Ganophyllite

Physical character
Terminated crystals of ganophyllite are not known at Franklin, but the mineral is found in rosettes of imperfect crystals or needles and in irregular grains embedded in bustamite. It is noteworthy for its extremely perfect micaceous cleavage, which makes it easily mistaken for mica in some specimens. In others the crystals are fibrous parallel to the orthoaxis, and in them the cleavage runs lengthwise and the resemblance to mica is lacking. Its color is light brown, its hardness is 4 to 4.5, and the specific gravity of the Harstigen mineral is 2.84.

Optical character

Ganophyllite is optically biaxial and negative; 2V = 26 ; r < v (easily perceptible); Z is normal to the cleavage; a = 1.573, b = 1.603, g = 1.604 (Larsen, 230). These optical data are different from those determined by Hamberg on the mineral from Harstigen, but determinations by Larsen on material from there agreed well with those made on material from Franklin.

Composition
Ganophyllite is an aluminum-manganese silicate, classed as a zeolite by its discoverer, Hamberg. The following analysis was made on only 73.5 milligrams of material and is slightly incomplete because of the small amount available.

**Analysis of ganophyllite**
	1	2
SiO₂	39.18	40.48
Al₂O₃	8.57	8.59
Fe₂O₃	Trace
MnO	36.33	41.83
ZnO	3.67
CaO	2.04
H₂O +	5.70	9.10*
H₂O -	1.65
	97.12	100.00

[* Figure represents combined water.]

1. Ganophyllite, Franklin. E. V. Shannon (230), analyst.

2. Composition computed from the formula given above.

Comparison of the incomplete analysis with the composition computed from the accepted formula serves definitely to identify the mineral as ganophyllite, and the analysis also shows that aluminum is an essential constituent of the mineral. The material from Franklin contains small amounts of zinc and calcium instead of the lead and alkali metals found in the mineral from Harstigen. As the molecular ratio, in the analysis, of MnO to ZnO plus CaO is very nearly 6 to 1, possibly (ZnO, CaO) is also one of the essential constituents.

Occurrence
Ganophyllite was first identified at Franklin by Palache (195) in 1909, a few needles being found on a single specimen of fibrous calamine sent for identification by the Foote Mineral Company. It was found again in 1921 in a few specimens that were described by Larsen and Shannon (222 and 230). The radiating needles of ganophyllite form rosettes, some of which are two-thirds of an inch in diameter, implanted on rhodonite in the open center of a thin vein, together with bustamite, axinite, barite, and willemite.

A rather different phase of the same association is shown by a specimen in the Harvard collection, in which a vein half an inch thick cuts the ordinary layered ore. Bustamite lines one wall of the vein, and upon it is implanted the ganophyllite, which shows on the fracture surface of the vein as slender brown cleavage surfaces. Axinite, in the customary yellow crystals, is later than both the other minerals.

Ganophyllite may be counted as one of the rarer minerals at Franklin, as it is at its only other known locality, the Harstigen mine, Pajsberg, Sweden.


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	This page created: January 12, 2001 6:19 PM