Plagioclase Feldspar

Over half of the Earth’s crust is composed of a single group of minerals known as the feldspars.  Feldspars are an important component of many building stones, but in economic terms these minerals are more important for what they become than what they are. As feldspars weather they produce clay minerals and aluminum ores, two resources used throughout our modern society.

Based on their chemical composition, feldspars can be subdivided into two mineral groups – plagioclase feldspars and potassium feldspars. Plagioclase feldspars are a continuous mineral series in which calcium and sodium substitute for one another in the same crystal structure. Because they share a similar crystal structure, all the plagioclase varieties exhibit very similar properties and in the field are often only identified as ‘plagioclase’. Plagioclase minerals are typically white or gray, hard blocky crystal masses that exhibit cleavage in two directions that are nearly at right angles. They are particularly common in igneous and metamorphic rocks, where they often form the bulk of the rock’s volume.

Description and Identifying Characteristics

‘Feldspar’ is derived from the German word feld, which means ‘field’, and spar, which means ‘crystal’. This name reflects the minerals’ abundance, as most crystalline material found in the field is feldspar. The plagioclase feldspar subgroup consists of a continuous mineral series that is arbitrarily subdivided into six mineral categories whose composition varies from being relatively pure sodium aluminum silicate (albite) to a relatively pure calcium aluminum silicate (anorthite). A continuous gradation exists between each of these subdivisions, and all exhibit very similar properties. As the subdivisions are only definitively distinguished by slight variations in their density and the way they refract light under an optical microscope, these minerals are typically lumped together as ‘plagioclase’ in the field.

Plagioclase minerals are hard, non-metallic minerals that exhibit two cleavage directions that meet at nearly right angles. The name ‘plagioclase’ refers to this cleavage pattern, coming from the Greek words for ‘oblique’ (plagios) and ‘to break’ (klasis). Apart from pegmatites, the plagioclase minerals seldom occur as well-defined isolated crystals, but instead occur as blocky to prismatic cleavable masses that may comprise most of an igneous rock’s volume. They are harder than glass; most often white to gray in color, and have well-developed cleavage planes that often exhibit fine parallel grooves or striations. Color is variable however, and in mafic igneous rocks, the more calcium-rich plagioclase may be dark gray to almost black. In general, the presence of striations is a more useful way to distinguish plagioclase minerals from the potassium feldspars with which they are most easily confused, although crystal twinning and compositional zonation in some potassium feldspar varieties may mimic striations.

Both feldspar groups commonly exhibit twinning and their crystal structures are so similar that they may actually form as thin alternating bands within a single specimen. This mixed variety of feldspar, consisting of alternating bands of plagioclase and potassium-feldspar, is called perthite.

Over 95% of the Earth’s crust consists of igneous rock or metamorphosed igneous rock and feldspar minerals compose the majority of these rocks. Plagioclase feldspars are an important, if not the dominant, component of nearly every igneous rock, especially intermediate to mafic igneous rocks. The crystals of plagioclase are often particularly well developed in pegmatites, coarsely crystalline igneous rocks that form during late water-wet stages of a magma cooling. They are also commonly found in many metamorphic schists, gneisses, and other contact or regionally metamorphosed rocks - especially those that formed from the alteration of shale or igneous rocks. Although plagioclase feldspars will weather to form clay minerals, they are resistant enough that plagioclase grains forms a significant component of many detrital sediments and sedimentary rocks.

Calcium-rich plagioclases tend to be characteristic of intermediate to mafic igneous rocks, intermediate- to high-grade metamorphic rocks, and metamorphosed carbonate rocks. In contrast, sodium-rich plagioclases are more common in felsic igneous rocks, low-grade metamorphic rocks and may occur as overgrowths on feldspar grains in sedimentary sandstones.

Although not as resistant as quartz, feldspar minerals are resistant enough that they form a significant component of many sand deposits and sandstones. Ultimately though, these minerals will break down to form clay minerals. These clay minerals form the bulk of the fine-grained muds, mudstones and shales that cover much of the land surface and shallow seafloor.

