MINERALS INDEX
Bustamite |
| CaMn(SiO3)2 |
| Triclinic |
Habit
Definite crystals of bustamite have not been seen, but it is found in parallel
growth with crystals of fowlerite and presenting the common forms of that
variety, also in prismatic groups and in masses of coarse fibers several inches
long. Its color is white to pale pink. Hey (269) observed a vivid red fluorescence
in bustamite from Franklin, but no fluorescence has been seen on any of the
specimens in the mineral collections at Harvard University.
Physical characters
Cleavages in four directions in one zone are parallel, respectively, to the
clinopinacoid (good, with strong pearly luster); to the unit prisms (slightly
poorer but with pearly luster); and to the orthopinacoid (with vitreous luster).
The cleavage parallel to the base, characteristic of fowlerite, is not developed
in bustamite. The two pinacoidal cleavages had not been recorded for rhodonite
until the description by Larsen and Shannon (220) appeared. They were identified
long ago by the author on a fibrous form of rhodonite known to Franklin collectors
as "manganese wollastonite" and were also measured by him on crystals
of fowlerite but with very weak development. This fibrous rhodonite now appears,
by analysis and by study of its optical constants, to be bustamite. The "keatingine"
of Shepard (112) likewise is bustamite.
The cleavage angles of bustamite were measured by Larsen and Shannon, as shown in the table given on page 69 [below].
Larsen and Shannon point out a close relation between the cleavage of bustamite and that of the monoclinic wollastonite.
The specific gravity of a bustamite determined by Hey is 3.302.
|
Bustamite, measured |
Rhodonite, computed |
|||
|
f |
r |
f |
r |
|
|
° ' |
° ' |
° ' |
||
| b(010) | 0°0'1°17' ; average of 7, 0°21' | 90 0 |
0 0 |
90 0 |
| m(110) | 44°45'-45°6' ; average of 7, 44° 58' | 90 0 |
45 53 |
90 0 |
| a(100) | 93°52'-95°12'; average of 8, 94°33' | 90 0 |
94 26 |
90 0 |
| M(110) | 139°14'140°21' ; average of 8, 139°53' | 90 0 |
138 21.5 |
90 0 |
Optical properties
The optical properties of bustamite have been studied by Larsen (220), by
Hey (269), and by Sundius (281). Hey also gives comparative values for rhodonite
from the Swedish localities. Bustamite is optically biaxial and negative.
Hey regards this property as the most convenient and natural one by which
to separate bustamite and rhodonite, as that mineral is optically positive.
The Franklin bustamite shows great dispersion of the bisectrices. The refractive
indices are:
| a = 1.662, b = 1.674, g = 1.676, all ±0.003 (Larsen). |
| a = 1.664, b = 1.675, g = 1.679, all ±0.003 (Hey). |
As determined by Larsen, 2V = 44° ±3° ; dispersion slight, r < v; crossed dispersion considerable.
The orientation of the axial ellipsoid was also determined by Hey and was stated in terms of the position angles on a projection on the clinopinacoid. In the following table some of Hey's data are given, transferred to the normal projection position as used by Dana.
|
Bustamite (Franklin) |
Rhodonite (Harstigen) |
|||
|
f |
r |
f |
r |
|
|
° |
° |
° |
° |
|
| X |
275 |
81 |
277 |
80 |
| Y |
12 |
70 |
14 |
62 |
| Z |
170 |
20 |
161 |
12 |
Composition
The following analyses show the composition of bustamite:
|
1 |
2 |
3 |
|
| SiO2 |
48.44 |
46.72 |
47.68 |
| Al2O3 |
1.34 |
0.25 |
|
| Fe2O3 |
0.06 |
||
| FeO |
0.27 |
0.46 |
|
| CaO |
25.20 |
22.24 |
24.86 |
| MnO |
25.20 |
26.51 |
27.65 |
| MgO |
0.65 |
1.27 |
0.03 |
| ZnO |
0.53 |
1.34 |
0.26 |
| Ignition |
0.34 |
0.06 |
|
|
100.63 |
99.88 |
100.85 |
| 1. E. V. Shannon (220), analyst. |
| 2. L. H. Bauer (243), analyst. |
| 3. M. H. Hey (269), analyst. |
The ratio derived from analysis 1 shows that molecularly CaSiO3 : MnSiO3 is approximately 4 : 3. On chemical grounds the mineral might therefore be regarded as either calcium rhodonite or as manganese wollastonite, but the triclinic crystallization shown by the cleavage inclines one to regard it as related to rhodonite.
The recent work by Sundius (281) shows that bustamite is a distinct species of rather definite chemical composition (as above given) with only a comparatively small range therein. Winchell likewise has pointed out this fact. Between rhodonite and bustamite an apparently well-established break in chemical continuity occurs—that is, rhodonite may have as high a ratio as Ca : Mn = 3 : 7. Bustamite may likewise approach the ratio Ca : Mn = 4 : 6. However, as Sundius has proved, no optical continuity can be shown. Nevertheless, the optical elements in both species (rhodonite, bustamite) lie in closely similar positions with respect to the crystallographic elements.
Occurrence
Most of what is certainly known of the occurrence of bustamite is the information
contained in the papers by Larsen and Shannon (220, 221), and Hey (269).
Their material was of two sorts. The first, which yielded the greater part
of the data given, was in prismatic grains embedded in albite with brown
garnet, biotite, and cyprine. It came from Franklin but nothing is known,
of the detailed locality. The second lot described consisted of zones of
bustamite coating crystals of fowlerite that lined seams in massive ore,
together with manganoaxinite, barite, willemite, and ganophyllite. Similar
specimens in the Harvard collections show the same association but with
bustamite so situated on the vein wall as to appear to have been the first
mineral to form after fowlerite. These specimens also came from Franklin.
It has, however, been shown by optical tests that the fibrous light-pink form of rhodonite, long known at Franklin as manganese wollastonite, is bustamite. The former abundance of this form of the mineral, first found during the sinking of the Parker shaft, suggests that bustamite may be much more widely developed there than has been suspected. In the lack of chemical analyses of material carefully studied optically, this cannot be stated positively, and more work is needed to decide the matter.
These specimens commonly contain hardystonite, and on transverse fractures there is in many specimens a whitish coating yielding a yellow fluorescence that resembles that of pectolite. The presence of pectolite could not, however, be proved optically.
|
|
||
|
Website
© by Herb Yeates 1997-2006.
|
||
|
This
page created: August 12, 2006 5:49 PM
|
||