78 PROFESSOR HEDDLE ON THE MINERALOGY OF SCOTLAND. 
TALC-CHLORITE. 
The composition of this mineral is expressed, as shown by Dana, by the 
same general formula as that of pyrosclerite ; but a considerable quantity of 
ferrous oxide here replaces magnesia. 
Its specific individuality cannot be said to have been hitherto absolutely 
determined ; it rests upon three closely accordant analyses by Martenac, and 
an optical determination by DESCLOIZEAUX. 
Dana, referring to the excess of its silica above that contained in ripidolite, 
remarks that “it is possibly ripidolite impure from mixture with talc;” but it is 
extremely improbable that DEScLoIzzEAux would not have detected such an 
admixture either while preparing plates for the polariscope, or during’ the 
employment of the instrument. 
The mineral, I now notice, goes a certain length im aiding in establishing the 
species. 
It was found forming a vein, which occurred on the foreshore, to the south 
of the Banks of the Nudister, at Hillswick, in Shetland. This vein was asso- 
ciated with a similar one of Biotite. The substance, being damp from recent 
marine submergence, could be raised in spadefuls. It formed a pulpy and 
slimy mass of minute glistening scales. These were floated in water to remove 
adhering salt, and being frequently examined during successive decantations, 
were seen to be absolutely free from talc, or any impurity. 
Their colour was grass-green, with a peculiar bronzy or golden lustre in 
certain directions. Their lustre was more pearly than that of chlorite or 
ripidolite. They were exceedingly smooth to the touch. 
1°38 grammes yielded— 
Marignac. 
silica,  . , ' yes cil 39 ° 87 
Alumina, . , 5 . 11°432 LOL 
Ferrous Oxide, . : Ort 134 
Manganous Oxide, . : * 259 
Lime, ; 2° 804 us 
Magnesia, ; : . 25° 648 28 + 76 
Potash, . ; ; 7 e203 
Soda, on 2, Ae 
Water, 7 Oils 7°98 
100-194 
The air-dried mineral lost at 212°, 10°945 of water in addition to the 
above ; a feature sufficing to distinguish it from either ripidolite or tale. 
