Properties

Definition

The ideal endmember composition of rossmanite is ([])(LiAl2)Al6(BO3)3Si6O18(OH)4, where [] is the cation vacancy in the X-site. The defining aspects of this phase are the dominance of vacancies in the X-site, and the dominance of Li and Al in the Y-site in the approximate proportions 2 Al to 1 Li.

Color

Rossmanite is not associated with any particular color of tourmaline. It has been found as colorless, pale pink, pink, purple-pink, and green crystals or portions of crystals. It has not been found as black or brown crystals.

The optical spectroscopy of rossmanite is essentially the same as elbaites of similar iron (green) and manganese (pink) contents.

Composition

To date, several rossmanites have been found with less Li and more Al than the ideal formula. These often contain an olenite [(Na)Al3Al6(BO3)3Si6O18(O)3(OH)] component.

The presence of OH has been confirmed with near-infrared spectroscopy. There has not been adequate study to determine if the OH pattern could be diagnostically useful for the identification of this species. Work in progress is attempting to address this issue. Many rossmanites contain a significant substitution of OH by F. A few of these have been reported to contain F dominant in the W-site and would, thus, be classified as the hypothetical end-member, fluor-rossmanite, according to the tourmaline classification scheme as proposed by Hawthorne and Henry (1999).

Crystal morphology

Rossmanite crystals resemble typical elbaite and liddicoatite crystals from their respective localities. No distinguishing features have been identified that will allow crystal morphology to be used to identify this species.

Indentification

The best way to identify rossmanite is through a complete chemical analysis. This may involve a combination of electron microprobe and secondary-ion mass spectroscopy (ion microprobe) analysis. A complete crystal structure refinement is a valuable adjunt to these identifications. These extensive and costly analyses are impractical for routine screening of tourmalines.

Density

Czech Republic:     measured: 3.00 g/cm3    calculated: 3.06 g/cm3

Indices of Refraction

Czech Republic:   uniaxial negative, ω = 1.645(1), ε = 1.624

Space Group

Trigonal, R3m

Unit Cell Parameters

Z = 3

Austria                 a = 15.802,         c = 7.088
Czech Republic    a = 15.770(2),     c = 7.085(1)

Hardness

Czech Republic:     7 


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