Appinite
Hornblende-rich intermediate plutonic rock (appinite suite), diorite to monzodiorite affinity
A group of coarse, water-rich plutonic rocks dominated by large hornblende crystals set in feldspar, intermediate between lamprophyre and diorite.
- Mohs hardness
- 5-6
- Color
- dark gray to greenish black, mottled with white feldspar
- Type
- igneous
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Overview
Appinite is a name for a suite of hornblende-rich plutonic and hypabyssal rocks in which large, well-formed crystals of hornblende are prominent in a feldspathic (plagioclase and/or alkali feldspar) matrix. Compositions span diorite, monzodiorite, and related intermediate rocks, all marked by an unusually high, water-rich amphibole content.
The term comes from Appin in the Scottish Highlands, where these rocks were first described. Appinites are closely linked to lamprophyres but occur as larger plugs, bosses, and intrusions rather than thin dikes.
They are valued chiefly as petrological subjects that record hydrous, mantle-derived magmatism.
Formation & geology
Appinites form from exceptionally water-rich intermediate to mafic magma that crystallizes large hornblende crystals as it cools at depth. The high volatile content stabilizes amphibole over pyroxene and can drive explosive brecciation, producing associated breccia pipes.
They are typically emplaced as plugs, stocks, and small intrusions in orogenic belts, often associated in space and time with granitic plutons and with lamprophyre dikes. Classic occurrences are in the Caledonian belt of Scotland (Appin, Ardara, Newer Granite suite) and in similar terranes worldwide.
How to identify it
Look for a dark, coarse-grained intrusive rock with conspicuous large black hornblende crystals, sometimes radiating or stout prisms, set in pale feldspar — giving a strongly mottled or speckled appearance. It is dense and moderately hard.
The defining feature is the abundance of large hornblende in a plutonic (coarse) rock. This separates appinite from ordinary diorite, which has finer, less dominant amphibole, and from lamprophyres, which occur as fine-grained dikes rather than coarse intrusions.
Look-alikes include hornblende diorite and gabbro; the combination of big hornblende crystals, intermediate feldspar content, and occurrence in plugs near granites points to appinite.
Uses & significance
Appinite has no notable commercial use beyond possible local use as crushed stone or rough building rock.
Its importance is scientific: appinites are key evidence for hydrous, mantle-derived magmatism in mountain belts and are studied for what they reveal about volatile-rich magmas, deep crustal processes, and the link between mafic and granitic intrusions. The Scottish type localities are classic field-study sites.
It has no gemstone or metaphysical tradition.
Frequently asked questions
What defines an appinite?
An abundance of large hornblende crystals in a coarse, feldspar-bearing intrusive rock of intermediate composition, reflecting very water-rich magma.
How is appinite related to lamprophyre?
They share a hydrous, hornblende-rich character, but appinites form coarse-grained plugs and stocks while lamprophyres form fine-grained dikes.
Where does the name appinite come from?
From Appin in the Scottish Highlands, where the rocks were first described.
Why are appinites scientifically important?
They record unusually water-rich, mantle-derived magmatism and are often linked to associated granite plutons and breccia pipes.
Appinite guides
In-depth guides for identifying, valuing, and understanding Appinite.
