8 Paired Tetrahedral Silicates and Related Minerals
8.1 Lawsonite
CaAl2Si2O7(OH)2·H2O
Occurrence—Lawsonite is a rare mineral found mainly in high-pressure metamorphic rocks of the blueschist facies, less commonly in eclogites.
Distinguishing Features— Lawsonite grains are colorless and subhedral to euhedral, with moderately high relief. Occurrence in high-pressure low-temperature rocks (blueschists, especially) is key to identifying this mineral.
Crystals are normally tabular or prismatic with rhombic, rectangular, or square cross sections (Figs 8.1.1 and 8.1.2). The mineral displays up to 2nd-order blue colors. Complex polysynthetic twins are common.
Lawsonite has two perfect and two fair cleavages. Two perpendicular cleavages may show in tabular or elongate section; two “rhombic” cleavages show in cross section.
Similar Minerals—Clinozoisite resembles lawsonite but often has anomalous interference colors and much higher relief. Prehnite has greater birefringence; tremolite has oblique extinction; and wollastonite has a smaller 2V (and is typically found in calc-silicates, not blueschists).
Optical; Properties
■■ Orthorhombic; biaxial (+)
■■ 2V = 79° to 85°
■■ α = 1.665, β = 1.674, γ = 1.685
■■ δ = 0.020
■■ Extinction appears symmetrical in end views; parallel in longitudinal views
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8.2 Vesuvianite (Idocrase)
Ca10Al4(Mg,Fe)2Si9O34(OH)4
Occurrence—Vesuvianite is a rare mineral found primarily in contact aureoles associated with impure limestone or dolomite.
Distinguishing Features—Vesuvianite crystals have high relief and are typically colorless, although some are light green, yellow, or brown. Pleochroism is absent or very weak.
Vesuvianite crystals occur as short tetragonal prisms, sometimes with rounded corners. It may also be in anhedral grains or radial, columnar, or fibrous aggregates. Vesuvianite has several poor cleavages but they are rarely visible in thin section. Twinning is absent.
In crossed polars, interference colors are no higher than 1st-order gray. Anomalous interference colors are common, often deep indigo blue but sometimes olive, yellow, or brown.
Vesuvianite is tetragonal and uniaxial.
Similar Minerals—Vesuvianite may be confused with zoisite or clinozoisite, but these minerals are biaxial and usually show cleavage. Epidote, also biaxial, has greater birefringence and is often faintly green or yellow. Andalusite is biaxial. Andalusite and apatite both have lower relief than vesuvianite.
Optical Properties
■■ Tetragonal; uniaxial (-)
■■ ω = 1.702-1.795, ε = 1.700-1.774
■■ δ = 0.001-0.021; interference colors are low 1st-order or anomalous
■■ Elongate crystals are length fast and display parallel extinction
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8.3 Epidote – Clinozoisite – Zoisite – Allanite
Ca2(Al2Fe3+)3Si3O12(OH) (epidote) – Ca2Al3Si3O12(OH) (clinozoisite; zoisite)
Occurrence—Epidote and clinozoisite form a solid solutions series but epidote always contains substantial ferric iron. They can be hard to tell apart. Zoisite is an orthorhombic polymorph of clinozoisite that contains little Fe.
These minerals are common in low- and medium-grade metamorphic rocks, especially greenschists, amphibolites, and calc-silicate rocks, as well as in high-pressure blueschists and eclogites. They may be minor accessory minerals or major primary minerals in a rock. Epidote can also be produced by alteration of feldspars in rocks of many different compositions.
Distinguishing Features—Clinozoisite and zoisite are normally colorless, but epidote can be faintly yellowish to pistachio green. Grains may be color zoned.
Individual prismatic crystals of these minerals are typically tabular, stubby, or elongate prisms. They can have a pseudohexagonal cross section. Columnar aggregates are common. Epidote and clinozoisite may be replacement minerals forming patches or aggregates. One good cleavage is apparent in some orientations; an additional poor cleavage is rarely seen. Simple twins may be present.
Epidote and clinozoisite have high relief and inclined extinction, although the extinction angle for clinozoisite can be very small. Zoisite has parallel extinction.
