2 = 69.19, TiO2 = 0.25, Al2O3 = 0.20, Fe2O3 = 4.88, FeO = 6.60, MnO = 0.29, MgO = 12.71, Na2O = 0.31, K2O = 5.17, total = 99.60 wt%, leading to the formula (based on O = 30, with Fe valences partitioned to give Si + A1 + Fe2+ + Fe3+ + Mg + Mn + Ti = 17)

(K1.14Na0.10)1.24(Mg3.29Fe0.672+Mn0.04)4.00(Fe0.643+Fe0.292+Al0.04Ti0.03)1.00Si12.00O30.00.

The idealized end-member is K(Mg,Fe2+)4Fe3+[Si12,O30], which (with Mg:Fe2+:Fe3+ = 3.32:0.99:0.69) requires SiO2 69.l5, Fe2O3 5.28,FeO 6.82,MgO 12.83, K2O 5.92,total 100.00 wt%

X-ray powder data prove chayesite to be a member of the osumilite group. It is related to roedderite,(Na,K)2(Mg,Fe)5[Si12O30], by the substitution Fe3+ + □ = Fe2+ + (Na,k). The strongespt powder lines are 7.14 (st) 002, 5.08 (vst) 110, 3.75 (vst) 202,3.24 (vst) l2l, 2.782 (st) 204. Chayesite is hexagonal, probably P6/mcc (by analogy with the osumilite group) with a: 10.153(4) Å, c: 14.388(6) Å, V: 1284.4 Å3, Z = 2.

Chayesite occurs microscopically as a late-crystallizing phase in a lamproite at Moon Canyon, Utah, U.S.A. The crystals are deep blue, transparent, and tabular; their streak is white, and their luster is vitreous. They show no cleavage. Dcalc = 2.68 g/cm3. Uniaxial positive with ω = 1.575(1) (O, sky blue) and ∊ = 1.578(l) (E, colorless). There is probably a second occrennce in a lamproite from Cancarix, Spain.

The name is for Dr. Felix Chayes, Geophysical Laboratory, Washington, D.C., U.S.A. Type material is deposited in the Institut fiir Mineralogie, Ruhr-Universitiit Bochum, F.R.G., and in the National Museum of Natural History (Smithsonian Institution), Washington, D.C., U.S.A., number NMNH 165807

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