Uncertain; apparently monoclinic.
G = 4·25. H = 6-7. Dark steel-grey; opaque.
Hackberry, Arizona.
Arrhenite.
Silico-tantalate of Yttrium metals, with Ce, Al, Fe, Ca, Be, aq. Yttr = 33·2; Cer = 2·6; ZrO₂ = 3·4%.
Amorphous.
G = 3·68. Red; translucent to opaque.
Ytterby, Sweden.
Astrophyllite.
Titano-silicate of Fe, Al, Mn, Zr, K, Na, with aq. ZrO₂ = 1·2-4·5; TiO₂ = 7-14%.