Chemical Mineralogy
The mineralogy of carbonates is comparatively simple since relatively few minerals occur in these rocks. The trace element or solid solution variations of some of these minerals, however, is complex and incompletely understood. They are diagnostic of depositional and diagenetic conditions, but difficult to detect and measure. To get you started, you will examine the petrography of some of the common minerals found in carbonate rocks.
1. calcite: Stains red with Alizarin Red "S".
a) This mineral occurs as a mosaic of polygonal spar crystals (sparry calcite or calspar) such as in (Mic/8/K/Tx-BZF) and as former fossil fragments in (B/38/Ord/Ky-CEO). Sketch the sparry calcite crystals from one of these slides.
b. calcite is precipitated by organisms as shells, etc. Look at the fossils in (B/36/Ord/Ohio-BAH). You will be identifying these later. Sketch the fabric now.
2. Aragonite: Similar in chemical composition to calcite, but with a different crystal system and trace element chemistry. This mineral is metastable, most often occurring as organism hard parts, and/or(inorganically?) precipitated cement, and micrite. Easily detected by using Feigl's solution which stains aragonite opaque black.
a. Unaltered skeletal aragonite is usually honey colored in plain light and skeletal microstructure is visible: (B/153/Hol/Aust-CGC). For example, sketch a) aragonitic shells b) aragonitic ooids. Find a large chambered fossil (Gastropod) with a fringe of aragonite needle cement. Is it honey colored? Why?
3. Magnesian calcite (MgCaCO3): This is calcite with a trace quantity of Mg which exceeds a critical value. The most commonly used (and the most arbitrary) limits are: low magnesian calcite = 0-5 mol %Mg; high magnesian calcite =10-18%. Most carbonate geologists recognize limits as: Low magnesian calcite less than 1%. High magnesian calcite 10-40%. Mg calcites with molar concentrations 5%<Mg<10% are rare. Magnesian calcite is common as skeletal remains, a precipitated cement, or micrite. Most easily detected by use of clayton yellow which stains this mineral yellow to red/orange. (B/2/Hol/Eni-26246) contains both aragonite and Mg calcite grains. Half of the slide is stained with clayton yellow. Compare single grain types, on the stained vs. unstained portions. Are the needle-like crystals Mg calcite or aragonite?
4. dolomite: The most enigmatic of the carbonate minerals, which varies in composition from about (Mg0.8Ca1.2)(CO3)2 to nearly stoichiometric (Mg Ca)(CO3)2. Distinguished by its common rhombic crystal habit and use of stains: Alizarin Red S stains both calcite and aragonite pink-red, but leaves dolomite unstained. (Dol/10/Jur/Saud- 25541) contains calcite and dolomite grains, distinguished by stain. Sketch a portion of the field of view; label your sketch.
5. Silica (quartz, chalcedony, etc.) These cannot be identified with stains but can be recognized by optical properties. They occur most often as detrital grains or as diagenetic replacement of other minerals.
a. Detrital quartz, some as ooid nuclei in (B/153/Hol/Aust-CGC).What is the grain size?
b. Chalcedony replaced oolite (O/41/Ord/Pa-DUB). Note how faithfully the silicification process mimics much of the original rock fabric.
6. Iron Minerals
a. (B/151/Camb/Tx-CBG).The trilobite/echinoderm biosparite with glauconite grains, typical of Cambrian platform (epieric) seas. glauconite grains are the green pellets, possibly fecal, composed of this iron-rich clay. glauconite is an indicator of shallow-water, nearshore, marine sedimentation.
b. (Bio/152/CLT?).The yellow/brown opaque mineral is hematite which has probably replaced some of these grains but it also may be primary. Look for evidence of either or both. Give an opinion of the origin of the hematite citing your evidence.
7. evaporites: Common, but often removed from the rock by careless thin section preparation. Look also for moldic pores which may indicate former presence of these important minerals differentiated by their birefringence and crystal habit.
a. (K/1/Pm/Eng-DQM) .Fibrous gypsum from a Permian evaporite sequence, (associated with the Zechstein salt). Note the birefringence and bladed habit common to gypsum. Compare to the properties of anhydrite. How do you distinguish between these two common evaporite minerals in thin section? See also Memoir 27, p. 133, 134.
b. (Dol/11/Mio/Iran).Blocky anhydrite in a dolomite from the Asmari limestone, a lagoonal/sabkha deposit which is the principal Iranian oil reservoir. Identify and sketch dolomite inclusions in anhydrite crystals. Why do some crystals have first-order gray birefringence? Is this gypsum? Explain.
8. For the hydrocarbon experts: (N/17/Pa/NMx-BRF). Pockets of "dead oil"; hydrocarbons that have been heated above the temperature at which liquid oil can exist. This brown translucent material is referred to as "organic matter" in many limestone descriptions. Note how it is isolated into pockets by limestone recrystallization or diagenesis.