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Adamite is a zinc arsenate hydroxide mineral, Zn2AsO4OH. It is a mineral that typically occurs in the oxidized or weathered zone above zinc ore occurrences. Pure adamite is colorless, but usually it possess yellow color due to Fe compounds admixture. Tints of green also occur and are connected with copper substitutions in the mineral structure. Olivenite is a copper arsenate that is isostructural with adamite and there is considerable substitution between zinc and copper resulting in an intermediate called cuproadamite. Zincolivenite is a recently discovered mineral being an intermediate mineral with formula CuZn(AsO4)(OH). Manganese, cobalt, and nickel also substitute in the structure. An analogous zinc phosphate, tarbuttite, is known.

Composition

The chemical formula of adamite is Zn2 (AsO4) (OH), identified its elemental component as zinc (Zn), arsenic (As), oxygen (O), and hydrogen (H). Adamite is assigned into the olivenite group included olivenite (basic copper arsenate), libethenite (basic copper phosphate), and eveite (basic manganese arsenate) all have the same general stoichiometry A2 (XO4) Zq (Richmond, 1940), where A= Zn, Cu, Mn; B = Po, As, V; Z= hydroxyl or halogen and q = variable quantifier. The most closely relate or might say could substitute for zinc in adamite is copper in olivenite Cu2 (AsO4) (OH) to form a complete solid-solution series. As Kokkoros (1938) stated when more than 80 percent copper of olivenite substitute for zinc in adamite, the adamite will become olivenite and cause symmetry of adamite from orthorhombic to monoclinic. This happened because olivenite is a bit less than adamite and has a high content of copper. Table 1 below summarize the weight percent of adamite which the cation of the molecule has two ion of Zn with consist charge of +4 and the group of anion consist of arsenite (AsO4) and hydroxyl (OH) with the charge relatively -3 and -1. Therefore, with the charge of 4+ from cation Zn and 4- from anion (AsO4) (OH), it provides the molecule with electrical balance within adamite structure.

Structure refinement

The adamite crystal structure can define by the tetrahedral arrangement of AsO with bond length varies from 1.66 and 1.70 A (Hawthorne, 1976) and the chain of edge sharing (Zn) octahedral view from [111] direction. As Kokkoros defined adamite's formula as Zn2 (AsO4) (OH), therefore, in adamite, one Zn atom coordinated with four oxygen and two hydroxyl to form an octahedral; one arsenic is with four oxygen and formed tetrahedral; while other Zn atom is also surrounded by four oxygen but only one hydroxyl so its shape is a trigonal dipyramidal. Apparently, the polyhedral distortion observed in adamite structure is the chain of edge sharing between Zn octahedral that extending parallel to c-axis, while the chains of O-O and OH-OH are linked together by arsenic tetrahedral to form a long channel. The structure of adamite with three short equatorial bond are equal and elongate, in addition, a strong elongation of the axial bond will produce the contraction effect of (O) edge that is share between the (Zn) trigonal dipyramid. Both the edge sharing between a strong anionic charge has caused the Zn cation decreased its stability in agreement with Pauling's third rule (Pauling, 1960).

Physical properties

The colors of adamite are variety when copper and other metal substitute for zinc. Pure adamite is colorless or white, and when contaminate by other element, it exhibits pale yellow, honey yellow, brownish yellow, reddish, pale green to vibrant lime green. Bright green is common in cuprian, bright pink, rose or purple is common in cobaltian. Adamite is typically transparent to translucent, and sometime may fluoresce and phosphoresce lemon yellow under SW and LW UV. Adamite usually exhibits good cleavage only along the single direction, when the c-axis is noticeable longer than other two; crystals are form as needle-like. Adamite is orthorhombic (2/m 2/m 2/m), elongated parallel to the b-axis and the form present good cleavage at {101}, and poor in {010} (Staples, 1935). This mineral has a complex orientation because of strong bonding along the octahedral chain and weak ionic bonding so it is typically elongated, may be tabular or equant but also forms in radial aggregate, fanlike rosettes or crystalline crust depend on its geologic occurrence. Adamite has relatively high specific gravity of 4.32-4.38 and it is soft as a copper penny with the hardness of 3.5.

Occurrence

Adamite usually occur closely with other oxidized minerals especially olivenite [Cu2 (AsO4) (OH)], the copper analog end member of adamite-olivenite solid solution series. Adamite only forms as a secondary mineral in the oxidized zone of zinc-and arsenic bearing hydrothermal mineral deposit. This specimen is favored in arid environments where mineralization is controlled by a system of underground rainwater river emerged from rocks’ interior. Although widely distributed, adamite only occurred at particular geologic setting where a series of underground rainwater dissolved acidic solutions into rock’s interior. This dissolving rocks neutralized by hydro-fluid that emerged from carbonated minerals, precipitating silicate and arsenate minerals (Hill, 1976). These replacement deposit rich in silver, lead, zinc, with less amounts of titanium and gold. Adamite became well known internationally and was named in honor of Gilbert-Joseph Adam, French mineralogist, who supplied the first specimen.