GYPSUM

Gypsum – CaSO4.2H2O – is the most common sulfate, one of the most abundant minerals in evaporite rocks. It is a very important ore for plaster, mineral fillers, fertilizers, and much more.

It is classified in the Gypsum Supergroup, being isostructural with ardealite, brushite, and pharmacolite. It can dehydrate to bassanite at high temperatures.

Among the varieties are selenite (transparent), satin spar (fibrous), alabaster (very fine grain), and ordite (pseudomorphs of gypsum on an unknown mineral).

1. Characteristics

Crystal system: Monoclinic prismatic.          

Color: Colorless to white, it can also be yellow, blue, pink, brown, reddish-brown, gray, and others.     

Habit: Massive, tabular, usually long prismatic crystals, acicular, earthy, massive, granular, etc. Many possible habits.       

Cleavage: {010} perfect, {100} distinct.       

Tenacity: Flexible, not elastic.        

Twinning: On {110}, very common, there are several others.       

Fracture: Conchoidal in {100}, splintery // at {001}

Mohs Hardness: 1.5 – 2

Parting: No.         

Streak: White.         

Lustre: Vitreous, silky, pearly, greasy.          

Diaphaneity: Transparent.           

Density (g/cm³): 2.31 – 2.33

 

2. Geology and Deposits

Gypsum is the most common sulfate, generally found in evaporites formed by the evaporation of seawater. It can form by hydration of anhydrite, a process that implies an increase in volume. The transition from gypsum to anhydrite implies a reduction in volume; it can form caves.

It occurs in volcanic fumaroles, hot springs, and in soils from anhydrite.

It is found in cavities in general, from caves (Naica Mine in Mexico), in mine walls (efflorescence), in caves where the air is dry enough (speleothems), and associated with other secondary minerals in vesicles in volcanic rocks (Ametista do Sul Mining District, Rio Grande do Sul state, Brazil).

Other forms of gypsum include “desert roses,” which are aggregates of gypsum crystals formed subsurface, slightly above the water table, the gypsum sand dunes at White Sands National Monument (New Mexico, USA), and the gypsum dunes in the northern polar region of Mars.

Its association with sulfide or oxidized ores is rare. It can be hydrothermal, but it is more common in oxidation zones (gossans), formed by the oxidation of sulfides such as pyrite.

 

3. Mineral Associations

In evaporites, it is associated with halite, anhydrite, and carbonates (calcite, aragonite, dolomite).

In oxidation zones (gossans), it occurs with native sulfur, native copper, celestine, pyrite, azurite, siderite, clay minerals, and many others. It is also associated with quartz and fluorite.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.519 – 1.521     nβ: 1.522 – 1.523    nγ: 1.529 – 1.530

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless, it has no color, much less pleochroism. 

Relief: Low.           

Cleavage: Perfect cleavage {010}, two distinct cleavages in {100} and {011}.           

Habits: Typically anhedral to subhedral crystals. Short to acicular prisms, thin or thick tabular, lenticular in rosettes, may be curved, deformed, fibrous, earthy, concretionary, granular or massive.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Low birefringence, of 0.01: gray to white interference colors, similar to feldspars and quartz.

Extinction: It tends to be oblique, up to 15º. In sections parallel to the b-axis, it is parallel. 

Elongation sign: ES(+) or ES(-) by cleavage, it is not diagnostic. 

Twins: Simple twins are very common macroscopically: contact twins in {011} forming crosses and “V”s, contact twins in {-101} forming butterfly or heart-shaped twins. During thin section preparation, heating can form polysynthetic twins in (100), which can be curved.

Zoning: No information available, probably not or very rare.

CONVERGENT LIGHT

Character: B(+)          

2V angle: 58º to 19º, decreases with increasing temperature.         

Alterations: It can dehydrate to anhydrite. Upon heating, during the preparation of the thin section, it can transform into bassanite. 

May be confused with: Several other colorless minerals with low relief.

Anhydrite occurs in association, but has pseudocubic cleavage, straight extinction, and much higher birefringence; its interference colors are very colorful, while gypsum is gray.

Chalcedony can be confused with gypsum if in fine aggregates, but chalcedony is U(+), a data point almost impossible to obtain due to the minute size of the crystals.

Barite occurs in the same paragenesis and may have similar forms, but has moderate relief.

Several zeolites are very similar to gypsum, almost indistinguishable: stilbite, scolecite, laumontite, and others. It is necessary to consider the paragenesis to exclude or not the possibility of the presence of zeolites.         

Gypsum 01: In PPL, an aggregate of gypsum with anhydrite. Both minerals are colorless and exhibit low relief.
Gypsum 01: The same section as of the previous image, now in XPL. Anhydrite exhibits strong, colorful, intense interference colors, while gypsum exhibits gray colors like quartz and feldspars.
In XPL, well-developed tabular crystals of gypsum (gray) and anhydrite (colored).
In XPL, microcrystals of gypsum, almost fibrous, in various orientations, forming a dense mass. In this case, identifying the mineral is more difficult.
In XPL, small rosette aggregate of gypsum (gray) and anhydrite (colored). These are radial groupings of tabular crystals.
In XPL, gypsum forming a dense aggregate of fine tabular crystals. This type is sold in "stone shops".
In XPL, one of several gypsum ores ("cocadinha") from the Araripe Gypsum Pole (State of Pernambuco, Brazil). These are approximately tabular crystals, parallel to each other, that form horizontal bands. Horizontal bands of wider crystals alternate (at the bottom of the image) with thinner and smaller crystals (almost at the top of the image). The bands are somewhat irregular, but easily visible both under a microscope and in a hand sample.
In XPL, large gypsum crystals are shown without orientation. The image diameter is 7 mm. In PPL, the crystals are completely colorless.
In XPL, small gypsum crystals are shown without orientation. The image diameter is 7 mm. In PPL, the crystals are completely colorless.

5. Reflected Light Microscopy

Reflected light microscopy is clearly not the recommended analytical method for identifying gypsum. However, it is important to prepare a polished slide or section to identify the opaque minerals that occur associated with gypsum.

Sample preparation: The low hardness of gypsum makes the production of polished sections quite difficult. Grinding must begin directly with a fine abrasive; during polishing, pressure should be avoided; the section is polished in a few minutes. The excellent cleavage {010} makes the production of sections perpendicular to the cleavage very difficult. It is almost impossible to avoid polishing grooves.

“Cleaning” the polished surface with a soft paper towel leaves the surface covered with grooves, and polishing needs to be redone. In polymineralic aggregates, gypsum crystals will likely exhibit strong negative relief.       

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray.       

Pleochroism: No.      

Reflectivity: ~5%        

Bireflectance: No.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: It does not have anisotropy.        

Internal reflections: Widespread, clear, milky and multicolored.

May be confused with: With many other colorless minerals, but the very low hardness is quite diagnostic.       

General Characteristics: 

Polishing grooves, very common in PPL, are no longer visible in XPL.

Cleavage will be visible in well-formed crystals if the cleavage is perpendicular to the polished section. In blocks made parallel to the {010} cleavage, no cleavage is observed.

Gypsum 01: Gypsum on PPL. The reflective color is a dark gray. Internal reflections are evident even in PPL, as is the large number of polishing grooves, practically unavoidable due to the low hardness of gypsum.
Gypsum 01: The same section as of the previous image, now in XPL. The internal reflections are colorless, and due to the excellent cleavage, areas with multicolored reflections appear.