Chloritoid occurs as greenish black plates (up to 1 × 20 mm.) in quartz-carbonate veins, at the Harvey Hill mine, 5 miles E. of Leeds, Megantic County, Quebec. Cleavage (001) good, (hhl) poor. H=612 G = 3.528. The monoclinic unit cell with space-group C2/c, has a=9.45, b = 5.48, c = 18.16 A, β = 101° 30/, and contains 4((Fe, Mg)2 Al2(Al2Si2) O10(OH)4]. Twinning lamellar, probably with twin axis [130], composition plane (001). Optically positive; X=b,Z:(001)= +15°, Z:[001]= +2612 α(greenish grey to olive green) = 1.716, β (bluish grey to idigo) = 1.719, γ (colourless to greenish yellow) = 1.725 (all ± 0.001 for Na); 2V = 66±2²,r > v.

Early crystal measurements indicate the forms c(001), m(110), e(401), i(401), k(114), h(111); an angle-table for these forms is given on a:b:c=1.7245:1:3.3141, β = 1O1°3O' (x-ray). The powder pattern of chloritoid is indexed and a review of 20 analyses establishes the composition (Fe",Mg)2(Al, Fe"')2(Al2Si2)O10(OH)4.

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