Subsilicates - Humite Group, Calamine Composition

The three minerals, humite, Mg3[Mg(F,OH)]2 [Si04]2, chondrodite, Mg5[Mg(F,oH) [Si04]a, and dinohumite, Mg5[Mg(F.OH)]2 [Si04]3, are closely related chemically and crystallographieally. They are characteristically found in crystalline limestones. Chondrodite is the most common in occurrence.

Ilvaite, or lievrite, HCaFc₂"Fe'''Si₂.O₉., is a rare mineral belonging in this section.

Calamine
Composition. Silicate of zinc, H2(Zn2O)Si04 = Silica 25, zinc oxide 67.5, water 7.5.

Crystallization. Orthorhombic; hemimorphic. Crystals usually tabular parallel to the brachypinacoid. They show prism faces and are terminated above usually by a combination of macrodomes and brachycomes and base, and below by a pyramid (Fig. 331).

Structure. Usually in crystal groups with the individuals attached at their lower (pyramidal) ends and lying with their brachypinacoid faces in common. Crystals often divergent, giving rounded groups with slight reentrant notches between the individual crystals, forming knuckle or coxcomb masses. Also marnmillary, stalaetitic, massive and granular.

Physical Properties. Prismatic cleavage. H. = 4.5-5. G. = 3.4-3.5. Vitreous luster. Color white, sometimes with faint bluish or greenish shade; also yellow to brown. Transparent to translucent. Strongly pyroelectric

Tests. Fusible with difficulty at 5. Soluble in hydrochloric acid and yields gelatinous silica on evaporation. Fused on charcoal with sodium carbonate gives a nonvolatile coating of zinc oxide (yellow when hot, white when cold). Gives water in C. T. Recognized usually by the characteristic grouping of its crystals, but may be obscure and to be determined only by above tests.

Occurrence. A mineral of secondary origin, found in the oxidized portion of zinc deposits, associated with smithsonite, sphalerite, crosstie, anglesite, galena, etc. Usually with limestone rocks. Occurs at Moresnet, Belgium; Aix-Ia-Chapelle, Germany; in Corinthian; Rumania; Sardinia; Cumberland and Derbyshire, England; Algeria; Chihuahua, Mexico; Sterling Hill, near Ogdensburg, New Jersey; Friedens Ville, Pennsylvania; Wythe County, Virginia; with the zinc deposits of southwestern Missouri; Leadville, Colorado; Organ Mts., New Mexico; Elkhorn Mts., Montana.

Name. Supposed to be derived from cadmic, a name given by the ancients to the silicate and carbonate of zinc. The mineral is calledis called by English mineralogists hemimorphite or electric calamine.

Use. An ore of zinc.