HOLDENITE Chapter15. Nesosilicates

HOLDENITE

Mn₆Zn₃(OH)₈(AsO₄)₂(SiO₄)
Orthorhombic, Abma, a = 11.99, b = 31.46, c = 8.697 Å, Z = 8.




	Figure 15-53. Crystal drawing of holdenite from Franklin. Drawing is from Palache (1935) who provided crystallographic data.

Holdenite was first described from Franklin by Palache (1921) and Palache and Shannon (1927) as a manganese zinc arsenate hydrate. Their analysis was of admittedly impure material, and they dismissed the low silica component of the analysis as willemite impurities. The crystallography of holdenite was later investigated by Prewitt-Hopkins (1949), but her space group was subsequently corrected by Moore and Araki (1977b). The discovery of holdenite at Sterling Hill in 1977 and again in subsequent years provided additional material for analytical studies by Dunn (1981e).

Crystal structure

Moore and Araki (1977b) described the crystal structure and showed that Si was essential to holdenite. They described the structure as having open tetrahedral sheets of composition [Zn₂SiO₆(OH)₂] which are further linked to AsO₄ tetrahedra to form slabs of composition [Zn₂As₂SiO₁₂(OH)₂]; insular ZnO₄ tetrahedra also occur. These slabs link to Mn(O,OH) condensed octahedra to form a framework structure based on cubic close-packing of oxygens.

Description




	Figure 15-54. Crystals of holdenite from Sterling Hill. Field of view is 0.2 mm in maximum dimension.

Holdenite crystals are up to 2-3 mm in size. Those from Franklin were depicted by Palache and Shannon (1927) and Palache (1935) (Figure 15-53).

The morphology of holdenite is similar to that of kolicite. Sterling Hill crystals are commonly distorted, as shown by Dunn (1981e) (Figure 15-54 and 15-55).

Holdenite is commonly light pink; crystals forming druses on other species may have false colors transmitted through the transparent holdenite. Holdenite has a vitreous luster. Palache and Shannon (1927) reported a poor cleavage {010}. The density is 4.11 g/cm³ (meas.) (Prewitt-Hopkins, 1949), 4.11 g/cm³ (calc.) (Moore and Araki, 1977).

Optically, holdenite has not been restudied. The known data given below were obtained by Esper Larsen and given by Palache and Shannon (1927). Holdenite is biaxial, positive, 2V = 30^o, with indices of refraction: a = 1.769, b = 1.770, g = 1.785. Dispersion is moderate, r > v. There is no discernible fluorescence in ultraviolet. Holdenite is brittle, and broken crystals resemble leucophoenicite, but the higher density is diagnostic. Holdenite also resembles some hodgkinsonite; it is easily distinguished by hodgkinsonite’s lower indices of refraction.




	Figure 15-55. Crude crystals of holdenite from Sterling Hill; this is the other side of the cluster shown in figure 15-54. Field of view is 0.2 mm in maximum dimension.

Because holdenite, a pink glassy mineral, is easily confused with leucophoenicite at Franklin or hodgkinsonite at either locality, specimens of holdenite may possibly be mislabeled in systematic collections, as first suggested by Albanese (1955), but the writer’s search for such mislabeled material has been fruitless.

Composition

Holdenite is a manganese zinc arsenate silicate hydroxide mineral. Because the original analysis was performed on a mixture and the crystal structure determination (Moore and Araki, 1977b) indicated that Si was essential to the species, holdenite was reinvestigated analytically by Dunn (1981e).




	Table 4. Chemical analyses of holdenite and kolicite.

Several analyses of holdenites from Franklin and Sterling Hill are provided in Table 4. They conform closely to the ideal composition, with approximately 2 weight percent MgO proxying for ZnO; no holdenite has been found to be Mg-free.

Occurrence and paragenesis

Holdenite was originally found at Franklin. It is a mineral of the secondary veins, and it remained a very rare species until the discovery of Sterling Hill material by Fred Parker in 1977. The Sterling Hill occurrence, which provided hundreds of specimens, was between the 1200 and 1300 levels in the central zincite zone. Like Franklin material, holdenite from Sterling Hill also accompanies slickensides and is also found with secondary willemite as banded vein-fillings. In some specimens, holdenite occurs within the ore adjacent to veins and locally may compose up to 10% by volume. Within such assemblages, kraisslite is present as thin plates. Sterling Hill holdenite is invariably pink. Holdenite in association with kolicite was noted by Dunn et al. (1979b).

Name

Holdenite was named in honor of Albert F. Holden, a mining engineer and collector, in whose collection the species was found.

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