Serpentine

MINERALS INDEX

Serpentine

H₄(Mg,Mn,Zn)₃Si₂O₉

Monoclinic

Physical character
Serpentine is found in massive, compact, cryptocrystalline, and fibrous forms, also as pseudomorphs. It is not uncommon at Franklin but does not seem to. have been found at Sterling Hill. It was early observed by Brush (95) as veinlets of chrysotile at the Hamburg mine. Similar fibrous chrysotile, consisting of radiate groups of silky fibers embedded in calcite and known locally as "wavellite", has been described by Foshag (241). The fibers are light-brown, and some of them are a quarter of an inch long. They are associated with massive brown serpentine and grains of franklinite. The fibrous serpentine is biaxial and negative; 2V medium; elongation positive; X and the plane of the optic axes are parallel to the length of the fibers; extinction parallel; a = 1.546, b = 1.550, g = 1.557. The optical characters and the composition given by the analysis show the material to be normal serpentine.

A much more common form of serpentine is translucent, hornlike, and brownish and is associated with a gray carbonate and with pale-green radiate willemite in secondary veins cutting the ore. Such veins were especially abundant in the Buckwheat mine, and some of the serpentine there was cut as an ornamental stone under the name "smithsonite." Possibly, however, much of the so-called "smithsonite" was really a form of bementite, which is found in the same association and can be distinguished from serpentine only by its greater hardness and higher indices of refraction.

The brown serpentine has been shown by analyses to be manganiferous. It is described by Larsen as optically a metacolloid and negative; 2V medium; a = 1.561, b = 1.567, g = 1.568.

The name "vorhauserite" has been used at Franklin for the manganiferous serpentine, and the usage is perhaps justified by the fact that, among the analyses of serpentine listed in Dana's "System of mineralogy" that of vorhauserite is the only one that shows manganese oxide. The variety is not well defined, however, and it seems unnecessary to revive the term. Some of the more highly manganiferous serpentine seems to have the character of the material that had been called "neotocite", and it is so described on pages 118-119.

Serpentine is also the product of the alteration of rhodonite and possibly of other manganesian minerals. Some of it is in grayish or yellowish pseudomorphs after rhodonite, in places very hard through the infiltration of silica. It is more commonly found, however, as translucent, brownish to reddish-brown cryptocrystalline masses intermingled with grains of franklinite or bits of feldspar, biotite, or garnet. Such material, locally often called "hydrorhodonite", was not uncommon on the dumps of the Trotter and Park shafts. It was not analyzed or studied in detail.

Composition
Serpentine is a hydrous magnesium silicate of rather diverse composition, as it may contain more or less of other elements in place of part of the magnesium. At Franklin, especially, it generally contains manganese and zinc.

**Analyses of serpentine**
	1	2	3	4	5	6	7
SiO₂	41.70	43.65	40.41	42.58	41.32	41.47	39.62
MgO	29.54	30.92	41.30	43.48	32.58	38.43	33.60
MnO	7.44	7.12	1.70		7.57	0.71	3.42
FeO			0.51		1.57	0.90	2.39
ZnO	4.10	3.62			0.14		4.14
CaO					0.96	0.83	0.37
Fe₂O₃	2.80
Al₂O₃					0.65	0.10	1.30
H₂O +	14.04	14.69	14.30	13.94	12.44	14.51	14.06
H₂O -					0.94	2.31	1.18
CO₂			1.41
Alkalis						0.42
	99.62	100.00	99.63	100.00	98.17	99.65	100.08

1. Serpentine, Franklin. G. A. Koenig (132), analyst.

2. Same, after deducting 4.08 percent of franklinite, equivalent to the Fe₂O₃ found, and recomputing to 100 percent.

3. Serpentine, Buckwheat mine. H. E. Merwin, analyst (unpublished).

4. Same, after deducting 3.63 percent of carbonates, equivalent to the CO₂ found, and recomputing to 100 percent.

5. Manganiferous serpentine, Franklin. E. V. Shannon (240), analyst.

6. Chrysotile, Franklin. W. F. Foshag (241), analyst.

7. Serpentine, Franklin. Jenkins and Bauer (243), analysts.


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