Chlotire Group – Clinochlore Penninite Composition & Structure

A somewhat ill-defined group of closey related micaceomineral is known as the Chorite Group or as the chlorites. They are so named on account of the characteristic green color that they show. They are silicates of aluminum with magnesium, ferrous iron and hydroxyl. Ferric iron may replace the aluminum in small amount. Chromium and manganese may occur. Calcium and the alkalies, which are characteristic of de micas proper, are practically absent. The composition of these minerals is not fully understood. Their crystals forms are similar to those of the micasand they show a perfect basal cleavage. Their lamine, however, are tough and inelastic. Clinochlore is the most common member of the group.

Clinochlore, Penninite

Composition. H8Mg5AL2Si3O18. see above

Crystallization. Monoclinic. In six-sided tabular crystals, with prominent basal planes. Similar in habit to the crystals of de mica gout, but distinct crystals rare. Penninite is pseudorhombohedral in symmetry, otherwise it is identical with clinochlore.

Structure. Usually foliated massive or in aggregates of minute scale, in finely disseminated particles; earthly.

Physical Properties. Perfect basal cleavage. Folia flexible but not elastic. H. = 2-2.5. G. = 2.65-2.75. Vitreous to pearly luster. Color green of various shades. Rarely pale green, yellow, white, rose-red. Transparent to opaque.

Test. Difficultly fusible, 5-5.5. unattached by hydrochloric acid. Decomposed by boiling concentrated sulfuric acid, giving a milk solution. Characterized by its green color, micaceous structure and cleavage and by the fact that the folia are not elastic.

Occurrence. A common and widespread mineral, usually of secondary origin and occurs in connection with chloritic and talcose rocks or schists and serpentines. It results from the alteration of silicates containing aluminum, ferrous iron and magnesium, such as pyroxene, amphibole, biotite, garnet, vesuvianite, etc. To be found where rocks, containing such minerals, are undergoing metamorphic change. The green color of many igneous rocks is due to the chlorite into witch the ferromagnesian silicates have altared. The green color of many schists and slates in due finely disseminated particles of the mineral.

Name. Clorite is deried from a Greek word meaning green, in allusion to the common color of the mineral.