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MINERALS INDEX
Hancockite |
| (Pb,Ca)2(AlOH)(Al,Fe,Mn)2(SiO4)3 |
| Monoclinic |
Forms
c(001), a(100), e(101), r(101), and n(111).
Habit
Hancockite is found in very small lath-shaped crystals, in drusy cellular aggregates, and
in compact masses. The crystals are striated parallel to their length and have rounded
faces that permit of only very poor measurement on the goniometer. The angles measured
approximate those of epidote.
Optical properties
The color of the massive mineral is dull brick-red, brownish-red, or maroon. That of the
crystals when separately examined is yellowish-brown with strong
pleochroism—yellowish-brown for vibrations parallel to the axis of symmetry, and
somewhat various for vibrations normal to that direction, being delicate rose-red at the
attached end (and in some crystals throughout) and grading to pale greenish-yellow at the
free end.
The axial plane is parallel to the clinopinacoid; 2V = 50° , about. The mineral is optically negative; r > v (perceptible); the absorption Z > X. a = 1.788 ±0.003, b = 1.81 ±0.01, g = 1.830 ±0.003 (Larsen). The luster is vitreous, the hardness 6.5 to 7, and the specific gravity 4.03.
Composition
Hancockite is a member of the epidote group, very distinct in containing lead and
strontium in place of part of the calcium of epidote. The presence of manganese allies it
to piedmontite.
Percent |
Molecular ratio |
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| SiO2 | 30.99 |
0.516 | = 6 x 0.084 |
| Al2O3 | 17.89 |
0.176* | |
| Fe2O3 | 12.33 |
0.077* | 0.262 = 3 x 0.087 |
| Mn2O3 | 1.38 |
0.009* | |
| PbO | 18.53 |
0.083† | |
| MnO | 2.12 |
0.030† | |
| MgO | 0.52 |
0.013† | 0.368 = 4 x 0.092 |
| CaO | 11.50 |
0.205† | |
| SrO | 3.89 |
0.037† | |
| H2O | 1.62 |
0.090 | =1 x 0.090 |
100.77 |
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| [* Figures reflected in the value 0.262 shown.] |
| [† Figures reflected in the value 0.368 shown.] |
The molecular ratio of SiO2 : R2O3 : RO : H2O is not far from 6 : 3 : 4 : 1, from which may be derived the formula adopted.
Occurrence
Hancockite was recognized as a new mineral and described by Penfield and Warren (179),
from whose paper the facts stated above are taken with slight additions. It was found in
considerable abundance in the Parker shaft in association with other rare and peculiar
silicates. As the dump of that mine was removed the mineral was found throughout its
extent in characteristic brick-red masses interspersed with granular garnet and scales of
biotite. In drusy parts of such masses its tiny crystals line the cavities and are
abundantly included in clear crystals of axinite, datolite, and willemite, to all of which
it lends its reddish-brown color.
Since Penfield's description of hancockite was published it has been found in but one other association in a specimen of barium feldspar from the same locality (see page 59), which is colored red by microscopic inclusions of hancockite.
The mineral was named for the late Mr. E. P. Hancock, formerly of Burlington, N.J., an enthusiastic and lifelong collector of Franklin minerals.
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Website
© by Herb Yeates 1997-2001.
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This
page created: January 12, 2001 6:22 PM
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