Holdenite

MINERALS INDEX

Holdenite

8MnO.4ZnO.As₂O₅.5H₂O

Orthorhombic

a : b : c = 0.3802 : 1 : 0.2755

Forms
c(001), b(010), a(100), m(110), l(120), n(130), e(011), f(031), d(102), p(111, q(211), r(311), s(131), w(151), t(251), x(182), and u(7.16.2)

Habit
Holdenite is found in crystals tabular parallel to the face taken as the macropinacoid, the largest crystal on the specimen being a third of an inch in greatest diameter. The crystals differ little in habit and about two-thirds of the forms are found on all of them. The base was seen but once, and n(130) and e(011) were each found well developed on but two crystals. The forms f(031), w(151), and x(182) were also found on only one crystal, the most complex measured, shown in figure 193. The pyramid u(7.16.2), seen on all the crystals, with relatively large faces has a considerable range in its angular position. As shown in the figure, it is in a zone with t(251) and s(131), and the angles measured on this one crystal agree well with the computed values. On other crystals, however, the angles are different and on some have values that correspond approximately to the simpler indices (491), but the deviation is considerable, and preference was given to the more complex symbol.

Physical properties
Holdenite has a poor cleavage parallel to the brachypinacoid. Its hardness is 4, and its specific gravity, determined by floating in Clerici solution, is 4.07. The color ranges from clear pink to deep red and yellowish red. The mineral is biaxial and positive; the plane of the optic axes is parallel to the brachypinacoid, with the acute bisectrix emerging normal to the macropinacoid. 2V = 30° 20' (measured), 28° 58' (computed); r > v (easily perceptible). a = 1.769 (parallel to c axis), b = 1.770 (parallel to b axis), g = 1.785 (parallel to a axis) (Larsen).

Composition
Holdenite is a basic arsenate of manganese and zinc in which the molecular ratio of manganese to zinc is approximately 2 to 1.

**Analysis of holdenite**
	1	2	3		4
SiO₂	2.01	0.033
As₂O₅	17.40	0.076	0.076	1 x 0.076	18.96
MnO	37.75	0.532	0.532*		46.78
FeO	1.80	0.025	0.025*
ZnO	28.08	0.345	0.279*	0.914 = 12 x 0.076	26.83
CaO	3.80	0.067	0.042*
MgO	1.45	0.036	0.036*
H₂O	6.62	0.367	0.367	5 x 0.073	7.43
PbO	Trace
Mn₂O₃	Trace
Al₂O₃	Trace
	98.91				100.00

[*These figures are reflected in the 0.914 value shown.]

1. Slightly impure material. E. V. Shannon (248), analyst.

2. Molecular equivalents of no. 1.

3. Molecular ratio of remainder after deducting 2.49 percent of calcite and 7.38 percent of willemite.

4. Composition computed from the derived formula.

About 0.42 gram of nearly pure material was prepared by Mr. Berman for analysis. The presence of calcite was proved optically and by the effervescence of grains on solution in acid, but the sample was not large enough to permit the determination of CO₂. The assumptions were made that the deficiency of the analysis, 1.09 percent, represents CO₂, that the SiO₂ was present in willemite, and that the material therefore contained 2.49 percent of calcite and 7.38 percent of willemite. After deducting the molecular equivalents of those constituents from column 2 the molecular ratio of the remainder leads to the formula 12RO.As₂O₅.5H₂O. As manganese and zinc oxides are present in the molecular ratio of about 2 to 1, the formula may be written as at the head of this article. The only other mineral at all resembling holdenite in composition is chlorophoenicite, also from Franklin, to which was assigned the formula 10RO.As₂O₅.7H₂O.

History
Holdenite was first described in 1927 by Palache and Shannon (248). It was named in honor of the late A. F. Holden, of Cleveland, in whose collection, now at Harvard University, the only known specimen of the mineral was discovered in 1913, mislabeled leucophoenicite. It is a slab of massive franklinite ore with a slickensided surface 4 by 3 inches, clearly one wall of a veinlet. The crystals of holdenite were attached for the most part directly to the vein wall or to a thin coating of manganiferous calcite.

Figure 193
Crystal of holdenite showing all the forms known for the mineral except c(001), l(120), and n(130). Franklin.

With them were minute amounts of barite, galena, pyrochroite, and fibrous willemite. Crystals were measured by the author in 1914, and practical certainty was reached that they were a new arsenate of manganese. After years of search for more of the mineral had proved fruitless, a part of the only specimen was sacrificed for analysis, the results of which confirmed the previous conclusion that the mineral is a hitherto unknown arsenate.


	Website © by Herb Yeates 1997-2001.
	This page created: January 12, 2001 6:25 PM