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Hidalgoite
Hidalgoite, PbAl3(AsO4)(SO4)(OH)4, is a rare member of the beudantite group and is usually classified as part of the alunite family. It was named after the place where it was first discovered, the Zimapán mining district, Hidalgo, Mexico. At Hidalgo where it was initially discovered, it was found as dense white masses in alternating dikes of quartz latite and quartz monzonite alongside other secondary minerals such as sphalerite, arsenopyrite, cerussite and trace amounts of angelsite and alamosite, it was then rediscovered at other locations such as Australia where it occurs on oxidized shear zones above greywacke shales especially on the anticline prospects of the area, and on fine grained quartz-spessartine rocks in Broken Hill, Australia. Hidalgoite specimens are usually associated with copper minerals, clay minerals, iron oxides and polymetallic sulfides in occurrence.
Hidalgoite is categorized under the trigonal crystal system and Rm space group and can be denoted by the H-M symbol (2/m). Physical properties of hidalgoite include its gray white to light gray color, a white streak, specific gravity of 3.96 and a hardness of 4.5. Hidalgoite specimens are usually translucent to sub opaque, and the brittle nature of mineral produces conchoidal fractures. It has an earthy luster.
==Composition==
Hidalgoite was collected from a quartz vein at the Zimapán mining area after which it was examined by Smith in the laboratories.〔Smith et al. 1953 Smith, Robert, S. Frank Simons, and C. Angelina Vlisidis, ''Hidalgoite, a New Mineral,'' American Mineralogist, 1953, pp. 1218-1224〕 Hidalgoite structure departs from the other beaudantite group members in the ion substitutions; lead is substituted for zinc, iron for aluminium and antimony for arsenic and these substitutions account for the presence of some ions in the chemical analysis for the specimen.〔 Other chemical analyses that were conducted on the hidalgoite specimen include determination of the sulfide using qualitative analysis in which zinc was precipitated as the sulfide and was then ignited to the oxide, iron content (Fe2O3) was determined colorimetrically with KCNS and water content which is only slightly expressed in the structure was determined by the Penfield method using anhydrous sodium tungstate as a flux.〔 The specific gravity of hidalgoite was determined as 3.96 using Adam-Johnston fused silica pycnometer.〔 Hidalgoite specimens also contained some limonitic impurities which according to Smith accounts for the excess water in the structure. Qualitative spectrographic analysis of hidalgoite specimen showed the presence of metallic cations such as Ca, V, Ti and Cu in very minute quantities and Mg, Sr, Ba, B, Cr and Sc in even smaller quantities.〔
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