30 BUTLER AND SCHALLER: MAGNESIOLUDWIGITE 



in rock debris near the Alta-Brighton trail north of Twin Lakes, 

 and is abundant in the South Columbus tunnel of the South 

 Hecla mine. It will doubtless be found at other localities in the 

 district. Magnesioludwigite was found only in the Mountain 

 Lake mine. 



Occurrence. Ludwigite occurs as a replacement of limestone 

 at or near the contact with intrusive rocks and is associated with 

 "contact minerals," including magnetite, forsterite, garnet, 

 diopside, muscovite, and sulphides of iron and copper. In the 

 Mountain Lake deposit magnetite and forsterite are the most 

 abundant original minerals associated with the ludwigite. 

 All of the minerals were probably formed at the same time, 

 though the presence of veinlets of magnetite in the ludwigite 

 and in the forsterite indicates that the magnetite continued 

 to form after the ludwigite and forsterite had ceased to form. 

 The sulphides also, in part at least, were formed probably later 

 than the ludwigite and forsterite. 



In the Mountain Lake deposit the ludwigite is in finely fibrous 

 crystals, forming radial and spherulitic groups which make up 

 large masses of the mineral. These, as seen with the unaided 

 eye or the hand lens, resemble tourmaline. There are exposed 

 in the outcrop and on the dump many tons of a mixture of 

 magnetite and ludwigite. The ludwigite occurs also as isolated 

 crystals and groups of crystals in metamorphosed limestone, 

 and from such occurrences well-defined crystals with natural 

 crystal faces have been isolated and measured. The ludwigite 

 is dull greenish black (Ridgway's color scale). In the outcrop 

 of the deposit there is also much material of ivy-green color 

 (Ridgway's color scale), which is the mineral for which the 

 name magnesioludwigite is proposed. Its occurrence is like 

 that of the ludwigite. 



Composition. Chemical analysis has shown that only 2.55 

 per cent of ferrous oxide is present in the ivy-green mineral, 

 whereas typical ludwigite contains 17.01 per cent ferrous oxide. 

 The analysis has also determined that the formula of the new 

 mineral is MgO.Fe2O3.3MgO.B2O3, in which a small amount of 

 ferrous oxide replaces some of the magnesia. The analysis 



