Hyalophane

MINERALS INDEX

Hyalophane

(K₂Ba)Al₂Si₄O₁₂

Monoclinic

Dark-red coarse-granular hyalophane, associated with black manganese-biotite and yellow garnet, was found in the old dump of the Parker shaft. In thin section it proved to be monoclinic and negative, with a small extinction angle; 2V large; and the refractive index b about 1.54. The specific gravity is 2.90 and the hardness is 6.

The material is very impure, the color being due to grains of hancockite and to a network of microscopic veinlets of an alteration product, apparently bementite. Analysis 1 below was made of this mixed material and in its interpretation the assumption was made that the sulphur was contained in barite, the lead in hancockite, and the manganese in bementite. After deducting 2.89 percent of barite, 5.48 percent of hancockite, 15.71 percent of bementite, and 1 percent of excess water, the remaining 75 percent, constituting the feldspar, was recomputed to 100 percent, as shown in column 2, and the molecular ratios computed therefrom are given in column 3.

The result approximates the composition of a sodium-rich hyalophane but is deficient in silica. The optical data are thus supported by the analysis. An attempt was made to purify some of the material, finely ground, in a heavy solution, but it was futile, as even the finest particles of the powder were penetrated by the network of alteration products. In view of the abundance of bementite the computed amount of impurity, 25 percent, does not seem too large.

The material containing the hyalophane was discovered by the chemists of the New Jersey Zinc Company, who also found other impure feldspars containing barium, the composition of which is given in columns 4 and 5 of the table. The chemists also supplied material for the optical tests made by Berman.

**Analyses of barium feldspars**
	1	2	3		4	5
SiO₂	45.40	49.51	0.824	0.824	62.33	52.34
Al₂O₃	20.82	25.48	0.250	0.250	20.94	19.35
Fe₂O₃					1.20
FeO	1.54					0.77
MnO	2.67				0.97	0.91
MgO	0.76				1.25	0.54
CaO	2.70				0.46	6.24
Na₂O	2.69	3.56	0.057	0.237*	9.06	1.02
K₂O	7.54	9.98	0.106		2.80	10.43
ZnO	1.36					1.00
PbO	1.15
SO₃	0.99				0.37	1.02
H₂O	2.35					0.16
BaO	10.58	11.47	0.074		1.34	6.05
	100.55	100.00			100.72	99.83

[* Figure reflects the 0.057 + 0.106 + 0.074 values shown.]

1. Hyalophane, dump of Parker shaft, Franklin. Bauer and Jenkins (245), analysts.

2. Same analysis recomputed to 100 percent after deducting 25.08 percent of impurities.

3. Molecular ratio computed from 2.

4. Anorthoclase, same locality. Bauer and Jenkins (243), analysts.

5. Grayish-white feldspar, same locality. Bauer and Jenkins (243), analysts.

The material of no. 4 is reported by Berman to be biaxial and negative; 2V is about 80°; a = 1.522, b = 1.525, g = 1.529. It shows polysynthetic twinning with a small extinction angle and appears to be anorthoclase with a small amount of barium.

The material of no. 5 is reported by Berman to be biaxial and negative; 2V is large; a = 1.525, b = 1.528, g = 1.530. This seems to be nearer hyalophane, but the barium content and the indices of refraction are too low for that species.

It is interesting to note that the discovery of hyalophane adds another mineral to the list of those common to the Franklin district and to the manganese mines of Langban and Jakobsberg, Sweden. The Swedish hyalophane is also a granular red feldspar and is associated with manganese-epidote.


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