Axinite |
| Varieties manganoaxinite and ferroaxinite |
| HAl2Ca2(Mn,Fe)Si4O16 |
| Triclinic |
Forms
In the list below, the letter and symbol preceding the equal sign are those
used by Dana, with crystals in the "Dana position", and the letter
and symbol following the equal sign are those used by Goldschmidt, with crystals
in the "Winkeltabellen position." [Web Ed. note:
The form U’ was presented as an uppercase Germanic ‘U’
in the original.]
[Forms on crystals of axinite; combinations on crystals of axinite]
Crystallography
Aminoff (215) has discussed the crystallography of the Franklin manganoaxinite in detail.
He computed the elements from the measurement of 11 crystals and found them almost the
same as those of the ferroaxinite from France. Hence the substitution of iron for
manganese in this mineral produces but slight morphotropic effect. The axial ratios of the
varieties are, for the ferroaxinite from Dauphiny (Goldschmidt, Winkeltabellen), a
: b : c = 0.7812 : 1 : 0.9771. a = 91° 49', b = 82° 1', g = 102° 38' ; and for the manganoaxinite from Franklin (Aminoff), a
: b : c = 0.7797 : 1 : 0.9764; a = 91° 55', b = 81° 51', g = 102° 53'.
Aminoff describes the new form U ’(534), which had been found and figured by the author also and is thus well confirmed. The forms listed by Aminoff are included in the above list.
Optical
properties
Axinite is optically biaxial and negative; 2V = 74° (measured), r <
v (strong); the crystals from Franklin are generally plates parallel to the
pyramid x(111) and the acute bisectrix emerges almost normal to that
face. When a crystal is oriented in the "Dana position" the directions
of the axes of elasticity, expressed in gnomonic coordinates, are for X,
f = 53°, r
=55° ; for Y, f =
156° r = 72° ;
for Z, f =
-93°, r = 42°,
all in sodium light. The refractive indices for sodium light are: a
= 1.684, b = 1.692,
g = 1.696, all
±0.001. The pleochroism is X = yellow, Y = yellow, Z
= colorless (Berman). Under the iron-arc spark most axinite from Franklin
shows no fluorescence, but a few specimens fluoresce with a vivid pinkish-red
tint.
Composition
Axinite is of rather complex composition, but it may be briefly characterized
as acid borosilicate of calcium, aluminum, and manganese with some iron and
zinc.
As the two analyses differ by less than 1 percent in the amount of each constituent, the Franklin axinite is apparently of rather uniform composition. It contains more manganese than any other axinite yet studied and differs but little in composition from the pure type of manganoaxinite as defined by Schaller.
Occurrence
Axinite has been found at Franklin at many localities. It was first discovered
in the Trotter mine, where it was found in the shaft in great abundance. It
formed lamellar masses and tiny brilliant crystals mixed chiefly with rose-colored
rhodonite, polyadelphite, biotite, and barite. The crystals are generally
pale yellow or yellowish green, but some are pale rose-colored or even quite
colorless. The yellow color is peculiar to the axinite of the locality. The
dominant habit of the crystals is tabular parallel to the pyramid x(111),
as shown in figures 147 and 148; the rare forms V(112) and X(021)
are seen, as well as the new form mentioned above, U ’(534).
| Figure
147 Crystal of axinite, variety manganoaxinite, showing the forms b(010), m(110), M(110), w(130), s(201), f(011), y(021), W(312), H ’(534), x(111), e(111), Y(131), r(111), and p(221). Trotter mine. |
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Figure
148 Crystal of axinite, variety manganoaxinite, showing the forms a(100), b(010), m(110), M(110), s(201), f(011), y(021), X(021), x(111), and r(111). Parker shaft. |
Penfield first described and figured the crystals in a paper by Genth (145), giving analyses of crystals and of massive material. Genth's material was reanalyzed by Ford (187), who showed that the original water determinations were slightly too low.
Yellow axinite was one of the more abundant minerals of the pneumatolytic deposit revealed in sinking the Parker shaft. It was generally in massive granular or lamellar form, and masses of many pounds in weight were not rare. Intimately mixed with it were polyadelphite, manganophyllite, and hancockite; barite and rhodonite were less abundant. A few crevices in the massive material are lined with drusy coatings of brilliant tabular crystals, some of which are attached by such slender bases as to be developed practically on all sides, as shown in figure 148. The axinite crystals show the same variations from the dominant honey-yellow color as those from the Trotter mine. Although not analyzed, they are doubtless of similar composition.
The development of the north end of the mine has revealed abundant axinite not different in general character from that just described but developed in larger and more showy groups of crystals. Some of the crystals are an inch across and are quite transparent, although badly striated and not measurable.
Ferroaxinite
Specimens obtained during the sinking of the Palmer
shaft showed a cavity in gneiss lined mainly with crystals of apophyllite.
Besides that mineral and pyrite, there is on one specimen a clove-brown axinite
in minute crystals, too poorly developed for measurement but apparently of
the ordinary habit of the mineral.
In the Hancock collection there is a single specimen, from the Gooseberry iron mine, containing axinite. It consists chiefly of massive crystalline epidote embedded in calcite. When the calcite was removed with acid the cavities revealed rough crystals of epidote, fibers of actinolite, and minute crystals of arsenopyrite, and also a few brilliant clove-brown crystals of axinite of the form illustrated in figure 149.
| Figure
149 Crystal or ferroaxinite showing the forms b(010), m(110), M(110), g(120), w(130), s(201), y(021), x(111), e(111), o(132), Y(131), z(112), and r(111). Gooseberry iron mine. A, Plan; B, clinographic projection. |
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They are entirely unlike the other crystals described and resemble those of the ferroaxinite of better-known localities, being prismatic and striated parallel to the vertical axis.
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© by Herb Yeates 1997-2006.
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