Because of their wide range of occurrence, the plagioclase feldspars may occur with most of the other important rock forming minerals. Calcium-rich plagioclase varieties dominate in mafic igneous rocks and are typically associated with biotite, amphiboles and pyroxenes. Sodium-rich plagioclases are more common in felsic igneous rocks where they are associated with quartz, muscovite and the potassium feldspars.

Many decorative igneous and metamorphic building stones are composed primarily of plagioclase minerals. The plagioclase component of these stones is hard enough, and resistant enough, to maintain a polish and the varied distribution of plagioclase and other minerals creates the patterns prized in these stones. By volume, however, the most significant use of plagioclase is that it comprises much of the sand and gravel used as aggregate in concrete and asphalt. Large volumes of plagioclase are also mined and processed to create a wide variety of ceramics, ranging from pottery to industrial uses. Ground into powder, plagioclases are also used in the manufacture of paints, rubber, glass, plastics, and are even used as a mild cleaning abrasive.

Indirectly, the feldspar group provides some crucial resources that form as feldspars weather and break down. Water will react with feldspars to produce a host of clay minerals upon which all ceramics and pottery, from the crudest mud-covered reed basket to the finest porcelain dinnerware and figurines. Without ceramics, fired clay, most human cultures would have had dramatically different histories. More recently, another weathering product of feldspar has had nearly as great an impact on our industrial society. As feldspars breakdown under the intense weathering conditions of tropical climates, almost all of the feldspar breaks down and is dissolved away except for some hydrous aluminum oxide minerals that are left behind. These concentrated deposits of aluminum ores are collectively called bauxite and are our major source of aluminum.

Plagioclase occurs as an important constituent of igneous and metamorphic rocks throughout the Upper Midwest. Concentrated plagioclase deposits are found in north-central Minnesota, as well as the northeast corner of Minnesota bordering Canada and North Dakota. Together with potassium feldspars, the plagioclase minerals form the bulk of the igneous and metamorphic decorative building stones found in the region, including the St. Cloud Granite, Morton Gneiss and Wausau Granite. Feldspars are also significant components of the Precambrian sandstones that filled the Mid-Continent Rift valleys, such as the Hinckley and Fond du Lac Sandstones.

All of the feldspar minerals exhibit a blocky crystal habit, hardness greater than glass, and two directions of cleavage that meet at nearly right angles. Together with their non-metallic luster and typically opaque character, these properties will usually distinguish feldspars from other common non-metallic minerals. Within the feldspar group, the plagioclase minerals are only differentiated from the potassium feldspars by the presence of parallel striations seen on some plagioclase cleavage surfaces.

Potassium Feldspar:

The various potassium feldspar minerals share a similar composition, crystal structure and origin with many of the plagioclase, so they are difficult to distinguish from one another. Typically, potassium feldspars tend to have more pink and reddish hues, while the color of plagioclase minerals usually ranges from white to gray. However, the hues of both mineral groups overlap, so color is seldom definitive. Fine parallel lines or grooves, called striations, occur on some cleavage surfaces in the plagioclase minerals and are a more definitive means to distinguish the two groups.

Perthite:

Because the crystal structures of both feldspar groups are so similar, the two may actually occur as thin alternating bands within a single specimen. This mixed variety of feldspar is called perthite. The thin alternating bands may be mistaken as striations, but the alternating bands usually have slightly different colors, and as you follow individual bands across the cleavage surfaces, they tend to be more irregular than striations, with their boundaries not being as strictly parallel.

Quartz:

Quartz is a common mineral in many metamorphic and felsic igneous rocks that, like plagioclase, also has a non-metallic luster and hardness greater than glass. Unlike plagioclase, however, quartz does not exhibit cleavage, which easily distinguishes it from feldspar minerals. Quartz also tends to be more transparent and typically occurs as clear glassy masses.

Calcite:

Although calcite and plagioclase may have similar colors and both exhibit well-developed cleavage, the two are easily distinguished as calcite is softer than glass or metal, reacts readily with dilute acid, and its rhombic cleavage surfaces do not meet at right angles.

Dolomite:

Although dolomite and plagioclase may have similar colors and both exhibit well-developed cleavage, the two are easily distinguished as dolomite is softer than glass or metal, and its rhombic cleavage surfaces do not meet at right angles.