Feldspathic Sandstone
Sandstone with abundant feldspar grains (>25% feldspar)
A feldspar-rich sandstone, often pink, that points to granitic source rocks eroded quickly in dry or cold climates.
- Mohs hardness
- 6-7 (component grains)
- Color
- pink, reddish, tan to gray
- Type
- sedimentary
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Overview
Feldspathic sandstone is a sandstone containing a high proportion of feldspar grains alongside quartz. When feldspar exceeds about 25 percent of the framework it is commonly called arkose; lower amounts are termed subarkose or feldspathic arenite.
The feldspar is usually pink or white and derived from the weathering of granite and gneiss. Because feldspar breaks down chemically faster than quartz, its survival in the sand indicates either rapid erosion and burial or deposition in a climate (arid or cold) that limited chemical weathering.
The result is often a handsome pinkish to reddish rock used decoratively and studied as a record of granitic mountains.
Formation & geology
Feldspathic sandstones form where granite and gneiss are eroded and the resulting sand is deposited before the feldspar can weather to clay. This favors arid, semi-arid, or glacial climates and steep, rapidly uplifting terrain near the source.
Typical environments include alluvial fans, braided river systems, and basins flanking mountain ranges. After deposition, compaction and cementation — often by iron oxides that add a red color, or by silica and calcite — lithify the sand.
Classic examples include the arkoses derived from the ancient granites of the Front Range in Colorado and many Precambrian and Triassic basin fills worldwide.
How to identify it
Look for a gritty, often pink or reddish sandstone in which a hand lens shows blocky, sometimes cleavable feldspar grains mixed with glassy quartz. The feldspar grains may show flat reflective cleavage faces, unlike the conchoidal quartz.
The pink to salmon color is a strong hint, though gray varieties exist. Compared with quartz arenite it looks less clean and more variegated; compared with lithic sandstone it has identifiable feldspar crystals rather than rock fragments.
Feldspar is slightly softer than quartz and may appear cloudy or weathered. A coarse, angular texture suggests short transport from a granitic source.
Uses & significance
Feldspathic sandstone and arkose are valued as dimension and decorative building stone, prized for their warm pink and red tones in paving, walls, and cladding. They have served as durable construction stone in many regions.
Geologically they are important provenance indicators, telling researchers that nearby granitic highlands were eroding under arid or cold conditions. They can also serve as petroleum reservoir rocks where porosity is preserved.
They carry no gem or notable metaphysical significance, but attractive slabs are used ornamentally.
Frequently asked questions
Is feldspathic sandstone the same as arkose?
Arkose is the most feldspar-rich variety (over about 25 percent feldspar). Feldspathic sandstone is a broader term that also includes less feldspar-rich types like subarkose.
Why is feldspathic sandstone often pink?
The pink color usually comes from potassium feldspar grains and from iron-oxide cement, both linked to its granitic source and arid depositional setting.
What does its feldspar content reveal?
Because feldspar weathers quickly, its abundance indicates rapid erosion and burial, often in dry or cold climates near granitic mountains.
How do I tell feldspar grains from quartz?
Feldspar grains are blocky with flat cleavage surfaces and may look cloudy, while quartz is glassy, harder, and breaks with curved conchoidal surfaces.
Is it good building stone?
Yes. Well-cemented feldspathic sandstones make attractive, durable decorative and dimension stone valued for their warm colors.
Feldspathic Sandstone guides
In-depth guides for identifying, valuing, and understanding Feldspathic Sandstone.
