igneous
Propylitic Altered Andesite
Hydrothermally Altered Andesite
Hardness: 5.5-6.5 (Mohs); Color: Mottled green, pink/rusty, and grey; Luster: Dull to sub-crystalline; Crystal Structure: Porphyritic/fine-grained; Cleavage: None/irregular; Specific Gravity: 2.5-2.8. Shows 'greenstone' alteration minerals such as chlorite and epidote.
- Hardness
- 5
- Color
- Mottled green, pink/rusty, and grey
- Luster
- Dull to sub-crystalline
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Physical properties
Hardness: 5.5-6.5 (Mohs); Color: Mottled green, pink/rusty, and grey; Luster: Dull to sub-crystalline; Crystal Structure: Porphyritic/fine-grained; Cleavage: None/irregular; Specific Gravity: 2.5-2.8. Shows 'greenstone' alteration minerals such as chlorite and epidote.
Formation & geological history
Formed through volcanic activity associated with subduction zones. The green color comes from low-grade hydrothermal alteration (propylitic alteration) where hot water replaces original minerals with chlorite, epidote, and calcite. The coordinates (Mindanao, Philippines) are located in a highly volcanic region rich in such specimens.
Uses & applications
Used primarily as construction aggregate, road base, and occasionally for lapidary work if highly silicified. Geologically, it is a key indicator for nearby porphyry copper or epithermal gold deposits.
Geological facts
The green tint is characteristic of many ancient volcanic rocks in the Philippines and is often called 'greenstone' by field geologists. The pink/orange areas are likely due to oxidation of iron-bearing minerals or secondary potassium feldspar.
Field identification & locations
Identify by its mottled green/grey color and relative hardness. Found in riverbeds and mountainous volcanic regions. In the field, look for the 'pistachio' green of epidote or the 'dark forest' green of chlorite on broken surfaces.
