250 The American Geologist. April. i899 
[Coiitributious to the Mineralogy of Minnesota. VI.l 
THE OPTICAL CHARACTERS OF JACKSONITE. 
By N. H. WixcHELL, Minneapolis, Minn. 
Jacksonite. In the collections of the Museum d'Histoire 
Xaturelle, Paris, is a fine specimen of the mineral named 
jacksonite by Whitney, and it is by the courtesy of Prof. A. 
Lacroix that a note on this specimen is here included. It was 
received in 1865 (No. 65.8) by purchase, and there is no sign 
of any artificial calcination which Dana suggests may have 
operated on that which w'as analyzed by Whitney. The speci- 
man consists of two or three laminations of jacksonite with 
green rock substance, and is recorded as from Isle Royale, la- 
belled "prehnite, sans eau." It is but rarethat a fine radiated 
spherulite is discoverable in the specimen, but the mass ap- 
pears compact and homogeneous. Its color is nearly white 
but has a faint tinge of pink. It is very hard and very firm. 
According to Whitney's description''' jacksonite is associated 
with wollastonite, both at the Clifif mine, Keweenaw point, and 
at Scovell point. Isle Royale. It was named by Whitney in 
honor of Dr. C. T. Jackson, "whose name is so well known in 
connection with the lake Superior region, and to whom we 
are so much indebted for our knowledge of its mineralogy." 
The description given by Whitney may be summarized as fol- 
lows : 
Jacksonite occurs in finely radiated and lamellar-radiated 
masses of a white color, slightly tinged with green . Its hard- 
ness is 6. Specific gravity 2.881. Lustre vitreous, translucent. 
The finely pulverized mineral is perfectly, though slowly, 
dissolved by chlorohydric acid, the silica separating in the form 
of a flocky powder. Before the blowpipe in the platinum 
forceps it fuses very readily with strong intumescence and 
emits a brilliant yellow light. x\ large quantity of soda dis- 
solves it readily; if more of the assay be added it swells up to 
an infusible slag. It gives with borax a colorless, transparent 
glass; with salt of phosphorus a glass enclosing a siliceous 
skeleton, faintly tinged with iron. 
The analysis of the "ignited" mineral gave: 
*Jour. Bost. Soc. Nat. Hist., V, 488, 1847. 