Clinozoisite may display anomalous or normal 1st-order colors. Epidote has greater birefringence than clinozoisite and commonly shows intense upper-2nd to 3rd-order colors, depending on composition. Individual epidote grains may appear brightly colored and patchy with lots of different colors, or like an archery target with concentric color rings, when viewed with crossed polars. Zoisite sometimes displays anomalous Berlin blue or patchy brownish-gray interference colors instead of 1st-order gray or white.
Similar Minerals—Epidote is distinguished from clinozoisite by being biaxial (-), having much higher birefringence, and having a yellow to yellow-green color. Greenish epidote is distinguished from most clinopyroxenes by cleavage, a smaller extinction angle, optic sign, and higher relief. Zoisite (the orthorhombic polymorph of clinozoisite) and clinozoisite have lower birefringence than epidote; zoisite has parallel extinction.
Fayalite is normally yellow or yellow-green and shows little cleavage. Vesuvianite is uniaxial (-). Allanite, a rare earth-bearing epidote-group mineral, is typically pleochroic brown, and can form cores to larger epidote grains. Piemontite is an Mn-rich variety of epidote with strong colors (yellow, orange, red, or violet) and marked pleochroism. Epidote and garnet have nearly the same relief, so they may appear similar in plane-polarized light, but garnet is isotropic.
Optical Properties
■■ Epidote and clinozoisite are monoclinic; epidote is biaxial (-); clinozoisite is biaxial (+). Zoisite is orthorhombic; biaxial (+)
■■ 2V = 69° to 89° (epidote); 2V = 14° to 90° (clinozoisite); 2V = 0° to 69° (zoisite)
■■ Refractive indices are high for these minerals (>1.71 for epidote, >1.67 for clinozoisite/zoisite)
■■ δ = 0.015-0.048 (epidote) or 0.005-0.015 (clinozoisite/zoisite)
■■ Cleavage fragments of epidote and clinozoisite often show an optic axis figure
■■ Extinction is inclined (epidote, clinozoisite) in longitudinal section, although the extinction angle may be small; it is parallel for zoisite
■■ Crystals may be length fast or length slow, depending on composition
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| Fig 8.3.7 Epidote and Allanite in Blueschist
This schist from Syros, Greece, contains large laths of high-relief colorless epidote cored by characteristically brown allanite (PP). Allanite is a rare earth rich variety of epidote. Moderate-relief glaucophane is optically zoned with darker blue cores (likely more Fe-rich) and light blue rims (PP). Colorless quartz and muscovite make up most of the rock. Fine-grained clots of high-relief rutile are also present. In XP, allanite is a muddy brown, epidote is anomalous lemon yellow, glaucophane is a beautiful 1st-order orange to blue, muscovite is 2nd order, and quartz is 1st-order gray to white. The dark color of rutile masks its high order interference colors. FOV = 1.4 mm. |
Fig 8.3.8 Clinozoisite in a Siliceous Carbonate
Colorless moderate-relief clinozoisite is a principal mineral in this silicate-rich layer in a calcite marble. Clear bladed tremolite is also present. There is no clear cleavage in the clinozoisite, but two cleavages intersecting at 60° and 120° are visible in the tremolite. Also visible in PP are pale brown phlogopite, low-relief colorless quartz, one small pale yellow tourmaline grain with a green core, and opaque graphite. In XP the clinozoisite has low, 1st-order gray colors. The tremolite and phlogopite show upper 1st- to low 2nd-order colors. FOV = 2.5 mm. |
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| Fig 8.3.9 Clinozoisite in a Metabasite
In PP, we see high-relief clinozoisite that is surrounded by low-relief quartz (and plagioclase, but the plagioclase cannot be easily distinguished). Some clinozoisite shows anomalous blue-green interference colors. Patchy interference colors are typical of epidote/clinozoisite. The high birefringence of one grain suggests that it is epidote. It was not possible to determine the optic sign of that grain, but the optic sign of a different grain is positive, suggesting it is clinozoisite. Both epidote and clinozoisite could be present, but chemical analysis is needed to tell for sure. |
Fig 8.3.10 Zoisite in Anorthosite
Zoisite forms an alteration patch in plagioclase in this altered anorthosite. Zoisite is essentially an orthorhombic, very low-iron epidote. As an alteration product, it commonly forms as masses or veins of irregular, ragged crystals in Ca-rich plagioclase. Zoisite has low-1st order birefringence, often displaying characteristic anomalous Berlin-blue (seen in this view) or anomalous brown interference colors. Photos from Dr. Kurt Hollocher. FOV = 3 mm. |
