Bustamite
| Bustamite | |
|---|---|
Bustamite (pink) with galena (grey) from Broken Hill, New South Wales, Australia. Specimen size 3.7 cm.
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| General | |
| Category | Inosilicate |
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Formula (repeating unit) |
CaMn2+Si2O6 |
| Strunz classification | 9.DG.05 (10 ed) 8/F.18.40 (8 ed) |
| Dana classification | 65.2.1.2 |
| Crystal system | Triclinic |
| Crystal class | Pinacoidal 1 (same H-M symbol) |
| Space group | I1 |
| Identification | |
| Color | Light pink to brownish red Pink color fades on exposure to sunlight |
| Crystal habit | Usually tabular or equant to prismatic; commonly massive, often compact and fibrous |
| Twinning | Rare. Simple twins with (110) as the composition plane |
| Cleavage | {100} perfect; {110} and {110} good; {010} poor |
| Mohs scale hardness | 5.5 to 6.5 |
| Luster | Vitreous |
| Streak | White |
| Diaphaneity | Translucent to transparent |
| Specific gravity | 3.32 to 3.43 (observed) 3.40 (calculated) |
| Optical properties | Biaxial (-), 2V=34° to 60° |
| Refractive index | nα = 1.640 - 1.695, nβ = 1.651 - 1.708, nγ = 1.653 - 1.710 |
| Birefringence | δ = 0.013 - 0.015 Dispersion r < v weak to strong |
| Pleochroism | Weak, X and Z orange, Y rose |
| Solubility | Partly soluble in HCl. |
| Other characteristics | Specimens from the Franklin Mine are fluorescent red in longwave ultraviolet light. Not radioactive |
| References | |
Bustamite is a calcium manganese inosilicate (chain silicate) and a member of the wollastonite group. Magnesium, zinc and iron are common impurities substituting for manganese. Bustamite is the high-temperature polymorph of CaMnSi2O6 and johannsenite is the low temperature polymorph. The inversion takes place at 830 °C (1,530 °F), but may be very slow.
Bustamite could be confused with light-colored rhodonite or pyroxmangite, but both these minerals are biaxial (+) whereas bustamite is biaxial (-).
There is considerable variety in the literature about the size and type of the unit cell, the formula to be used, and the value of Z, the number of formula units per unit cell.
Bustamite is a triclinic mineral, which could be described by a primitive unit cell, but the larger A-centered cell is often preferred, in order to facilitate comparison with the similar mineral wollastonite.
The formula for bustamite is CaMn(SiO3)2 but it is sometimes written (Ca,Mn)SiO3, and changing the formula in this way will change the value of Z. The structure is chains of SiO4 tetraheda with repeat unit of three tetrahedra, unlike the pyroxenes where the repeat unit is two. Ca++ and Mn++ are positioned between the chains. There are 12 tetrahedra in the A-centered unit cell.
The unit cell, the formula and Z cannot be taken separately; they are interlinked and form a consistent set of values. In this article we adopt the A-centred unit cell (space group A1) with a = 7.736 Å, b = 7.157 Å and c = 13.824 Å, the formula CaMn(SO3)2 and Z = 6. Deer et al. take the formula as (Mn,Ca,Fe)[SiO3] so their value of Z is doubled to 12. Mindat apparently gives the lattice parameters for a face-centred cell, although they give the space group as P1.
The type locality was originally taken as Tetela de Jonotla, Puebla, Mexico, and the mineral was named for General Anastasio Bustamante (1780–1853), three times President of Mexico. The material from Puebla, however, was later found to be a mixture of johannsenite and rhodonite, so the type locality is now the Franklin Mine, Franklin, Sussex County, New Jersey, US.
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