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, A light-coloured, igneous rock with an extremely coarse texture, made mainly of quartz, feldspar and mica, and often containing very large crystals. It forms from the last, water-rich melt of a cooling granite body.
Plutonic Igneous. Cooled extremely slowly at depth in the Earth's crust, resulting in very large crystals.
Source location: South Westland.
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Note: Images are examples of specimens only. Sizes and shapes will vary
More detail
Structure
Granite pegmatite is an intrusive igneous rock made of very large, interlocking crystals. It forms from the last, water-rich melt of a cooling granite body, so the crystals have space and time to grow unusually large — often centimetres to metres across.
Type
Plutonic (intrusive) igneous rock with a granitic (felsic) composition — essentially a very coarse-grained, late-stage granite.
Physical properties
- Colour: Usually light – white, light grey, pink or cream — with visible large crystals of quartz, feldspar and mica; darker mica flakes can give a speckled look.
- Lustre: Crystalline and often quite shiny, especially on fresh surfaces; large mica plates can look sparkly or pearly.
- Hardness: Generally hard, around 6–7 on the Mohs scale, because quartz and feldspar dominate.
- Density: Typically about 2.6–2.8 g/cm³, similar to ordinary granite.
General properties
- Composition: Mainly quartz, alkali feldspar and mica (muscovite and/or biotite), sometimes with accessory minerals like tourmaline, beryl, garnet or lithium minerals in more complex pegmatites.
- Extremely Coarse Texture: Defined by 'giant' crystals — much larger than in normal granite — due to slow cooling of a water-rich residual melt.
- Durability: Generally hard and durable, but the very large crystals and zoning can make some pegmatites more prone to splitting along crystal boundaries than uniform granite.
- Heat Resistance: Good heat resistance, similar to granite.
- Porosity: Usually low when fresh, though cracks along large crystal contacts can let water in as the rock weathers.
Uses in New Zealand and worldwide
- Construction and Aggregate: Can be crushed for concrete aggregate, road base and landscaping fill where it is quarried, though it’s less commonly used this way than more uniform granites.
- Dimension Stone and Decorative Stone: Cut and polished for feature cladding, interior panels, benchtops and decorative elements where the dramatic, very coarse texture is wanted.
- Landscaping: Used as decorative rock or feature boulders in rockeries and gardens, especially for its light colour and striking crystals.
- Monuments and Memorials: Occasionally used for monuments and headstones where local pegmatite has an attractive pattern, though its variable texture limits widespread use.
- Sculpture: Valued by some sculptors for its large crystals and distinctive look, but can be trickier to work than more even-grained stone.
- Industrial Minerals and Rare Elements: Worldwide, pegmatites are important sources of feldspar, quartz and mica for glass, ceramics and electronics, and in some cases of lithium, tantalum, beryllium and gem minerals like tourmaline, beryl and topaz.
- Geological Studies: In New Zealand, granitic pegmatites (for example in the Alpine Schist) are studied to understand how the crust melted and evolved as Zealandia broke away from Gondwana.