Walthierite and huangite, two new members of the alunite group, were found in the Tambo mining district and the El Indio gold, silver, and copper deposit, Coquimbo region, Chile, respectively. Walthierite, Ba0.50.5Al3(SO4)2(OH)6, is associated with alunite, jarosite, baite, quartz, and pyrite in veins. Huangite, Ca0.50.5Al3(SO4)2(OH)6, coexists with kaolinite, pyrite, and woodhouseite in altered wall rocks. The new alunite group minerals have space group R3m, Z = 6, with lattice parameters a = 6.992(4) Å, c = 34.443(8) Å, Dcalc = 3.02 g/cm3 for walthierite, and a = 6.983(5) Å, c = 33.517(9) Å, Dcalc = 2.80 g/cm3 for huangite. The strongest lines in the X-ray powder diffraction patterns are (d, I, hkl): 5.73, (50),(006); 3.49,(55),(110); 2.98,(100),(116); 2.283,(80),(1,0,14); 1.909,(70),(0,0,18) for walthierite, and 4.91,(75),(014); 2.97,(100),(116); 2.231,(51),(1,0,14); 1.899,(43),(306); 1.375,(40),(3,1,14) for huangite. Both minerals show superlattice reflections with c that is double that of alunite, resulting from the ordering of cations and vacancies on l2-fold coordinated M sites. Both minerals are transparent, white to liglt yellowish in color, and have perfect {001} cleavage. Optically, they are uniaxial positive with negative elongation. The indices of refraction are ω = 1.588(2), = 1.604(2) for walthierite. Minamiite is now considered to be a Na-dominant and Ca-bearing member, rather than a Ca-dominant end-member of the alunite group; huangite is now considered to be the Ca end-member. Wide miscibility gaps may exist between walthierite and natroalunite and between huangite and alunite. Extensive solid solution probably occurs between walthierite and alunite, huangite and natroalunite, and huangite and woodhouseite. The nature ofoccurrence and stability relations for walthierite and huangite indicate that they, like other alunite group minerals, are stable only under acid sulfate hydrothermal conditions.

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