Pectolite |
| HNa(Ca,Mn)2(SiO3)3 |
| Monoclinic |
Forms
a(100), c(001), h(540), q(340), w(140),
v(101)
Habit
Pectolite is found as transparent colorless crystals and grains implanted
on and mixed with prehnite and as pinkish coarse-fibrous aggregates associated
with brown garnet, willemite, and biotite. The crystals are elongated parallel
to the vertical axis, unlike all those of this mineral hitherto described.
| Figure
86 Crystal of pectolite showing the forms a(100), c(001), h(540), q(340), w(140) and v(101). Franklin. |
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The cleavage is highly perfect parallel to the base and to the orthopinacoid. The orthopinacoidal cleavage is the better (Berman). Most of the crystals were coated with a thin film of powdery barite, which dulled the faces and hindered measurement.
The forms listed above were established on the following measurements made on three crystals:
|
Computed |
Measured |
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|
f |
r |
f |
r |
|||||
|
° |
´ |
° |
´ |
° |
´ |
° |
´ |
|
| a (100) |
90 |
0 |
90 |
0 |
91 |
0 |
90 |
0 |
| c (001) |
90 |
0 |
5 |
20 |
90 |
0 |
5 |
17 |
| h (540) |
48 |
25 |
90 |
0 |
48 |
24 |
90 |
0 |
| q (340) |
34 |
4 |
90 |
0 |
33 |
45 |
90 |
0 |
| w (140) |
12 |
42 |
90 |
0 |
13 |
40 |
90 |
0 |
| v (101) |
90 |
0 |
44 |
30 |
90 |
0 |
45a |
0 |
| a Approximate |
Recent research has shown (Warren and Biscoe, 281b) that pectolite is triclinic. In a study by M. A. Peacock, shortly to be published in the Zeitschrift fur Kristallographie, this conclusion is confirmed and pectolite is shown to be more closely related to wollastonite than to the monoclinic pyroxenes. Peacock adopts an orientation showing this relation, which changes the form w(140) to the side pinacoid b(010) and the form h(540) to the unit prism H(110). The changes in symbols for the forms listed above and shown in figure 86 will then be as follows:
|
Symbols of forms of pectolite in triclinic interpretation |
|
|
The letters H and Q correspond to analogous forms of wollastonite. (See appendix, d, page 130.)
Optical
properties
Under the iron-arc spark pectolite gives a pure-yellow fluorescence. It is
optically biaxial and positive; a /\
c = 19°, g
/\
b = 13° ; 2V medium; r > v (perceptible); a
= 1.604, b
= 1.610, g
= 1.636.
Composition
Crystals of pectolite were crushed and separated as completely as possible
from the associated minerals by heavy solution. The material analyzed was
not entirely pure, however, the alumina found indicating some prehnite and
the barium oxide found showing a trace of barium as present in some form.
After correction by eliminating the BaO and SrO and 3.7 percent of prehnite
the remainder gives a fairly close approximation to the theoretical ratio
of pectolite—that is, SiO2 : CaO : Na2O : H2O
= 6 : 4 : 1 : 1.
| SiO2 | 52.04 |
| Al2O3 | 0.92 |
| CaO | 31.15 |
| MgO | 0.05 |
| MnO | 2.31 |
| FeO | 1.29 |
| ZnO | 0.26 |
| BaO | 0.13 |
| SrO | 0.12 |
| Na2O | 7.97 |
| K2O | 0.90 |
| H2O | 3.07 |
| 100.21 |
Occurrence
Pectolite was first determined in fibrous form on a specimen sent by the
Foote Mineral Company for examination. The crystals were on a large specimen
of prehnite sent by Mr. W. F. Ferrier for determination, which he kindly
presented for study on being told of the puzzling problem regarding the
nature of the prehnite and the then unknown mineral mixed with it. Both
specimens undoubtedly came from the Parker shaft, but pectolite appears
to be among the rarest and least known minerals there.
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Website
© by Herb Yeates 1997-2006.
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This
page created: August 12, 2006 6:45 PM
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