92 The Philippine Journal of Scioice i»i8 



in the old Santa Barbara copper mine, Mancayan (Lepanto), 

 Mountain Province, Luzon; it also occurs on Camaguin Island, 

 north of Luzon. 



Plagioclase— (NaAlSigO, to CaALSi,OJ. 



This series of closely related minerals is one of the commonest 

 of all the constituents of igneous rocks in the Philippines. It 

 embraces the following species : Albite, anorthite, andesine, and 

 labradorite. It has no known economic value as such. 



Platinum — Pt (metallic). 



Platinum is found in minute flattened grains in placer-test 

 borings near Penaranda in Nueva Ecija; on the Mariquina River 

 in Rizal Province, Luzon; and in Agusan Valley, Mindanao. 

 There is no production in the Philippines. 



Prochlorite— H,(Mg, Fe)„ALSiO,. 



Prochlorite occurs in dirty green leaves which are flexible but 

 not elastic. It is found in the metamorphic area of Ilocos Norte, 

 Luzon. No use is made of it. 



Psilomelane — H.,MnO-. The manganese is commonly replaced in 

 part by barium or potassium. 



Psilomelane occurs associated with pyrolusite. It is a very 

 impure ore of manganese, often containing only 40 per cent of 

 manganese (see Pyrolusite). 



Pyrite— FeS,. 



Pyrite occurs both massive and crystalline. The mineral 

 appears as disseminated grains and as large crystals. Pyrite 

 is often cupriferous. It is persistent in quartz veins and occa- 

 sionally in calcite veins. Frequently it is one of the early min- 

 erals to ci-ystallize. It is often associated with galena and zinc, 

 but generally precedes them. Pyritization of vein walls occurs 

 by reaction of the magnetite with HjS gas from the fissure. 

 Frequently this mineral is disseminated in volcanic rocks. Spec- 

 imens of large secondary (?) crystals are in the Bureau of 

 Science collection from Malaguit River, Camarines. Pyrite 

 forms under conditions ranging from deep to surface. It is 

 the most widely distributed metallic mineral. It is found in 

 almost all rocks. This mineral is especially abundant in quartz 

 veins, and is frequently associated with gold. There is no com- 

 mercial use of pyrite in the Philippines. 



Pyrolusite— MnO, with 2% H,0. 



Pyrolusite is the principal ore of manganese in the Philippines. 



