CAHNITE   

Ca₂B(AsO₄)(OH)₄
Tetragonal, I4, a = 7.0952, c = 6.1904 Å, Z = 2

	Figure 25-11. Crystal drawings of twinned cahnite from Franklin. Drawings are from Palache (1935) who provided crystallographic data.

Cahnite was first announced by Palache (1921) and described from Franklin by Palache and Bauer (1927b); it has not been found at Sterling Hill. The crystal class was determined by Palache (1941a). Embrey (1960) gave powder data and suggested the crystal structure relations. An historical summary was given by Palache (1935).

Crystal structure

The crystal structure was solved by Prewitt and Buerger (1960, 1961). They reported both arsenic and boron to be tetrahedrally coordinated and calcium to be eight-coordinated. The structure is similar to those of xenotime and zircon.

Description

Cahnite is known primarily in colorless crystals, which are pseudo-tetrahedral or pseudo-octahedral in habit (Figures 25-12, 25-15, 25-16, 25-18, and 25-19), but is most commonly found complexly twinned (Figures 25-11 and 25-14), some on {110}. Most crystals are 1-2 mm or less in size, but crystal twins up to 18 mm are known (Palache, 1935; figure 195). The forms present are {100}, {110}, {111}, {111}, and {311}. Crystals twinned on {110} commonly have a rectangular or square cross-section (Palache, 1935, fig. 194).

	Figure 25-12. Pseudo-tetrahedral crystals of cahnite on rhodonite from Franklin. Field of view is 0.8 mm in maximum dimension.

Cahnite is colorless to white or light-yellow; Frondel (1972) has reported light green material. It is transparent and has perfect cleavage on {110}, a vitreous luster, and a density of 3.156 g/cm³. Optically, it is uniaxial, positive, with w = 1.662,  e = 1.663, and very low birefringence.

Cahnite fluoresces in longwave and shortwave ultraviolet with a moderate yellowish white color. When massive, it can be confused with barite, but is easily distinguished by its unique cleavage or by using optical or X-ray methods. Additional descriptive information was given by King (1993).

Composition

Cahnite is a calcium boron arsenate hydroxide mineral. Few analyses exist, but they are in close agreement (Palache and Bauer, 1927a; Palache, 1935). The best of these is given in Table 25.

Occurrence and paragenesis

	Figure 25-13. Crystal drawing of cahnite from Franklin. Drawing is from Palache (1941a) who provided crystallographic data.

Cahnite was found in superb crystals in a number of assemblages from the Parker Mine. Palache and Bauer (1927a) reported it occurring with manganaxinite, barite, and friedelite and also with calcite, willemite, friedelite, and barite.

Palache and Bauer (1927a)  also reported a 1927 find with manganaxinite, calcite, rhodonite, barite, hedyphane and willemite, these minerals being followed in sequence of formation by cahnite and finally datolite.

Cahnite may be much more common than previously recognized; it is found sparingly with a number of rare minerals in vein assemblages.

	Figure 25-14. Twinned cahnite crystals from Franklin. These are “front” and “back” views of the same twinned crystals. Field of view is 0.8 mm in maximum dimension.		Figure 25-15. Isolated pseudo-tetrahedral crystal of cahnite on rhodonite from Franklin. Field of view is 0.6 mm in maximum dimension.

A number of unique assemblages are known. Cahnite occurs with samfowlerite on a light pink, lustrous, rough-surfaced, unstudied garnet which lines vugs in franklinite; the twinned 1 mm crystals resemble those of Palache (1935) (Figure 25-14).

	Figure 25-16. Cahnite crystals on rhodonite from Franklin. Field of view is 0.7 mm in maximum dimension.		Figure 25-17. Typical clustering of cahnite crystals from Franklin, exhibiting some parallel growth. Field of view is 0.5 mm in maximum dimension.

Cahnite of pseudo-octahedral habit occurs in vugs in brown andradite, associated with hetaerolite, hausmannite, groutite, and kentrolite. Cahnite was reported by Cook (1973) from the Buckwheat Dump, associated with andradite, flinkite, and “carminite” (later found to be jarosewichite) (Figure 25-26). Cahnite is also associated with a druse of hodgkinsonite on franklinite-willemite  ore, and Frondel (1972) has reported it in cleavable 25 mm masses with roeblingite.

	Figure 25-18. Randomly scattered cahnite crystals on rhodonite from Franklin. Field of view is 0.4 mm in maximum dimension.

It is also known from vugs in light green andradite, associated with mica and copper.    

Name

Cahnite was named in honor of Lazard Cahn, a tireless investigator of the rare minerals at Franklin and a dedicated collector and mineralogist. Roe (1972) described Cahn’s unique tombstone, carved in the shape of a giant cahnite crystal (Figure 25-19).

	Figure 25-19. The largest known crystal-model of a Franklin mineral. This replica of a cahnite crystal is the tombstone of Lazard Cahn (1865-1940) in the Evergreen Cemetery in Colorado Springs, Colorado. It was discussed by Dr. Arthur Roe in The Mineralogical Record, 3, page 99 (1972).

 

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